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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Kyle Huey <me@kylehuey.com>
Cc: Kees Cook <keescook@chromium.org>,
	"Eric W. Biederman" <ebiederm@xmission.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 <bpf@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	linux-hardening@vger.kernel.org,
	"Robert O'Callahan" <rocallahan@gmail.com>
Subject: Re: [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers
Date: Wed, 17 Nov 2021 17:11:14 -0800	[thread overview]
Message-ID: <CAHk-=wiCRbSvUi_TnQkokLeM==_+Tow0GsQXnV3UYwhsxirPwg@mail.gmail.com> (raw)
In-Reply-To: <CAP045ApYXxhiAfmn=fQM7_hD58T-yx724ctWFHO4UAWCD+QapQ@mail.gmail.com>

On Wed, Nov 17, 2021 at 4:37 PM Kyle Huey <me@kylehuey.com> wrote:
>
> This fixes most of the issues with rr, but it still changes the ptrace
> behavior for the double-SIGSEGV case

Hmm. I think that's because of how "force_sigsgv()" works.

I absolutely detest that function.

So we have signal_setup_done() doing that

        if (failed)
                force_sigsegv(ksig->sig);

and then force_sigsegv() has that completely insane

        if (sig == SIGSEGV)
                force_fatal_sig(SIGSEGV);
        else
                force_sig(SIGSEGV);

behavior.

And I think I know the _reason_ for that complete insanity: when
SIGSEGV takes a SIGSEGV, and there is a handler, we need to stop
trying to send more SIGSEGV's.

But it does mean that with my change, that second SIGSEGV now ends up
being that SA_IMMUTABLE kind, so yeah, it broke the debugger test -
where catching the second SIGSEGV is actually somewhat sensible (ok,
not really, but at least understandable)

End result: I think we want not a boolean, but a three-way choice for
that force_sig_info_to_task() thing:

 - unconditionally fatal (for things that just want to force an exit
and used to do do_exit())

 - ignore valid and unblocked handler (for that SIGSEGV recursion
case, aka force "sigdfl")

 - catching signal ok

So my one-liner isn't sufficient. It wants some kind of nasty enum.

At least the enum can be entirely internal to kernel/signal.c, I
think. No need to expose this all to anything else.

            Linus

  reply	other threads:[~2021-11-18  1:11 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
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 [this message]
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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