linux-hardening - Re: [REGRESSION] 5.16rc1: SA

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Date:   Wed, 17 Nov 2021 16:37:21 -0800
From:   Kyle Huey <me@...ehuey.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrea Righi <andrea.righi@...onical.com>,
        Shuah Khan <shuah@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org,
        "Robert O'Callahan" <rocallahan@...il.com>
Subject: Re: [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers

On Wed, Nov 17, 2021 at 3:24 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Wed, Nov 17, 2021 at 1:54 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > 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.
>
> I think this basically shows that the conversion from do_exit() to
> fatal_signal() was just wrong. The "do_exit()" wasn't really a signal,
> and can't be treated as such.
>
> That said, instead of reverting, maybe we can just mark the cases
> where it really is about sending a synchronous signal, vs sending an
> explicitly fatal signal.
>
> It's basically the "true" condition to force_sig_info_to_task(), so
> the fix might be just
>
>   @@ -1323,7 +1323,8 @@ force_sig_info_to_task(struct kernel_siginfo
> *info, struct task_struct *t, bool
>         blocked = sigismember(&t->blocked, sig);
>         if (blocked || ignored || sigdfl) {
>                 action->sa.sa_handler = SIG_DFL;
>   -             action->sa.sa_flags |= SA_IMMUTABLE;
>   +             if (sigdfl)
>   +                     action->sa.sa_flags |= SA_IMMUTABLE;
>                 if (blocked) {
>                         sigdelset(&t->blocked, sig);
>                         recalc_sigpending_and_wake(t);
>
> Kyle, does that fix your test-case? And Kees - yours?

This fixes most of the issues with rr, but it still changes the ptrace
behavior for the double-SIGSEGV case (yes, we have a test for that
too). The second SIGSEGV isn't reported to the ptracer and the program
just skips straight to the PTRACE_EVENT_EXIT. This is visible in gdb
as well (only the first SIGSEGV is caught).

- Kyle