| Message ID | 20220210025321.787113-2-keescook@chromium.org |
|---|---|
| State | Accepted |
| Commit | 5c72263ef2fbe99596848f03758ae2dc593adf2c |
| Headers | show |
| Series | [1/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE | expand |
On Thu, Feb 10, 2022 at 10:09 PM Kees Cook <keescook@chromium.org> wrote: > On Thu, Feb 10, 2022 at 07:01:39PM +0100, Jann Horn wrote: > > On Thu, Feb 10, 2022 at 6:37 PM Kees Cook <keescook@chromium.org> wrote: > > > On Thu, Feb 10, 2022 at 05:18:39PM +0100, Jann Horn wrote: > > > > On Thu, Feb 10, 2022 at 3:53 AM Kees Cook <keescook@chromium.org> wrote: > > > > > Fatal SIGSYS signals were not being delivered to pid namespace init > > > > > processes. Make sure the SIGNAL_UNKILLABLE doesn't get set for these > > > > > cases. > > > > > > > > > > Reported-by: Robert Święcki <robert@swiecki.net> > > > > > Suggested-by: "Eric W. Biederman" <ebiederm@xmission.com> > > > > > Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed") > > > > > Cc: stable@vger.kernel.org > > > > > Signed-off-by: Kees Cook <keescook@chromium.org> > > > > > --- > > > > > kernel/signal.c | 5 +++-- > > > > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/kernel/signal.c b/kernel/signal.c > > > > > index 38602738866e..33e3ee4f3383 100644 > > > > > --- a/kernel/signal.c > > > > > +++ b/kernel/signal.c > > > > > @@ -1342,9 +1342,10 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, > > > > > } > > > > > /* > > > > > * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect > > > > > - * debugging to leave init killable. > > > > > + * debugging to leave init killable, unless it is intended to exit. > > > > > */ > > > > > - if (action->sa.sa_handler == SIG_DFL && !t->ptrace) > > > > > + if (action->sa.sa_handler == SIG_DFL && > > > > > + (!t->ptrace || (handler == HANDLER_EXIT))) > > > > > t->signal->flags &= ~SIGNAL_UNKILLABLE; > > > > > > > > You're changing the subclause: > > > > > > > > !t->ptrace > > > > > > > > to: > > > > > > > > (!t->ptrace || (handler == HANDLER_EXIT)) > > > > > > > > which means that the change only affects cases where the process has a > > > > ptracer, right? That's not the scenario the commit message is talking > > > > about... > > > > > > Sorry, yes, I was not as accurate as I should have been in the commit > > > log. I have changed it to: > > > > > > Fatal SIGSYS signals (i.e. seccomp RET_KILL_* syscall filter actions) > > > were not being delivered to ptraced pid namespace init processes. Make > > > sure the SIGNAL_UNKILLABLE doesn't get set for these cases. > > > > So basically force_sig_info() is trying to figure out whether > > get_signal() will later on check for SIGNAL_UNKILLABLE (the SIG_DFL > > case), and if so, it clears the flag from the target's signal_struct > > that marks the process as unkillable? > > > > This used to be: > > > > if (action->sa.sa_handler == SIG_DFL) > > t->signal->flags &= ~SIGNAL_UNKILLABLE; > > > > Then someone noticed that in the ptrace case, the signal might not > > actually end up being consumed by the target process, and added the > > "&& !t->ptrace" clause in commit > > eb61b5911bdc923875cde99eb25203a0e2b06d43. > > > > And now Robert Swiecki noticed that that still didn't accurately model > > what'll happen in get_signal(). > > > > This seems hacky to me, and also racy: What if, while you're going > > through a SECCOMP_RET_KILL_PROCESS in an unkillable process, some > > other thread e.g. concurrently changes the disposition of SIGSYS from > > a custom handler to SIG_DFL? > > Do you mean after force_sig_info_to_task() has finished but before > get_signal()? SA_IMMUTABLE will block changes to the action. Yeah, that's what I meant. Thanks, I missed SA_IMMUTABLE. Ugh, this is not pretty code...
diff --git a/kernel/signal.c b/kernel/signal.c index 38602738866e..33e3ee4f3383 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1342,9 +1342,10 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, } /* * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect - * debugging to leave init killable. + * debugging to leave init killable, unless it is intended to exit. */ - if (action->sa.sa_handler == SIG_DFL && !t->ptrace) + if (action->sa.sa_handler == SIG_DFL && + (!t->ptrace || (handler == HANDLER_EXIT))) t->signal->flags &= ~SIGNAL_UNKILLABLE; ret = send_signal(sig, info, t, PIDTYPE_PID); spin_unlock_irqrestore(&t->sighand->siglock, flags);
Fatal SIGSYS signals were not being delivered to pid namespace init processes. Make sure the SIGNAL_UNKILLABLE doesn't get set for these cases. Reported-by: Robert Święcki <robert@swiecki.net> Suggested-by: "Eric W. Biederman" <ebiederm@xmission.com> Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed") Cc: stable@vger.kernel.org Signed-off-by: Kees Cook <keescook@chromium.org> --- kernel/signal.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)