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From: Andy Lutomirski <luto@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-arch@vger.kernel.org
Cc: Oleg Nesterov <oleg@redhat.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH 13/20] signal: Implement force_fatal_sig
Date: Mon, 25 Oct 2021 15:41:01 -0700	[thread overview]
Message-ID: <9416e8d7-5545-4fc4-8ab0-68fddd35520b@kernel.org> (raw)
In-Reply-To: <CAHk-=whe-ixeDp_OgSOsC4H+dWTLDSuNDU2a0sE3p8DapNeCuQ@mail.gmail.com>

On 10/20/21 13:05, Linus Torvalds wrote:
> On Wed, Oct 20, 2021 at 7:45 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>
>> Add a simple helper force_fatal_sig that causes a signal to be
>> delivered to a process as if the signal handler was set to SIG_DFL.
>>
>> Reimplement force_sigsegv based upon this new helper.
> 
> Can you just make the old force_sigsegv() go away? The odd special
> casing of SIGSEGV was odd to begin with, I think everybody really just
> wanted this new "force_fatal_sig()" and allow any signal - not making
> SIGSEGV special.
> 

I'm rather nervous about all this, and I'm also nervous about the 
existing code.  A quick skim is finding plenty of code paths that assume 
force_sigsegv (or a do_exit that this series touches) are genuinely 
unrecoverable.  For example:

- rseq: the *kernel* will be fine if a signal is handled, but the 
userspace process may be in a very strange state.

- bprm_execve: The comment says it best:

         /*
          * If past the point of no return ensure the code never
          * returns to the userspace process.  Use an existing fatal
          * signal if present otherwise terminate the process with
          * SIGSEGV.
          */
         if (bprm->point_of_no_return && !fatal_signal_pending(current))
                 force_sigsegv(SIGSEGV);

- vm86: already discussed

Now force_sigsegv() at least tries to kill the task, but not very well. 
With the whole series applied and force_sigsegv() gone, these errors 
become handleable, and that needs real care.

(I don't think bprm_execve() is exploitable.  It looks like it's 
attackable in the window between setting point_of_no_return and 
unshare_sighand(), but I'm not seeing any useful way to attack it unless 
a core dump is already in progress or a *different* fatal signal is 
already pending, and in either of those cases we're fine.)

  parent reply	other threads:[~2021-10-25 22:41 UTC|newest]

