signal: Don't send signals to tasks that don't exist

Recently syzbot reported crashes in send_sigio_to_task and
send_sigurg_to_task in linux-next.  Despite finding a reproducer
syzbot apparently did not bisected this or otherwise track down the
offending commit in linux-next.

I happened to see this report and examined the code because I had
recently changed these functions as part of making PIDTYPE_TGID a real
pid type so that fork would does not need to restart when receiving a
signal.  By examination I see that I spotted a bug in the code
that could explain the reported crashes.

When I took Oleg's suggestion and optimized send_sigurg and send_sigio
to only send to a single task when type is PIDTYPE_PID or PIDTYPE_TGID
I failed to handle pids that no longer point to tasks.  The macro
do_each_pid_task simply iterates for zero iterations.  With pid_task
an explicit NULL test is needed.

Update the code to include the missing NULL test.

Fixes: 019191342fec ("signal: Use PIDTYPE_TGID to clearly store where file signals will be sent")
Reported-by: [email protected]
Signed-off-by: "Eric W. Biederman" <[email protected]>

diff --git a/fs/fcntl.c b/fs/fcntl.c
index a04accf6847f301b06dad01d278f2bf2d528c1d8..4137d96534a6cb6a73d7a70dc75db543b12a3441 100644 (file)
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -791,7 +791,8 @@ void send_sigio(struct fown_struct *fown, int fd, int band)
        if (type <= PIDTYPE_TGID) {
                rcu_read_lock();
                p = pid_task(pid, PIDTYPE_PID);
-               send_sigio_to_task(p, fown, fd, band, type);
+               if (p)
+                       send_sigio_to_task(p, fown, fd, band, type);
                rcu_read_unlock();
        } else {
                read_lock(&tasklist_lock);
@@ -830,7 +831,8 @@ int send_sigurg(struct fown_struct *fown)
        if (type <= PIDTYPE_TGID) {
                rcu_read_lock();
                p = pid_task(pid, PIDTYPE_PID);
-               send_sigurg_to_task(p, fown, type);
+               if (p)
+                       send_sigurg_to_task(p, fown, type);
                rcu_read_unlock();
        } else {
                read_lock(&tasklist_lock);