[34-longterm 207/209] Prevent rt_sigqueueinfo and rt_tgsigqueueinfo from spoofing the signal code

[Paul Gortmaker]

From: Julien Tinnes <jln@xxxxxxxxxx>

  =====================================================================
  | This is a commit scheduled for the next v2.6.34 longterm release. |
  | If you see a problem with using this for longterm, please comment.|
  =====================================================================

commit da48524eb20662618854bb3df2db01fc65f3070c upstream.

Userland should be able to trust the pid and uid of the sender of a
signal if the si_code is SI_TKILL.

Unfortunately, the kernel has historically allowed sigqueueinfo() to
send any si_code at all (as long as it was negative - to distinguish it
from kernel-generated signals like SIGILL etc), so it could spoof a
SI_TKILL with incorrect siginfo values.

Happily, it looks like glibc has always set si_code to the appropriate
SI_QUEUE, so there are probably no actual user code that ever uses
anything but the appropriate SI_QUEUE flag.

So just tighten the check for si_code (we used to allow any negative
value), and add a (one-time) warning in case there are binaries out
there that might depend on using other si_code values.

Signed-off-by: Julien Tinnes <jln@xxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
 kernel/signal.c |   16 ++++++++++++----
 1 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index edabc2f..704c030 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2409,9 +2409,13 @@ SYSCALL_DEFINE3(rt_sigqueueinfo, pid_t, pid, int, sig,
 		return -EFAULT;
 
 	/* Not even root can pretend to send signals from the kernel.
-	   Nor can they impersonate a kill(), which adds source info.  */
-	if (info.si_code >= 0)
+	 * Nor can they impersonate a kill()/tgkill(), which adds source info.
+	 */
+	if (info.si_code != SI_QUEUE) {
+		/* We used to allow any < 0 si_code */
+		WARN_ON_ONCE(info.si_code < 0);
 		return -EPERM;
+	}
 	info.si_signo = sig;
 
 	/* POSIX.1b doesn't mention process groups.  */
@@ -2425,9 +2429,13 @@ long do_rt_tgsigqueueinfo(pid_t tgid, pid_t pid, int sig, siginfo_t *info)
 		return -EINVAL;
 
 	/* Not even root can pretend to send signals from the kernel.
-	   Nor can they impersonate a kill(), which adds source info.  */
-	if (info->si_code >= 0)
+	 * Nor can they impersonate a kill()/tgkill(), which adds source info.
+	 */
+	if (info->si_code != SI_QUEUE) {
+		/* We used to allow any < 0 si_code */
+		WARN_ON_ONCE(info->si_code < 0);
 		return -EPERM;
+	}
 	info->si_signo = sig;
 
 	return do_send_specific(tgid, pid, sig, info);
-- 
1.7.4.4

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