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Subject[PATCH] 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: syzkaller-bugs@googlegroups.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/fcntl.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index a04accf6847f..4137d96534a6 100644
--- 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);
--
2.17.1
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