| Message ID | 1408611405-8943-3-git-send-email-takahiro.akashi@linaro.org |
|---|---|
| State | New |
| Headers | show |
On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro <takahiro.akashi@linaro.org> wrote: > If tracer specifies -1 as a syscall number, this traced system call should > be skipped with a value in x0 used as a return value. > This patch enables this semantics, but there is a restriction here: > > when syscall(-1) is issued by user, tracer cannot skip this system call > and modify a return value at syscall entry. > > In order to ease this flavor, we need to treat whatever value in x0 as > a return value, but this might result in a bogus value being returned, > especially when tracer doesn't do anything at this syscall. > So we always return ENOSYS instead, while we have another chance to change > a return value at syscall exit. > > Please also note: > * syscall entry tracing and syscall exit tracing (ftrace tracepoint and > audit) are always executed, if enabled, even when skipping a system call > (that is, -1). > In this way, we can avoid a potential bug where audit_syscall_entry() > might be called without audit_syscall_exit() at the previous system call > being called, that would cause OOPs in audit_syscall_entry(). > > * syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected > in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS) > is not used in this case, we may neglect the case. > > Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org> > --- > arch/arm64/include/asm/ptrace.h | 8 ++++++++ > arch/arm64/kernel/entry.S | 4 ++++ > arch/arm64/kernel/ptrace.c | 20 ++++++++++++++++++++ > 3 files changed, 32 insertions(+) > > diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h > index 501000f..a58cf62 100644 > --- a/arch/arm64/include/asm/ptrace.h > +++ b/arch/arm64/include/asm/ptrace.h > @@ -65,6 +65,14 @@ > #define COMPAT_PT_TEXT_ADDR 0x10000 > #define COMPAT_PT_DATA_ADDR 0x10004 > #define COMPAT_PT_TEXT_END_ADDR 0x10008 > + > +/* > + * used to skip a system call when tracer changes its number to -1 > + * with ptrace(PTRACE_SET_SYSCALL) > + */ > +#define RET_SKIP_SYSCALL -1 > +#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1) > + > #ifndef __ASSEMBLY__ > > /* sizeof(struct user) for AArch32 */ > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > index f0b5e51..fdd6eae 100644 > --- a/arch/arm64/kernel/entry.S > +++ b/arch/arm64/kernel/entry.S > @@ -25,6 +25,7 @@ > #include <asm/asm-offsets.h> > #include <asm/errno.h> > #include <asm/esr.h> > +#include <asm/ptrace.h> > #include <asm/thread_info.h> > #include <asm/unistd.h> > > @@ -671,6 +672,8 @@ ENDPROC(el0_svc) > __sys_trace: > mov x0, sp > bl syscall_trace_enter > + cmp w0, #RET_SKIP_SYSCALL // skip syscall? > + b.eq __sys_trace_return_skipped > adr lr, __sys_trace_return // return address > uxtw scno, w0 // syscall number (possibly new) > mov x1, sp // pointer to regs > @@ -685,6 +688,7 @@ __sys_trace: > > __sys_trace_return: > str x0, [sp] // save returned x0 > +__sys_trace_return_skipped: // x0 already in regs[0] > mov x0, sp > bl syscall_trace_exit > b ret_to_user > diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c > index 8876049..c54dbcc 100644 > --- a/arch/arm64/kernel/ptrace.c > +++ b/arch/arm64/kernel/ptrace.c > @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs, > > asmlinkage int syscall_trace_enter(struct pt_regs *regs) > { > + unsigned int saved_syscallno = regs->syscallno; > + > if (test_thread_flag(TIF_SYSCALL_TRACE)) > tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); > > + if (IS_SKIP_SYSCALL(regs->syscallno)) { > + /* > + * RESTRICTION: we can't modify a return value of user > + * issued syscall(-1) here. In order to ease this flavor, > + * we need to treat whatever value in x0 as a return value, > + * but this might result in a bogus value being returned. > + */ > + /* > + * NOTE: syscallno may also be set to -1 if fatal signal is > + * detected in tracehook_report_syscall_entry(), but since > + * a value set to x0 here is not used in this case, we may > + * neglect the case. > + */ > + if (!test_thread_flag(TIF_SYSCALL_TRACE) || > + (IS_SKIP_SYSCALL(saved_syscallno))) > + regs->regs[0] = -ENOSYS; > + } > + I don't have a runtime environment yet for arm64, so I can't test this directly myself, so I'm just trying to eyeball this. :) Once the seccomp logic is added here, I don't think using -2 as a special value will work. Doesn't this mean the Oops is possible by the user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and the user passed -2 as the syscall, audit will be called only on entry, and then skipped on exit? -Kees > if (test_thread_flag(TIF_SYSCALL_TRACEPOINT)) > trace_sys_enter(regs, regs->syscallno); > > -- > 1.7.9.5 >
