linux-hardening - Re: [PATCH 1/2] signal: Don't always set SA

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Date:   Thu, 18 Nov 2021 15:54:02 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, Kyle Huey <me@...ehuey.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        linux-hardening@...r.kernel.org,
        Robert O'Callahan <rocallahan@...il.com>,
        Oliver Sang <oliver.sang@...el.com>, lkp@...ts.01.org,
        lkp@...el.com
Subject: Re: [PATCH 1/2] signal: Don't always set SA_IMMUTABLE for forced
 signals

On Thu, Nov 18, 2021 at 04:04:58PM -0600, Eric W. Biederman wrote:
> 
> Recently to prevent issues with SECCOMP_RET_KILL and similar signals
> being changed before they are delivered SA_IMMUTABLE was added.
> 
> Unfortunately this broke debuggers[1][2] which reasonably expect to be
> able to trap synchronous SIGTRAP and SIGSEGV even when the target
> process is not configured to handle those signals.
> 
> Update force_sig_to_task to support both the case when we can
> allow the debugger to intercept and possibly ignore the
> signal and the case when it is not safe to let userspace
> known about the signal until the process has exited.
> 
> Reported-by: Kyle Huey <me@...ehuey.com>
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Cc: stable@...r.kernel.org
> [1] https://lkml.kernel.org/r/CAP045AoMY4xf8aC_4QU_-j7obuEPYgTcnQQP3Yxk=2X90jtpjw@mail.gmail.com
> [2] https://lkml.kernel.org/r/20211117150258.GB5403@xsang-OptiPlex-902
> Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  kernel/signal.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 7c4b7ae714d4..02058c983bd6 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1298,6 +1298,12 @@ int do_send_sig_info(int sig, struct kernel_siginfo *info, struct task_struct *p
>  	return ret;
>  }
>  
> +enum sig_handler {
> +	HANDLER_CURRENT, /* If reachable use the current handler */
> +	HANDLER_SIG_DFL, /* Always use SIG_DFL handler semantics */
> +	HANDLER_EXIT,	 /* Only visible as the proces exit code */

Oh, I just noticed this typo "proces" -> "process"

-Kees

-- 
Kees Cook