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From: Kees Cook <keescook@chromium.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Kyle Huey <me@kylehuey.com>,
	Andrea Righi <andrea.righi@canonical.com>,
	Shuah Khan <shuah@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Will Drewry <wad@chromium.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>,
	bpf@vger.kernel.org, open list <linux-kernel@vger.kernel.org>,
	linux-hardening@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Robert O'Callahan <rocallahan@gmail.com>
Subject: Re: [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers
Date: Wed, 17 Nov 2021 13:54:14 -0800	[thread overview]
Message-ID: <202111171341.41053845C3@keescook> (raw)
In-Reply-To: <87k0h6334w.fsf@email.froward.int.ebiederm.org>

On Wed, Nov 17, 2021 at 03:04:31PM -0600, Eric W. Biederman wrote:
> Kyle Huey <me@kylehuey.com> writes:
> 
> > On Wed, Nov 17, 2021 at 11:05 AM Kyle Huey <me@kylehuey.com> wrote:
> >>
> >> On Wed, Nov 17, 2021 at 10:51 AM Kees Cook <keescook@chromium.org> wrote:
> >> >
> >> > On Wed, Nov 17, 2021 at 10:47:13AM -0800, Kyle Huey wrote:
> >> > > rr, a userspace record and replay debugger[0], is completely broken on
> >> > > 5.16rc1. I bisected this to 00b06da29cf9dc633cdba87acd3f57f4df3fd5c7.
> >> > >
> >> > > That patch makes two changes, it blocks sigaction from changing signal
> >> > > handlers once the kernel has decided to force the program to take a
> >> > > signal and it also stops notifying ptracers of the signal in the same
> >> > > circumstances. The latter behavior is just wrong. There's no reason
> >> > > that ptrace should not be able to observe and even change
> >> > > (non-SIGKILL) forced signals.  It should be reverted.
> >> > >
> >> > > This behavior change is also observable in gdb. If you take a program
> >> > > that sets SIGSYS to SIG_IGN and then raises a SIGSYS via
> >> > > SECCOMP_RET_TRAP and run it under gdb on a good kernel gdb will stop
> >> > > when the SIGSYS is raised, let you inspect program state, etc. After
> >> > > the SA_IMMUTABLE change gdb won't stop until the program has already
> >> > > died of SIGSYS.
> >> >
> >> > Ah, hm, this was trying to fix the case where a program trips
> >> > SECCOMP_RET_KILL (which is a "fatal SIGSYS"), and had been unobservable
> >> > before. I guess the fix was too broad...
> >>
> >> Perhaps I don't understand precisely what you mean by this, but gdb's
> >> behavior for a program that is SECCOMP_RET_KILLed was not changed by
> >> this patch (the SIGSYS is not observed until after program exit before
> >> or after this change).

The SA_IMMUTABLE change was to deal with failures seen in the seccomp
test suite after the recent fatal signal refactoring. Mainly that a
process that should have effectively performed do_exit() was suddenly
visible to the tracer.

> > Ah, maybe that behavior changed in 5.15 (my "before" here is a 5.14
> > kernel).  I would argue that the debugger seeing the SIGSYS for
> > SECCOMP_RET_KILL is desirable though ...
> 
> This is definitely worth discussing, and probably in need of fixing (aka
> something in rr seems to have broken).
> 
> We definitely need protection against the race with sigaction.
> 
> The fundamental question becomes does it make sense and is it safe
> to allow a debugger to stop at, and possibly change these signals.

I have no problem with a debugger getting notified about a fatal
(SECCOMP_RET_KILL*-originated) SIGSYS. But whatever happens, the kernel
needs to make sure the process does not continue. (i.e. signal can't be
changed/removed/etc.)

> Stopping at something SA_IMMUTABLE as long as the signal is allowed to
> continue and kill the process when PTRACE_CONT happens seems harmless.
> 
> Allowing the debugger to change the signal, or change it's handling
> I don't know.

Right -- I'm fine with a visibility change (the seccomp test suite is
just checking for various expected state machine changes across the
various signal/death cases: as long as it _dies_, that's what we want.
If a extra notification appears before it dies, that's okay, it just
needs the test suite to change).

> [...]
> Kees I am back to asking the question I had before I figured out
> SA_IMMUTABLE.  Are there security concerns with debuggers intercepting
> SECCOMP_RET_KILL.

I see no problem with allowing a tracer to observe the signal, but the
signalled process must have no way to continue running. If we end up in
such a state, then a seccomp process with access to clone() and
ptrace() can escape the seccomp sandbox. This is why seccomp had been
using the big do_exit() hammer -- I really want to absolutely never have
a bug manifest with a bypassed SECCOMP_RET_KILL: having a completely
unavoidable "dying" state is needed.

-- 
Kees Cook

  reply	other threads:[~2021-11-17 21:54 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-17 18:47 Kyle Huey
2021-11-17 18:51 ` Kees Cook
2021-11-17 19:05   ` Kyle Huey
2021-11-17 19:09     ` Kyle Huey
2021-11-17 21:04       ` Eric W. Biederman
2021-11-17 21:54         ` Kees Cook [this message]
2021-11-17 23:24           ` Linus Torvalds
2021-11-18  0:05             ` Kees Cook
2021-11-18  0:15               ` Linus Torvalds
2021-11-18  0:37             ` Kyle Huey
2021-11-18  1:11               ` Linus Torvalds
2021-11-18  1:20                 ` Kyle Huey
2021-11-18  1:32                   ` Kees Cook
2021-11-18 16:10                     ` Eric W. Biederman
2021-11-19 16:07                       ` Kyle Huey
2021-11-19 16:35                         ` Kees Cook
2021-11-19 16:58                           ` Kyle Huey
2021-11-18 21:58                     ` [PATCH 0/2] SA_IMMUTABLE fixes Eric W. Biederman
2021-11-18 22:04                       ` [PATCH 1/2] signal: Don't always set SA_IMMUTABLE for forced signals Eric W. Biederman
2021-11-18 23:52                         ` Kees Cook
2021-11-18 23:54                         ` Kees Cook
2021-11-19 15:08                           ` Eric W. Biederman
2021-11-19  1:13                         ` Kyle Huey
2021-11-19 15:03                           ` Eric W. Biederman
2021-11-18 22:05                       ` [PATCH 2/2] signal: Replace force_fatal_sig with force_exit_sig when in doubt Eric W. Biederman
2021-11-18 23:53                         ` Kees Cook
2021-11-19  1:12                       ` [PATCH 0/2] SA_IMMUTABLE fixes Kyle Huey
2021-11-19 15:41                         ` [GIT PULL] SA_IMMUTABLE fixes for v5.16-rc2 Eric W. Biederman
2021-11-19 19:46                           ` pr-tracker-bot
2021-11-17 22:29         ` [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers Kyle Huey
2021-11-18  5:43 ` Thorsten Leemhuis
2021-11-20  6:13   ` Thorsten Leemhuis

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