linux-kernel - Re: [PATCH AUTOSEL 5.9 026/111] seccomp: kill process instead of thread for unknown actions

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Message-ID: <202010191627.EAD97F5E@keescook>
Date:   Mon, 19 Oct 2020 16:28:19 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Rich Felker <dalias@...c.org>
Subject: Re: [PATCH AUTOSEL 5.9 026/111] seccomp: kill process instead of
 thread for unknown actions

Hi Sasha,

I'd prefer this was not backported -- it's not a bug fix, and if the
behavioral change is actually disruptive, I'd like to keep the fall-out
contained. :)

Thanks!

-Kees

On Sun, Oct 18, 2020 at 03:16:42PM -0400, Sasha Levin wrote:
> From: Rich Felker <dalias@...c.org>
> 
> [ Upstream commit 4d671d922d51907bc41f1f7f2dc737c928ae78fd ]
> 
> Asynchronous termination of a thread outside of the userspace thread
> library's knowledge is an unsafe operation that leaves the process in
> an inconsistent, corrupt, and possibly unrecoverable state. In order
> to make new actions that may be added in the future safe on kernels
> not aware of them, change the default action from
> SECCOMP_RET_KILL_THREAD to SECCOMP_RET_KILL_PROCESS.
> 
> Signed-off-by: Rich Felker <dalias@...c.org>
> Link: https://lore.kernel.org/r/20200829015609.GA32566@brightrain.aerifal.cx
> [kees: Fixed up coredump selection logic to match]
> Signed-off-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  kernel/seccomp.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 676d4af621038..f754c1087e413 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -1020,7 +1020,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>  	default:
>  		seccomp_log(this_syscall, SIGSYS, action, true);
>  		/* Dump core only if this is the last remaining thread. */
> -		if (action == SECCOMP_RET_KILL_PROCESS ||
> +		if (action != SECCOMP_RET_KILL_THREAD ||
>  		    get_nr_threads(current) == 1) {
>  			kernel_siginfo_t info;
>  
> @@ -1030,10 +1030,10 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>  			seccomp_init_siginfo(&info, this_syscall, data);
>  			do_coredump(&info);
>  		}
> -		if (action == SECCOMP_RET_KILL_PROCESS)
> -			do_group_exit(SIGSYS);
> -		else
> +		if (action == SECCOMP_RET_KILL_THREAD)
>  			do_exit(SIGSYS);
> +		else
> +			do_group_exit(SIGSYS);
>  	}
>  
>  	unreachable();
> -- 
> 2.25.1
> 

-- 
Kees Cook