On 08/22/2014 02:08 AM, Kees Cook wrote: > On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro > <takahiro.akashi@linaro.org> wrote: >> If tracer specifies -1 as a syscall number, this traced system call should >> be skipped with a value in x0 used as a return value. >> This patch enables this semantics, but there is a restriction here: >> >> when syscall(-1) is issued by user, tracer cannot skip this system call >> and modify a return value at syscall entry. >> >> In order to ease this flavor, we need to treat whatever value in x0 as >> a return value, but this might result in a bogus value being returned, >> especially when tracer doesn't do anything at this syscall. >> So we always return ENOSYS instead, while we have another chance to change >> a return value at syscall exit. >> >> Please also note: >> * syscall entry tracing and syscall exit tracing (ftrace tracepoint and >> audit) are always executed, if enabled, even when skipping a system call >> (that is, -1). >> In this way, we can avoid a potential bug where audit_syscall_entry() >> might be called without audit_syscall_exit() at the previous system call >> being called, that would cause OOPs in audit_syscall_entry(). >> >> * syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected >> in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS) >> is not used in this case, we may neglect the case. >> >> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org> >> --- >> arch/arm64/include/asm/ptrace.h | 8 ++++++++ >> arch/arm64/kernel/entry.S | 4 ++++ >> arch/arm64/kernel/ptrace.c | 20 ++++++++++++++++++++ >> 3 files changed, 32 insertions(+) >> >> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h >> index 501000f..a58cf62 100644 >> --- a/arch/arm64/include/asm/ptrace.h >> +++ b/arch/arm64/include/asm/ptrace.h >> @@ -65,6 +65,14 @@ >> #define COMPAT_PT_TEXT_ADDR 0x10000 >> #define COMPAT_PT_DATA_ADDR 0x10004 >> #define COMPAT_PT_TEXT_END_ADDR 0x10008 >> + >> +/* >> + * used to skip a system call when tracer changes its number to -1 >> + * with ptrace(PTRACE_SET_SYSCALL) >> + */ >> +#define RET_SKIP_SYSCALL -1 >> +#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1) >> + >> #ifndef __ASSEMBLY__ >> >> /* sizeof(struct user) for AArch32 */ >> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S >> index f0b5e51..fdd6eae 100644 >> --- a/arch/arm64/kernel/entry.S >> +++ b/arch/arm64/kernel/entry.S >> @@ -25,6 +25,7 @@ >> #include <asm/asm-offsets.h> >> #include <asm/errno.h> >> #include <asm/esr.h> >> +#include <asm/ptrace.h> >> #include <asm/thread_info.h> >> #include <asm/unistd.h> >> >> @@ -671,6 +672,8 @@ ENDPROC(el0_svc) >> __sys_trace: >> mov x0, sp >> bl syscall_trace_enter >> + cmp w0, #RET_SKIP_SYSCALL // skip syscall? >> + b.eq __sys_trace_return_skipped >> adr lr, __sys_trace_return // return address >> uxtw scno, w0 // syscall number (possibly new) >> mov x1, sp // pointer to regs >> @@ -685,6 +688,7 @@ __sys_trace: >> >> __sys_trace_return: >> str x0, [sp] // save returned x0 >> +__sys_trace_return_skipped: // x0 already in regs[0] >> mov x0, sp >> bl syscall_trace_exit >> b ret_to_user >> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c >> index 8876049..c54dbcc 100644 >> --- a/arch/arm64/kernel/ptrace.c >> +++ b/arch/arm64/kernel/ptrace.c >> @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs, >> >> asmlinkage int syscall_trace_enter(struct pt_regs *regs) >> { >> + unsigned int saved_syscallno = regs->syscallno; >> + >> if (test_thread_flag(TIF_SYSCALL_TRACE)) >> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); >> >> + if (IS_SKIP_SYSCALL(regs->syscallno)) { >> + /* >> + * RESTRICTION: we can't