Re: siginfo pid not populated from ptrace?

[Kees Cook]

On Tue, Nov 27, 2018 at 8:44 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>
> Kees Cook <keescook@xxxxxxxxxxxx> writes:
>
> > On Tue, Nov 27, 2018 at 4:38 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >> On Tue, Nov 27, 2018 at 3:21 PM, Tycho Andersen <tycho@xxxxxxxx> wrote:
> >>> On Mon, Nov 12, 2018 at 12:24:43PM -0700, Tycho Andersen wrote:
> >>>> On Mon, Nov 12, 2018 at 11:55:38AM -0700, Tycho Andersen wrote:
> >>>> > I haven't manage to reproduce it on stock v4.20-rc2, unfortunately.
> >>>>
> >>>> Ok, now I have,
> >>>>
> >>>> seccomp_bpf.c:2736:global.syscall_restart:Expected getpid() (1493) == info._sifields._kill.si_pid (0)
> >>>> global.syscall_restart: Test failed at step #22
> >>>
> >>> Seems like this is still happening on v4.20-rc4,
> >>>
> >>> [ RUN      ] global.syscall_restart
> >>> seccomp_bpf.c:2736:global.syscall_restart:Expected getpid() (1901) == info._sifields._kill.si_pid (0)
> >>> global.syscall_restart: Test failed at step #22
> >>
> >> This fails every time for me -- is it still racey for you?
> >>
> >> I'm attempting a bisect, hoping it doesn't _become_ racey for me. ;)
> >
> > This bisect to here for me:
> >
> > commit f149b31557446aff9ca96d4be7e39cc266f6e7cc
> > Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> > Date:   Mon Sep 3 09:50:36 2018 +0200
> >
> >     signal: Never allocate siginfo for SIGKILL or SIGSTOP
> >
> >     The SIGKILL and SIGSTOP signals are never delivered to userspace so
> >     queued siginfo for these signals can never be observed.  Therefore
> >     remove the chance of failure by never even attempting to allocate
> >     siginfo in those cases.
> >
> >     Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >     Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> >
> > They are certainly visible via seccomp ;)
>
> Well SIGSTOP is visible via PTRACE_GETSIGINFO.
>
> I see what is happening now.  Since we don't have queued siginfo
> we generate some as:
>                 /*
>                  * Ok, it wasn't in the queue.  This must be
>                  * a fast-pathed signal or we must have been
>                  * out of queue space.  So zero out the info.
>                  */
>                 clear_siginfo(info);
>                 info->si_signo = sig;
>                 info->si_errno = 0;
>                 info->si_code = SI_USER;
>                 info->si_pid = 0;
>                 info->si_uid = 0;
>
> Which allows last_signfo to be set,
> so despite not really having any siginfo PTRACE_GET_SIGINFO
> has something to return so does not return -EINVAL.
>
> Reconstructing my context that was part of removing SEND_SIG_FORCED
> so this looks like it will take a little more than a revert to fix
> this.
>
> This is definitely a change that is visible to user space.  The logic in
> my patch was definitely wrong with respect to SIGSTOP and
> PTRACE_GETSIGINFO.  Is there something in userspace that actually cares?
> AKA is the idiom that the test seccomp_bpf.c is using something that
> non-test code does?

I think this would be needed by any ptracer that handled multiple
threads. It needs to figure out which pid stopped. I think it's worth
fixing, yes.

> The change below should restore the old behavior.  I am just wondering
> if this is something we want to do.  siginfo is allocated with
> GFP_ATOMIC so if your machine is under memory pressure there is a real
> chance the allocation can fail.  Which would cause whatever is breaking
> now to break less deterministically then.

I think memory pressure that would block a 128 byte GFP_ATOMIC
allocation would mean the system was about to seriously fall over.
Given the user-facing behavior change and that an existing test was
already checking for this means we need to fix it.

> If we need to fix this do we need to make siginfo allocation more
> reliable?

I don't think so -- we'd already get a WARN() if allocation failed.

> Eric
>
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 4fd431ce4f91..5c34c55bfea4 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1057,10 +1057,10 @@ static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struc
>
>         result = TRACE_SIGNAL_DELIVERED;
>         /*
> -        * Skip useless siginfo allocation for SIGKILL SIGSTOP,
> +        * Skip useless siginfo allocation for SIGKILL,
>          * and kernel threads.
>          */
> -       if (sig_kernel_only(sig) || (t->flags & PF_KTHREAD))
> +       if ((sig == SIGKILL) || (t->flags & PF_KTHREAD))
>                 goto out_set;
>
>         /*
>

This fixes it for me!

Reported-by: Tycho Andersen <tycho@xxxxxxxx>
Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Fixes: f149b3155744 ("signal: Never allocate siginfo for SIGKILL or SIGSTOP")

Thanks!

-- 
Kees Cook