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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Andrea Righi <andrea.righi@canonical.com>
Cc:     Kees Cook <keescook@chromium.org>, Shuah Khan <shuah@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Will Drewry <wad@chromium.org>,
        linux-kselftest@vger.kernel.org, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
References: <YXrN+Hnl9pSOsWlA@arighi-desktop> <202110280955.B18CB67@keescook>
        <878rydm56l.fsf@disp2133> <202110281136.5CE65399A7@keescook>
        <87k0hvkgvj.fsf_-_@disp2133> <YX7VA1JZaYkTQeSi@arighi-desktop>
Date:   Mon, 01 Nov 2021 17:28:07 -0500
In-Reply-To: <YX7VA1JZaYkTQeSi@arighi-desktop> (Andrea Righi's message of
        "Sun, 31 Oct 2021 18:40:19 +0100")
Message-ID: <87tugva4ug.fsf@disp2133>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
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Subject: Re: [PATCH] signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed
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Andrea Righi <andrea.righi@canonical.com> writes:

> On Fri, Oct 29, 2021 at 10:09:04AM -0500, Eric W. Biederman wrote:
>> 
>> As Andy pointed out that there are races between
>> force_sig_info_to_task and sigaction[1] when force_sig_info_task.  As
>> Kees discovered[2] ptrace is also able to change these signals.
>> 
>> In the case of seeccomp killing a process with a signal it is a
>> security violation to allow the signal to be caught or manipulated.
>> 
>> Solve this problem by introducing a new flag SA_IMMUTABLE that
>> prevents sigaction and ptrace from modifying these forced signals.
>> This flag is carefully made kernel internal so that no new ABI is
>> introduced.
>> 
>> Longer term I think this can be solved by guaranteeing short circuit
>> delivery of signals in this case.  Unfortunately reliable and
>> guaranteed short circuit delivery of these signals is still a ways off
>> from being implemented, tested, and merged.  So I have implemented a much
>> simpler alternative for now.
>> 
>> [1] https://lkml.kernel.org/r/b5d52d25-7bde-4030-a7b1-7c6f8ab90660@www.fastmail.com
>> [2] https://lkml.kernel.org/r/202110281136.5CE65399A7@keescook
>> Cc: stable@vger.kernel.org
>> Fixes: 307d522f5eb8 ("signal/seccomp: Refactor seccomp signal and coredump generation")
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>
> FWIW I've tested this patch and I confirm that it fixes the failure that
> I reported with the seccomp_bpf selftest.
>
> Tested-by: Andrea Righi <andrea.righi@canonical.com>

Sigh.  Except for the extra 0 in the definition of SA_IMMUTABLE
that caused it to conflict with the x86 specific signal numbers.

Eric