Re: [BUG] ptraced process waiting on syscall may return kernel internal errnos

[Roland McGrath <roland@redhat.com>]

Oleg and I were just discussing this issue in relation to other problems.
We established that it is never safe to clear TIF_SIGPENDING on another
thread.  But I hadn't really thought through that it's sometimes not safe
to clear your own TIF_SIGPENDING either.  That is, any time you are not
positive you cannot be in a syscall that will return a -ERESTART* code.
(I had the ptrace_stop case lurking in the back of my mind but hadn't
considered how it would really come up.)

I have a general recollection of thinking that dequeue_signal could only
be called on current and that it mattered somehow.  But aside from
avoiding recalc_sigpending, and kernel threads with notifier_mask set, I
can't see off hand what it is.  I won't testify that I think signalfd is
necessarily on safe ground, though.


Thanks,
Roland

---

[PATCH] 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*.
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>
---
 kernel/signal.c |   40 ++++++++++++++++++++++++----------------
 1 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index acdfc05..dc5797c 100644  
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -105,7 +105,11 @@ static int recalc_sigpending_tsk(struct 
 		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 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 *t
 			}
 		}
 	}
-	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, i
 	/*
 	 * 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)
-
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