modify a return value of user >> + * issued syscall(-1) here. In order to ease this flavor, >> + * we need to treat whatever value in x0 as a return value, >> + * but this might result in a bogus value being returned. >> + */ >> + /* >> + * NOTE: syscallno may also be set to -1 if fatal signal is >> + * detected in tracehook_report_syscall_entry(), but since >> + * a value set to x0 here is not used in this case, we may >> + * neglect the case. >> + */ >> + if (!test_thread_flag(TIF_SYSCALL_TRACE) || >> + (IS_SKIP_SYSCALL(saved_syscallno))) >> + regs->regs[0] = -ENOSYS; >> + } >> + > > I don't have a runtime environment yet for arm64, so I can't test this > directly myself, so I'm just trying to eyeball this. :) > > Once the seccomp logic is added here, I don't think using -2 as a > special value will work. Doesn't this mean the Oops is possible by the > user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and > the user passed -2 as the syscall, audit will be called only on entry, > and then skipped on exit? Oops, you're absolutely right. I didn't think of this case. syscall_trace_enter() should not return a syscallno directly, but always return -1 if syscallno < 0. (except when secure_computing() returns with -1) This also implies that tracehook_report_syscall() should also have a return value. Will, is this fine with you? -Takahiro AKASHI > -Kees > >> if (test_thread_flag(TIF_SYSCALL_TRACEPOINT)) >> trace_sys_enter(regs, regs->syscallno); >> >> -- >> 1.7.9.5 >> > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Fri, Aug 22, 2014 at 01:35:17AM +0100, AKASHI Takahiro wrote: > On 08/22/2014 02:08 AM, Kees Cook wrote: > > On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro > > <takahiro.akashi@linaro.org> wrote: > >> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c > >> index 8876049..c54dbcc 100644 > >> --- a/arch/arm64/kernel/ptrace.c > >> +++ b/arch/arm64/kernel/ptrace.c > >> @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs, > >> > >> asmlinkage int syscall_trace_enter(struct pt_regs *regs) > >> { > >> + unsigned int saved_syscallno = regs->syscallno; > >> + > >> if (test_thread_flag(TIF_SYSCALL_TRACE)) > >> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); > >> > >> + if (IS_SKIP_SYSCALL(regs->syscallno)) { > >> + /* > >> + * RESTRICTION: we can't modify a return value of user > >> + * issued syscall(-1) here. In order to ease this flavor, > >> + * we need to treat whatever value in x0 as a return value, > >> + * but this might result in a bogus value being returned. > >> + */ > >> + /* > >> + * NOTE: syscallno may also be set to -1 if fatal signal is > >> + * detected in tracehook_report_syscall_entry(), but since > >> + * a value set to x0 here is not used in this case, we may > >> + * neglect the case. > >> + */ > >> + if (!test_thread_flag(TIF_SYSCALL_TRACE) || > >> + (IS_SKIP_SYSCALL(saved_syscallno))) > >> + regs->regs[0] = -ENOSYS; > >> + } > >> + > > > > I don't have a runtime environment yet for arm64, so I can't test this > > directly myself, so I'm just trying to eyeball this. :) > > > > Once the seccomp logic is added here, I don't think using -2 as a > > special value will work. Doesn't this mean the Oops is possible by the > > user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and > > the user passed -2 as the syscall, audit will be called only on entry, > > and then skipped on exit? > > Oops, you're absolutely right. I didn't think of this case. > syscall_trace_enter() should not return a syscallno directly, but always > return -1 if syscallno < 0. (except when secure_computing() returns with -1) > This also implies that tracehook_report_syscall() should also have a return value. > > Will, is this fine with you? Well, the first thing that jumps out at me is why this is being done completely differently for arm64 and arm. I thought adding the new ptrace requests would reconcile the differences? Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On 08/27/2014 02:51 AM, Will Deacon wrote: > On Fri, Aug 22, 2014 at 01:35:17AM +0100, AKASHI Takahiro wrote: >> On 08/22/2014 02:08 AM, Kees Cook wrote: >>> On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro >>> <takahiro.akashi@linaro.org> wrote: >>>> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c >>>> index 8876049..c54dbcc 100644 >>>> --- a/arch/arm64/kernel/ptrace.c >>>> +++ b/arch/arm64/kernel/ptrace.c >>>> @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs, >>>> >>>> asmlinkage int syscall_trace_enter(struct pt_regs *regs) >>>> { >>>> + unsigned int saved_syscallno = regs->syscallno; >>>> + >>>> if (test_thread_flag(TIF_SYSCALL_TRACE)) >>>> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); >>>> >>>> + if (IS_SKIP_SYSCALL(regs->syscallno)) { >>>> + /* >>>> + * RESTRICTION: we can't modify a return value of user >>>> + * issued syscall(-1) here. In order to ease this flavor, >>>> + * we need to treat whatever value in x0 as a return value, >>>> + * but this might result in a bogus value being returned. >>>> + */ >>>> + /* >>>> + * NOTE: syscallno may also be set to -1 if fatal signal is >>>> + * detected in tracehook_report_syscall_entry(), but since >>>> + * a value set to x0 here is not used in this case, we may >>>> + * neglect the case. >>>> + */ >>>> + if (!test_thread_flag(TIF_SYSCALL_TRACE) || >>>> + (IS_SKIP_SYSCALL(saved_syscallno))) >>>> + regs->regs[0] = -ENOSYS; >>>> + } >>>> + >>> >>> I don't have a runtime environment yet for arm64, so I can't test this >>> directly myself, so I'm just trying to eyeball this. :) >>> >>> Once the seccomp logic is added here, I don't think using -2 as a >>> special value will work. Doesn't this mean the Oops is possible by the >>> user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and >>> the user passed -2 as the syscall, audit will be called only on entry, >>> and then skipped on exit? >> >> Oops, you're absolutely right. I didn't think of this case. >> syscall_trace_enter() should not return a syscallno directly, but always >> return -1 if syscallno < 0. (except when secure_computing() returns with -1) >> This also implies that tracehook_report_syscall() should also have a return value. >> >> Will, is this fine with you? > > Well, the first thing that jumps out at me is why this is being done > completely differently for arm64 and arm. I thought adding the new ptrace > requests would reconcile the differences? I'm not sure what portion of my code you mentioned as "completely different", but 1) setting x0 to -ENOSYS is necessary because, otherwise, user-issued syscall(-1) will return a bogus value when audit tracing is on. Please note that, on arm, not traced traced ------ ------ syscall(-1) aborted OOPs(BUG_ON) syscall(-3000) aborted aborted syscall(1000) ENOSYS ENOSYS So, anyhow, its a bit difficult and meaningless to mimic these invalid cases. 2) branching a new label, syscall_trace_return_skip (see entry.S), after syscall_trace_enter() is necessary in order to avoid OOPS in audit_syscall_enter() as we discussed. Did I make it clear? -Takahiro AKASHI > Will > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Wed, Aug 27, 2014 at 06:55:46AM +0100, AKASHI Takahiro wrote: > On 08/27/2014 02:51 AM, Will Deacon wrote: > > On Fri, Aug 22, 2014 at 01:35:17AM +0100, AKASHI Takahiro wrote: > >> Oops, you're absolutely right. I didn't think of this case. > >> syscall_trace_enter() should not return a syscallno directly, but always > >> return -1 if syscallno < 0. (except when secure_computing() returns with -1) > >> This also implies that tracehook_report_syscall() should also have a return value. > >> > >> Will, is this fine with you? > > > > Well, the first thing that jumps out at me is why this is being done > > completely differently for arm64 and arm. I thought adding the new ptrace > > requests would reconcile the differences? > > I'm not sure what portion of my code you mentioned as "completely different", but > > 1) > setting x0 to -ENOSYS is necessary because, otherwise, user-issued syscall(-1) will > return a bogus value when audit tracing is on. > > Please note that, on arm, > not traced traced > ------ ------ > syscall(-1) aborted OOPs(BUG_ON) > syscall(-3000) aborted aborted > syscall(1000) ENOSYS ENOSYS > > So, anyhow, its a bit difficult and meaningless to mimic these invalid cases. I'm not suggesting we make ourselves bug-compatible with ARM. Instead, I'd rather see a series of patches getting the ARM code working correctly, before we go off doing something different for arm64. > 2) > branching a new label, syscall_trace_return_skip (see entry.S), after syscall_trace_enter() > is necessary in order to avoid OOPS in audit_syscall_enter() as we discussed. > > Did I make it clear? Sure. So let's fix ARM, then look at the arm64 port after that. I really want to avoid divergence in this area. