signal: Allow cifs and drbd to receive their terminating signals

[ Upstream commit 33da8e7c814f77310250bb54a9db36a44c5de784 ]

My recent to change to only use force_sig for a synchronous events
wound up breaking signal reception cifs and drbd.  I had overlooked
the fact that by default kthreads start out with all signals set to
SIG_IGN.  So a change I thought was safe turned out to have made it
impossible for those kernel thread to catch their signals.

Reverting the work on force_sig is a bad idea because what the code
was doing was very much a misuse of force_sig.  As the way force_sig
ultimately allowed the signal to happen was to change the signal
handler to SIG_DFL.  Which after the first signal will allow userspace
to send signals to these kernel threads.  At least for
wake_ack_receiver in drbd that does not appear actively wrong.

So correct this problem by adding allow_kernel_signal that will allow
signals whose siginfo reports they were sent by the kernel through,
but will not allow userspace generated signals, and update cifs and
drbd to call allow_kernel_signal in an appropriate place so that their
thread can receive this signal.

Fixing things this way ensures that userspace won't be able to send
signals and cause problems, that it is clear which signals the
threads are expecting to receive, and it guarantees that nothing
else in the system will be affected.

This change was partly inspired by similar cifs and drbd patches that
added allow_signal.

Reported-by: ronnie sahlberg <ronniesahlberg@gmail.com>
Reported-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Tested-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Cc: Steve French <smfrench@gmail.com>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Fixes: 247bc9470b1e ("cifs: fix rmmod regression in cifs.ko caused by force_sig changes")
Fixes: 72abe3bcf091 ("signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of force_sig")
Fixes: fee109901f39 ("signal/drbd: Use send_sig not force_sig")
Fixes: 3cf5d076fb4d ("signal: Remove task parameter from force_sig")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
index f5c24459fc5c172c545a3f5ac6ab5b7295eb0400..daa9cef96ec66c1c98a834c99859e9a14918e272 100644 (file)
--- a/drivers/block/drbd/drbd_main.c
+++ b/drivers/block/drbd/drbd_main.c
@@ -332,6 +332,8 @@ static int drbd_thread_setup(void *arg)
                 thi->name[0],
                 resource->name);
 
+       allow_kernel_signal(DRBD_SIGKILL);
+       allow_kernel_signal(SIGXCPU);
 restart:
        retval = thi->function(thi);
 

diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index 7d46025d5e899c332d8f3168507185c817ecde50..751bdde6515d5ac853d83d774ac80172bc14a159 100644 (file)
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -885,7 +885,7 @@ cifs_demultiplex_thread(void *p)
                mempool_resize(cifs_req_poolp, length + cifs_min_rcv);
 
        set_freezable();
-       allow_signal(SIGKILL);
+       allow_kernel_signal(SIGKILL);
        while (server->tcpStatus != CifsExiting) {
                if (try_to_freeze())
                        continue;

diff --git a/include/linux/signal.h b/include/linux/signal.h
index 5308304993bea584105a9747d85c9abcbadcc5da..ffa58ff53e22580146988c6540d628780cb3ed2c 100644 (file)
--- a/include/linux/signal.h
+++ b/include/linux/signal.h
@@ -313,6 +313,9 @@ extern void signal_setup_done(int failed, struct ksignal *ksig, int stepping);
 extern void exit_signals(struct task_struct *tsk);
 extern void kernel_sigaction(int, __sighandler_t);
 
+#define SIG_KTHREAD ((__force __sighandler_t)2)
+#define SIG_KTHREAD_KERNEL ((__force __sighandler_t)3)
+
 static inline void allow_signal(int sig)
 {
        /*
@@ -320,7 +323,17 @@ static inline void allow_signal(int sig)
         * know it'll be handled, so that they don't get converted to
         * SIGKILL or just silently dropped.
         */
-       kernel_sigaction(sig, (__force __sighandler_t)2);
+       kernel_sigaction(sig, SIG_KTHREAD);
+}
+
+static inline void allow_kernel_signal(int sig)
+{
+       /*
+        * Kernel threads handle their own signals. Let the signal code
+        * know signals sent by the kernel will be handled, so that they
+        * don't get silently dropped.
+        */
+       kernel_sigaction(sig, SIG_KTHREAD_KERNEL);
 }
 
 static inline void disallow_signal(int sig)

diff --git a/kernel/signal.c b/kernel/signal.c
index 30914b3c76b21d3e7dd04dc7f5889463d4a99da2..57fadbe69c2e621c2ed307d6d682c1db43e48336 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -79,6 +79,11 @@ static int sig_task_ignored(struct task_struct *t, int sig, bool force)
            handler == SIG_DFL && !(force && sig_kernel_only(sig)))
                return 1;
 
+       /* Only allow kernel generated signals to this kthread */
+       if (unlikely((t->flags & PF_KTHREAD) &&
+                    (handler == SIG_KTHREAD_KERNEL) && !force))
+               return true;
+
        return sig_handler_ignored(handler, sig);
 }