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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: 019191342f ("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>
hifive-unleashed-5.1
Eric W. Biederman 2018-08-15 21:20:46 -05:00
parent c3ad2c3b02
commit 84fe4cc09a
1 changed files with 4 additions and 2 deletions
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6
fs/fcntl.c
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@ -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);