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On 09/01/2014 08:37 PM, Will Deacon wrote: > On Wed, Aug 27, 2014 at 06:55:46AM +0100, AKASHI Takahiro wrote: >> On 08/27/2014 02:51 AM, Will Deacon wrote: >>> On Fri, Aug 22, 2014 at 01:35:17AM +0100, AKASHI Takahiro wrote: >>>> Oops, you're absolutely right. I didn't think of this case. >>>> syscall_trace_enter() should not return a syscallno directly, but always >>>> return -1 if syscallno < 0. (except when secure_computing() returns with -1) >>>> This also implies that tracehook_report_syscall() should also have a return value. >>>> >>>> Will, is this fine with you? >>> >>> Well, the first thing that jumps out at me is why this is being done >>> completely differently for arm64 and arm. I thought adding the new ptrace >>> requests would reconcile the differences? >> >> I'm not sure what portion of my code you mentioned as "completely different", but >> >> 1) >> setting x0 to -ENOSYS is necessary because, otherwise, user-issued syscall(-1) will >> return a bogus value when audit tracing is on. >> >> Please note that, on arm, >> not traced traced >> ------ ------ >> syscall(-1) aborted OOPs(BUG_ON) >> syscall(-3000) aborted aborted >> syscall(1000) ENOSYS ENOSYS >> >> So, anyhow, its a bit difficult and meaningless to mimic these invalid cases. > > I'm not suggesting we make ourselves bug-compatible with ARM. Instead, I'd > rather see a series of patches getting the ARM code working correctly, > before we go off doing something different for arm64. I see. >> 2) >> branching a new label, syscall_trace_return_skip (see entry.S), after syscall_trace_enter() >> is necessary in order to avoid OOPS in audit_syscall_enter() as we discussed. >> >> Did I make it clear? > > Sure. So let's fix ARM, then look at the arm64 port after that. I really > want to avoid divergence in this area. Okey, I will start with fixing the issue on arm. -Takahiro AKASHI > Will > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Will, When I was looking into syscall_trace_exit() more closely, I found another (big) problem. There are two system calls, execve() and rt_sigreturn(), which change 'syscallno' in pt_regs to -1 in start_thread() and restore_sigframe(), respectively. Since syscallno is not valid anymore in syscall_trace_exit() for these system calls, we cannot create a correct syscall exit record for tracepoint in trace_sys_exit() (=> ftrace_syscall_exit()) and for audit in audit_syscall_exit(). This does not happen on arm because syscall numbers are kept in thread_info on arm. How can we deal with this issue? -Takahiro AKASHI On 08/27/2014 02:51 AM, Will Deacon wrote: > On Fri, Aug 22, 2014 at 01:35:17AM +0100, AKASHI Takahiro wrote: >> On 08/22/2014 02:08 AM, Kees Cook wrote: >>> On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro >>> <takahiro.akashi@linaro.org> wrote: >>>> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c >>>> index 8876049..c54dbcc 100644 >>>> --- a/arch/arm64/kernel/ptrace.c >>>> +++ b/arch/arm64/kernel/ptrace.c >>>> @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs, >>>> >>>> asmlinkage int syscall_trace_enter(struct pt_regs *regs) >>>> { >>>> + unsigned int saved_syscallno = regs->syscallno; >>>> + >>>> if (test_thread_flag(TIF_SYSCALL_TRACE)) >>>> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); >>>> >>>> + if (IS_SKIP_SYSCALL(regs->syscallno)) { >>>> + /* >>>> + * RESTRICTION: we can't modify a return value of user >>>> + * issued syscall(-1) here. In order to ease this flavor, >>>> + * we need to treat whatever value in x0 as a return value, >>>> + * but this might result in a bogus value being returned. >>>> + */ >>>> + /* >>>> + * NOTE: syscallno may also be set to -1 if fatal signal is >>>> + * detected in tracehook_report_syscall_entry(), but