Restrict clearing TIF_SIGPENDING

This patch should get a few birds.  It prevents sigaction calls from
clearing TIF_SIGPENDING in other threads, which could leak -ERESTART*.
And It fixes ptrace_stop not to clear it, which done at the syscall exit
stop could leak -ERESTART*.  It probably removes the harm from signalfd,
at least assuming it never calls dequeue_signal on kernel threads that
might have used block_all_signals.

Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.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 acdfc0549c6f..fe590e00db8d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -105,7 +105,11 @@ static int recalc_sigpending_tsk(struct task_struct *t)
                 set_tsk_thread_flag(t, TIF_SIGPENDING);
                 return 1;
         }
-        clear_tsk_thread_flag(t, TIF_SIGPENDING);
+        /*
+         * We must never clear the flag in another thread, or in current
+         * when it's possible the current syscall is returning -ERESTART*.
+         * So we don't clear it here, and only callers who know they should do.
+         */
         return 0;
 }
 
@@ -121,7 +125,9 @@ void recalc_sigpending_and_wake(struct task_struct *t)
 
 void recalc_sigpending(void)
 {
-        recalc_sigpending_tsk(current);
+        if (!recalc_sigpending_tsk(current))
+                clear_thread_flag(TIF_SIGPENDING);
+
 }
 
 /* Given the mask, find the first available signal that should be serviced. */
@@ -385,7 +391,8 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
                         }
                 }
         }
-        recalc_sigpending_tsk(tsk);
+        if (likely(tsk == current))
+                recalc_sigpending();
         if (signr && unlikely(sig_kernel_stop(signr))) {
                 /*
                  * Set a marker that we have dequeued a stop signal.  Our
@@ -1580,8 +1587,9 @@ static void ptrace_stop(int exit_code, int nostop_code, siginfo_t *info)
         /*
          * Queued signals ignored us while we were stopped for tracing.
          * So check for any that we should take before resuming user mode.
+         * This sets TIF_SIGPENDING, but never clears it.
          */
-        recalc_sigpending();
+        recalc_sigpending_tsk(current);
 }
 
 void ptrace_notify(int exit_code)