Thread overview: 110+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-20 17:32 [PATCH 00/20] exit cleanups Eric W. Biederman
2021-10-20 17:32 ` [OpenRISC] " Eric W. Biederman
2021-10-20 17:32 ` Eric W. Biederman
2021-10-20 17:43 ` [PATCH 01/20] exit/doublefault: Remove apparently bogus comment about rewind_stack_do_exit Eric W. Biederman
2021-10-21 16:02   ` Kees Cook
2021-10-20 17:43 ` [PATCH 02/20] exit: Remove calls of do_exit after noreturn versions of die Eric W. Biederman
2021-10-20 17:43   ` [OpenRISC] " Eric W. Biederman
2021-10-21 16:02   ` Kees Cook
2021-10-21 16:02     ` [OpenRISC] " Kees Cook
2021-10-21 16:25     ` Eric W. Biederman
2021-10-21 16:25       ` [OpenRISC] " Eric W. Biederman
2021-10-20 17:43 ` [PATCH 03/20] reboot: Remove the unreachable panic after do_exit in reboot(2) Eric W. Biederman
2021-10-21 16:05   ` Kees Cook
2021-10-20 17:43 ` [PATCH 04/20] signal/sparc32: Remove unreachable do_exit in do_sparc_fault Eric W. Biederman
2021-10-21 16:05   ` Kees Cook
2021-10-20 17:43 ` [PATCH 05/20] signal/mips: Update (_save|_restore)_fp_context to fail with -EFAULT Eric W. Biederman
2021-10-21 16:06   ` Kees Cook
2021-10-24  4:24   ` Maciej W. Rozycki
2021-10-25 20:55     ` Eric W. Biederman
2021-10-24 15:27   ` Thomas Bogendoerfer
2021-10-20 17:43 ` [PATCH 06/20] signal/sh: Use force_sig(SIGKILL) instead of do_group_exit(SIGKILL) Eric W. Biederman
2021-10-20 19:57   ` Linus Torvalds
2021-10-27 14:24     ` Rich Felker
2021-10-21 16:08   ` Kees Cook
2021-10-20 17:43 ` [PATCH 07/20] signal/powerpc: On swapcontext failure force SIGSEGV Eric W. Biederman
2021-10-20 17:43   ` Eric W. Biederman
2021-10-21 16:09   ` Kees Cook
2021-10-21 16:09     ` Kees Cook
2021-10-20 17:43 ` [PATCH 08/20] signal/sparc: In setup_tsb_params convert open coded BUG into BUG Eric W. Biederman
2021-10-21 16:12   ` Kees Cook
2021-10-20 17:43 ` [PATCH 09/20] signal/vm86_32: Replace open coded BUG_ON with an actual BUG_ON Eric W. Biederman
2021-10-21 16:15   ` Kees Cook
2021-11-12 15:40   ` Eric W. Biederman
2021-11-12 17:51     ` Brian Gerst
2021-11-12 19:57       ` Eric W. Biederman
2021-11-12 20:40         ` Linus Torvalds
2021-11-12 21:03           ` Eric W. Biederman
2021-11-12 21:23             ` Linus Torvalds
2021-11-12 21:24               ` Linus Torvalds
2021-11-12 21:37                 ` [GIT PULL ] signal/vm86_32: Remove pointless test in BUG_ON Eric W. Biederman
2021-11-13 19:15                   ` pr-tracker-bot
2021-11-12 21:43                 ` [PATCH 09/20] signal/vm86_32: Replace open coded BUG_ON with an actual BUG_ON Eric W. Biederman
2021-10-20 17:43 ` [PATCH 10/20] signal/vm86_32: Properly send SIGSEGV when the vm86 state cannot be saved Eric W. Biederman
2021-10-21 16:16   ` Kees Cook
2021-10-21 17:02     ` Eric W. Biederman
2021-10-21 20:33       ` Kees Cook
2021-10-21 23:08   ` Andy Lutomirski
2021-10-24 16:06     ` Eric W. Biederman
     [not found]   ` <875ytkygfj.fsf_-_@disp2133>
2021-10-25 21:12     ` [PATCH v2 10/32] " Linus Torvalds
2021-10-25 21:28       ` Eric W. Biederman
2021-10-25 22:25     ` Andy Lutomirski
2021-10-25 23:45       ` Linus Torvalds
2021-10-26  0:21         ` Andy Lutomirski
2021-10-20 17:43 ` [PATCH 11/20] signal/s390: Use force_sigsegv in default_trap_handler Eric W. Biederman
2021-10-21 16:17   ` Kees Cook
2021-10-26  9:38   ` Christian Borntraeger
2021-10-28 15:56     ` Eric W. Biederman
2021-10-29 19:32       ` Eric W. Biederman
2021-10-20 17:43 ` [PATCH 12/20] exit/kthread: Have kernel threads return instead of calling do_exit Eric W. Biederman
2021-10-21 11:12   ` Christoph Hellwig
2021-10-21 15:11     ` Eric W. Biederman
2021-10-21 16:21   ` Kees Cook
2021-10-20 17:43 ` [PATCH 13/20] signal: Implement force_fatal_sig Eric W. Biederman
2021-10-20 20:05   ` Linus Torvalds
2021-10-20 21:25     ` Eric W. Biederman
2021-10-25 22:41     ` Andy Lutomirski [this message]
2021-10-25 23:15       ` Linus Torvalds
2021-10-26  4:45         ` Eric W. Biederman
2021-10-26  4:57         ` Eric W. Biederman
2021-10-26 16:15           ` Linus Torvalds
2021-10-28 16:33             ` Eric W. Biederman
2021-10-21 16:24   ` Kees Cook
2021-10-21 16:33     ` Eric W. Biederman
2021-10-21 16:39       ` Kees Cook
2021-10-20 17:44 ` [PATCH 14/20] exit/syscall_user_dispatch: Send ordinary signals on failure Eric W. Biederman
2021-10-21 16:25   ` Kees Cook
2021-10-21 16:37     ` Eric W. Biederman
2021-10-21 16:40       ` Kees Cook
2021-10-21 17:05         ` Eric W. Biederman
2021-10-25 22:32     ` Andy Lutomirski
2021-10-21 16:35   ` Gabriel Krisman Bertazi
2021-10-20 17:44 ` [PATCH 15/20] signal/sparc32: Exit with a fatal signal when try_to_clear_window_buffer fails Eric W. Biederman
2021-10-21 16:34   ` Kees Cook
2021-10-21 16:56     ` Eric W. Biederman
2021-10-20 17:44 ` [PATCH 16/20] signal/sparc32: In setup_rt_frame and setup_fram use force_fatal_sig Eric W. Biederman
2021-10-21 16:34   ` Kees Cook
2021-10-20 17:44 ` [PATCH 17/20] signal/x86: In emulate_vsyscall force a signal instead of calling do_exit Eric W. Biederman
2021-10-21 16:36   ` Kees Cook
2021-10-20 17:44 ` [PATCH 18/20] exit/rtl8723bs: Replace the macro thread_exit with a simple return 0 Eric W. Biederman
2021-10-21  7:06   ` Greg KH
2021-10-21 15:06     ` Eric W. Biederman
2021-10-21 16:37   ` Kees Cook
2021-10-20 17:44 ` [PATCH 19/20] exit/rtl8712: " Eric W. Biederman
2021-10-21  7:07   ` Greg KH
2021-10-21 16:37   ` Kees Cook
2021-10-20 17:44 ` [PATCH 20/20] exit/r8188eu: " Eric W. Biederman
2021-10-21  7:07   ` Greg KH
2021-10-21 16:37   ` Kees Cook
2021-10-20 21:51 ` [PATCH 21/20] signal: Replace force_sigsegv(SIGSEGV) with force_fatal_sig(SIGSEGV) Eric W. Biederman
2021-10-20 21:51   ` [OpenRISC] " Eric W. Biederman
2021-10-20 21:51   ` Eric W. Biederman
2021-10-21  8:09   ` Geert Uytterhoeven
2021-10-21  8:09     ` [OpenRISC] " Geert Uytterhoeven
2021-10-21  8:09     ` Geert Uytterhoeven
2021-10-21 13:33     ` Eric W. Biederman
2021-10-21 13:33       ` [OpenRISC] " Eric W. Biederman
2021-10-21 13:33       ` Eric W. Biederman
2021-10-21  8:32   ` Philippe Mathieu-Daudé
2021-10-21  8:32     ` [OpenRISC] " Philippe =?unknown-8bit?q?Mathieu-Daud=C3=A9?=
2021-10-21  8:32     ` Philippe Mathieu-Daudé

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