since >>>> + * a value set to x0 here is not used in this case, we may >>>> + * neglect the case. >>>> + */ >>>> + if (!test_thread_flag(TIF_SYSCALL_TRACE) || >>>> + (IS_SKIP_SYSCALL(saved_syscallno))) >>>> + regs->regs[0] = -ENOSYS; >>>> + } >>>> + >>> >>> I don't have a runtime environment yet for arm64, so I can't test this >>> directly myself, so I'm just trying to eyeball this. :) >>> >>> Once the seccomp logic is added here, I don't think using -2 as a >>> special value will work. Doesn't this mean the Oops is possible by the >>> user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and >>> the user passed -2 as the syscall, audit will be called only on entry, >>> and then skipped on exit? >> >> Oops, you're absolutely right. I didn't think of this case. >> syscall_trace_enter() should not return a syscallno directly, but always >> return -1 if syscallno < 0. (except when secure_computing() returns with -1) >> This also implies that tracehook_report_syscall() should also have a return value. >> >> Will, is this fine with you? > > Well, the first thing that jumps out at me is why this is being done > completely differently for arm64 and arm. I thought adding the new ptrace > requests would reconcile the differences? > > Will > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Wed, Oct 01, 2014 at 12:08:05PM +0100, AKASHI Takahiro wrote: > Will, > > When I was looking into syscall_trace_exit() more closely, I found > another (big) problem. > There are two system calls, execve() and rt_sigreturn(), which change > 'syscallno' in pt_regs to -1 in start_thread() and restore_sigframe(), > respectively. > > Since syscallno is not valid anymore in syscall_trace_exit() for these > system calls, we cannot create a correct syscall exit record for tracepoint > in trace_sys_exit() (=> ftrace_syscall_exit()) and for audit in audit_syscall_exit(). > > This does not happen on arm because syscall numbers are kept in > thread_info on arm. > > How can we deal with this issue? How is this handled on other architectures? x86, for example, seems to zero orig_ax when restoring the sigcontext, but leaves it alone in start_thread. What is the impact of this problem? AFAICT, we just miss some exits, right (as opposed to an OOPs or the like)? Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On 10/04/2014 12:23 AM, Will Deacon wrote: > On Wed, Oct 01, 2014 at 12:08:05PM +0100, AKASHI Takahiro wrote: >> Will, >> >> When I was looking into syscall_trace_exit() more closely, I found >> another (big) problem. >> There are two system calls, execve() and rt_sigreturn(), which change >> 'syscallno' in pt_regs to -1 in start_thread() and restore_sigframe(), >> respectively. I need to correct my mis-understandings here: >> Since syscallno is not valid anymore in syscall_trace_exit() for these >> system calls, we cannot create a correct syscall exit record for tracepoint >> in trace_sys_exit() (=> ftrace_syscall_exit()) This is true, but since rt_sigreturn() doesn't have a syscall tracepoint (and so there is no entry under /sys/kernel/tracing/events/syscalls/), it cannot be traced anyway. >> and for audit in audit_syscall_exit(). not true. Since a syscall number is saved as 'major' in a per-thread audit context at audit_syscall_exit(), we will see a correct audit log for both system calls. >> This does not happen on arm because syscall numbers are kept in >> thread_info on arm. >> >> How can we deal with this issue? > > How is this handled on other architectures? x86, for example, seems to zero > orig_ax when restoring the sigcontext, but leaves it alone in start_thread. > What is the impact of this problem? AFAICT, we just miss some exits, right > (as opposed to an OOPs or the like)? So the impacts here are: 1) We just miss a syscall exit for execve tracepoint (syscalls:sys_exit_execve). (no fatal errors like kernel panic) (FYI, on x86, there is no tracepoint entry for execve nor sigreturn.) 