signals: check_kill_permission(): don't check creds if same_thread_group()

Andrew Tridgell reports that aio_read(SIGEV_SIGNAL) can fail if the
notification from the helper thread races with setresuid(), see
http://samba.org/~tridge/junkcode/aio_uid.c

This happens because check_kill_permission() doesn't permit sending a
signal to the task with the different cred->xids.  But there is not any
security reason to check ->cred's when the task sends a signal (private or
group-wide) to its sub-thread.  Whatever we do, any thread can bypass all
security checks and send SIGKILL to all threads, or it can block a signal
SIG and do kill(gettid(), SIG) to deliver this signal to another
sub-thread.  Not to mention that CLONE_THREAD implies CLONE_VM.

Change check_kill_permission() to avoid the credentials check when the
sender and the target are from the same thread group.

Also, move "cred = current_cred()" down to avoid calling get_current()
twice.

Note: David Howells pointed out we could relax this even more, the
CLONE_SIGHAND (without CLONE_THREAD) case probably does not need
these checks too.

Roland said:
: The glibc (libpthread) that does set*id across threads has
: been in use for a while (2.3.4?), probably in distro's using kernels as old
: or older than any active -stable streams.  In the race in question, this
: kernel bug is breaking valid POSIX application expectations.

Reported-by: Andrew Tridgell <tridge@samba.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Eric Paris <eparis@parisplace.org>
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: <stable@kernel.org>		[all kernel versions]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 825a3f24ad76..2ee9573ed11f 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -642,7 +642,7 @@ static inline bool si_fromuser(const struct siginfo *info)
 static int check_kill_permission(int sig, struct siginfo *info,
                                  struct task_struct *t)
 {
-        const struct cred *cred = current_cred(), *tcred;
+        const struct cred *cred, *tcred;
         struct pid *sid;
         int error;
 
@@ -656,8 +656,10 @@ static int check_kill_permission(int sig, struct siginfo *info,
         if (error)
                 return error;
 
+        cred = current_cred();
         tcred = __task_cred(t);
-        if ((cred->euid ^ tcred->suid) &&
+        if (!same_thread_group(current, t) &&
+            (cred->euid ^ tcred->suid) &&
             (cred->euid ^ tcred->uid) &&
             (cred->uid  ^ tcred->suid) &&
             (cred->uid  ^ tcred->uid) &&