Prevent rt_sigqueueinfo and rt_tgsigqueueinfo from spoofing the signal code

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@google.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 4e3cff10fdc..31751868de8 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2421,9 +2421,13 @@ 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.  */
@@ -2437,9 +2441,13 @@ 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);