2) From the viewpoint of my seccomp patch, we cannot skip some syscall exit tracing for invalid system calls by adding a check for syscallno in the following way: (I'm not quite sure this might cause a threat with DDoS attach as Russell suggested.) syscall_trace_exit(struct pt_regs *regs) { if (regs->syscallno < NR_syscalls) { /* Adding this check */ audit_syscall_exit(regs); if (test_thread_flags(TIF_SYSCALL_TRACEPOINT)) trace_sys_exit(regs, regs_return_value(regs)); } ... } As you can imagine, any system call after execve() will hit BUG_ON() in audit_syscall_exit() since audit_syscall_exit() is not called for execve(). Thanks, -Takahiro AKASHI > Will > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h index 501000f..a58cf62 100644 --- a/arch/arm64/include/asm/ptrace.h +++ b/arch/arm64/include/asm/ptrace.h @@ -65,6 +65,14 @@ #define COMPAT_PT_TEXT_ADDR 0x10000 #define COMPAT_PT_DATA_ADDR 0x10004 #define COMPAT_PT_TEXT_END_ADDR 0x10008 + +/* + * used to skip a system call when tracer changes its number to -1 + * with ptrace(PTRACE_SET_SYSCALL) + */ +#define RET_SKIP_SYSCALL -1 +#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1) + #ifndef __ASSEMBLY__ /* sizeof(struct user) for AArch32 */ diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S index f0b5e51..fdd6eae 100644 --- a/arch/arm64/kernel/entry.S +++ b/arch/arm64/kernel/entry.S @@ -25,6 +25,7 @@ #include <asm/asm-offsets.h> #include <asm/errno.h> #include <asm/esr.h> +#include <asm/ptrace.h> #include <asm/thread_info.h> #include <asm/unistd.h> @@ -671,6 +672,8 @@ ENDPROC(el0_svc) __sys_trace: mov x0, sp bl syscall_trace_enter + cmp w0, #RET_SKIP_SYSCALL // skip syscall? + b.eq __sys_trace_return_skipped adr lr, __sys_trace_return // return address uxtw scno, w0 // syscall number (possibly new) mov x1, sp // pointer to regs @@ -685,6 +688,7 @@ __sys_trace: __sys_trace_return: str x0, [sp] // save returned x0 +__sys_trace_return_skipped: // x0 already in regs[0] mov x0, sp bl syscall_trace_exit b ret_to_user diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c index 8876049..c54dbcc 100644 --- a/arch/arm64/kernel/ptrace.c +++ b/arch/arm64/kernel/ptrace.c @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs, asmlinkage int syscall_trace_enter(struct pt_regs *regs) { + unsigned int saved_syscallno = regs->syscallno; + if (test_thread_flag(TIF_SYSCALL_TRACE)) tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); + if (IS_SKIP_SYSCALL(regs->syscallno)) { + /* + * RESTRICTION: we can't modify a return value of user + * issued syscall(-1) here. In order to ease this flavor, + * we need to treat whatever value in x0 as a return value, + * but this might result in a bogus value being returned. + */ + /* + * NOTE: syscallno may also be set to -1 if fatal signal is + * detected in tracehook_report_syscall_entry(), but since + * a value set to x0 here is not used in this case, we may + * neglect the case. + */ + if (!test_thread_flag(TIF_SYSCALL_TRACE) || + (IS_SKIP_SYSCALL(saved_syscallno))) + regs->regs[0] = -ENOSYS; + } + if (test_thread_flag(TIF_SYSCALL_TRACEPOINT)) trace_sys_enter(regs, regs->syscallno);
If tracer specifies -1 as a syscall number, this traced system call should be skipped with a value in x0 used as a return value. This patch enables this semantics, but there is a restriction here: when syscall(-1) is issued by user, tracer cannot skip this system call and modify a return value at syscall entry. In order to ease this flavor, we need to treat whatever value in x0 as a return value, but this might result in a bogus value being returned, especially when tracer doesn't do anything at this syscall. So we always return ENOSYS instead, while we have another chance to change a return value at syscall exit. Please also note: * syscall entry tracing and syscall exit tracing (ftrace tracepoint and audit) are always executed, if enabled, even when skipping a system call (that is, -1). In this way, we can avoid a potential bug where audit_syscall_entry() might be called without audit_syscall_exit() at the previous system call being called, that would cause OOPs in audit_syscall_entry(). * syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS) is not used in this case, we may neglect the case. Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org> --- arch/arm64/include/asm/ptrace.h | 8 ++++++++ arch/arm64/kernel/entry.S | 4 ++++ arch/arm64/kernel/ptrace.c | 20 ++++++++++++++++++++ 3 files changed, 32 insertions(+)