arm64: Fix syscall restarting around signal suppressed by tracer

Message ID 1528371125-24505-1-git-send-email-Dave.Martin@arm.com
State New
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Series
  • arm64: Fix syscall restarting around signal suppressed by tracer
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Commit Message

Dave Martin June 7, 2018, 11:32 a.m.
Commit 17c2895 ("arm64: Abstract syscallno manipulation") abstracts
out the pt_regs.syscallno value for a syscall cancelled by a tracer
as NO_SYSCALL, and provides helpers to set and check for this
condition.  However, the way this was implemented has the
unintended side-effect of disabling part of the syscall restart
logic.

This comes about because the second in_syscall() check in
do_signal() re-evaluates the "in a syscall" condition based on the
updated pt_regs instead of the original pt_regs.  forget_syscall()
is explicitly called prior to the second check in order to prevent
restart logic in the ret_to_user path being spuriously triggered,
which means that the second in_syscall() check always yields false.

This triggers a failure in
tools/testing/selftests/seccomp/seccomp_bpf.c, when using ptrace to
suppress a signal that interrups a nanosleep() syscall.

Misbehaviour of this type is only expected in the case where a
tracer suppresses a signal and the target process is either being
single-stepped or the interrupted syscall attempts to restart via
-ERESTARTBLOCK.

This patch restores the old behaviour by performing the
in_syscall() check only once at the start of the function.

Fixes: 17c289586009 ("arm64: Abstract syscallno manipulation")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>

Reported-by: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: <stable@vger.kernel.org> # 4.14.x-
---
 arch/arm64/kernel/signal.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

-- 
2.1.4

Comments

Catalin Marinas June 8, 2018, 12:22 p.m. | #1
On Thu, Jun 07, 2018 at 12:32:05PM +0100, Dave P Martin wrote:
> Commit 17c2895 ("arm64: Abstract syscallno manipulation") abstracts

> out the pt_regs.syscallno value for a syscall cancelled by a tracer

> as NO_SYSCALL, and provides helpers to set and check for this

> condition.  However, the way this was implemented has the

> unintended side-effect of disabling part of the syscall restart

> logic.

> 

> This comes about because the second in_syscall() check in

> do_signal() re-evaluates the "in a syscall" condition based on the

> updated pt_regs instead of the original pt_regs.  forget_syscall()

> is explicitly called prior to the second check in order to prevent

> restart logic in the ret_to_user path being spuriously triggered,

> which means that the second in_syscall() check always yields false.

> 

> This triggers a failure in

> tools/testing/selftests/seccomp/seccomp_bpf.c, when using ptrace to

> suppress a signal that interrups a nanosleep() syscall.

> 

> Misbehaviour of this type is only expected in the case where a

> tracer suppresses a signal and the target process is either being

> single-stepped or the interrupted syscall attempts to restart via

> -ERESTARTBLOCK.

> 

> This patch restores the old behaviour by performing the

> in_syscall() check only once at the start of the function.

> 

> Fixes: 17c289586009 ("arm64: Abstract syscallno manipulation")

> Signed-off-by: Dave Martin <Dave.Martin@arm.com>

> Reported-by: Sumit Semwal <sumit.semwal@linaro.org>

> Cc: Will Deacon <will.deacon@arm.com>

> Cc: Catalin Marinas <catalin.marinas@arm.com>

> Cc: <stable@vger.kernel.org> # 4.14.x-


Applied. Thanks.

-- 
Catalin

Patch

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 154b7d3..f212090 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -830,11 +830,12 @@  static void do_signal(struct pt_regs *regs)
 	unsigned long continue_addr = 0, restart_addr = 0;
 	int retval = 0;
 	struct ksignal ksig;
+	bool syscall = in_syscall(regs);
 
 	/*
 	 * If we were from a system call, check for system call restarting...
 	 */
-	if (in_syscall(regs)) {
+	if (syscall) {
 		continue_addr = regs->pc;
 		restart_addr = continue_addr - (compat_thumb_mode(regs) ? 2 : 4);
 		retval = regs->regs[0];
@@ -886,7 +887,7 @@  static void do_signal(struct pt_regs *regs)
 	 * Handle restarting a different system call. As above, if a debugger
 	 * has chosen to restart at a different PC, ignore the restart.
 	 */
-	if (in_syscall(regs) && regs->pc == restart_addr) {
+	if (syscall && regs->pc == restart_addr) {
 		if (retval == -ERESTART_RESTARTBLOCK)
 			setup_restart_syscall(regs);
 		user_rewind_single_step(current);