linux-kernel - Re: [PATCH] signal: Make the si_code check in rt

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Date:	Mon, 12 Oct 2015 17:54:44 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Amanieu d'Antras" <amanieu@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo
	stricter

On 10/12, Amanieu d'Antras wrote:
>
> rt_sigqueueinfo and rt_tgsigqueueinfo check the value of si_code to
> prevent a process from spoofing a kernel-generated signal or one
> generated by kill/tgkill.
>
> Unfortunately this check failed to take into account the fact that
> the si_code value seen by a user process is only the low 16 bits of
> the value in the kernel. It was still possible to spoof any si_code
> by ORing 0xffff into the top 16 bits.

Confused...

> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2989,7 +2989,7 @@ static int do_rt_sigqueueinfo(pid_t pid, int sig, siginfo_t *info)
>  	/* Not even root can pretend to send signals from the kernel.
>  	 * Nor can they impersonate a kill()/tgkill(), which adds source info.
>  	 */
> -	if ((info->si_code >= 0 || info->si_code == SI_TKILL) &&
> +	if (((short)info->si_code >= 0 || (short)info->si_code == SI_TKILL) &&
>  	    (task_pid_vnr(current) != pid))
>  		return -EPERM;
>
> @@ -3037,7 +3037,7 @@ static int do_rt_tgsigqueueinfo(pid_t tgid, pid_t pid, int sig, siginfo_t *info)
>  	/* Not even root can pretend to send signals from the kernel.
>  	 * Nor can they impersonate a kill()/tgkill(), which adds source info.
>  	 */
> -	if ((info->si_code >= 0 || info->si_code == SI_TKILL) &&
> +	if (((short)info->si_code >= 0 || (short)info->si_code == SI_TKILL) &&
>  	    (task_pid_vnr(current) != pid))
>  		return -EPERM;

Yes, copy_siginfo_to_user() does __put_user((short)from->si_code).
But SI_FROMUSER/SI_FROMKERNEL are internal kernel checks, we mostly
use them in copy_siginfo_to_user().

And note that if ->si_code < 0 we simply do __copy_to_user(), so
userspace can't see something which looks like "from kernel", in
this case we do not truncate ->si_code.

So I do not think this patch is right.

Oleg.

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