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

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Date:	Mon, 12 Oct 2015 16:33:54 +0100
From:	Amanieu d'Antras <amanieu@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Amanieu d'Antras <amanieu@...il.com>
Subject: [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo stricter

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.

The check is tightened by checking the value of si_code that will
be seen by a user program instead of the one in the kernel.

Signed-off-by: Amanieu d'Antras <amanieu@...il.com>
---
 kernel/signal.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 0f6bbbe..f3d4f39 100644
--- 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;
 
-- 
2.6.1

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