Re: KCSAN: data-race in exit_signals / prepare_signal

[Christian Brauner <christian.brauner@ubuntu.com>]

[+Cc Will]

On Mon, Oct 21, 2019 at 03:34:07AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    d724f94f x86, kcsan: Enable KCSAN for x86
> git tree:       https://github.com/google/ktsan.git kcsan
> console output: https://syzkaller.appspot.com/x/log.txt?x=13eab79f600000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80
> dashboard link: https://syzkaller.appspot.com/bug?extid=492a4acccd8fc75ddfd0
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> 
> Unfortunately, I don't have any reproducer for this crash yet.
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+492a4acccd8fc75ddfd0@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KCSAN: data-race in exit_signals / prepare_signal

This traces back to Oleg fixing a race between a group stop and a thread
exiting before it notices that it has a pending signal or is in the middle of
do_exit() already, causing group stop to get wacky.
The original commit to fix this race is
commit d12619b5ff56 ("fix group stop with exit race") which took sighand
lock before setting PF_EXITING on the thread.

Later on in
commit 5dee1707dfbf ("move the related code from exit_notify() to exit_signals()")
an improvement was made for the single-threaded case and the
case where the group stop is already in progress. This removed the
sighand lock around the PF_EXITING assignment.

If the race really matters and given how tsk->flags is currently accessed
everywhere the simple fix for now might be:

diff --git a/kernel/signal.c b/kernel/signal.c
index c4da1ef56fdf..cf61e044c4cc 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2819,7 +2819,9 @@ void exit_signals(struct task_struct *tsk)
        cgroup_threadgroup_change_begin(tsk);

        if (thread_group_empty(tsk) || signal_group_exit(tsk->signal)) {
+               spin_lock_irq(&tsk->sighand->siglock);
                tsk->flags |= PF_EXITING;
+               spin_unlock_irq(&tsk->sighand->siglock);
                cgroup_threadgroup_change_end(tsk);
                return;
        }

Christian