On Fri, Dec 15, 2017 at 7:56 AM, Eric W. Biederman <[email protected]> wrote: > Cong Wang <[email protected]> writes: > >> syzbot reported we have a use-after-free when mqueue_evict_inode() >> is called on __cleanup_mnt() path, where the ipc ns is already >> freed by the previous exit_task_namespaces(). We can just move >> it after after exit_task_work() to avoid this use-after-free. > > How does that possibly work. (I haven't seen this syzbot report). > > Looking at the code we have get_ns_from_inode. Which takes the mq_lock, > sees if the pointer is NULL and takes a reference if it is non-NULL. > > Meanwhile put_ipc_ns calls mq_clear_sbinfo(ns) with the mq_lock held > when the count drops to zero. > > Where is the race in that? > > The rest of mqueue_evict_inode uses the returned pointer and > tests that the pointer is non-NULL before user it. > > So either szbot is giving you a bad report or there is a subtle race > there I am not seeing. The change below is not at all the proper way to > fix a subtle race. > > Eric
Cong, what was that report? Searching by "exit_task_work|exit_task_namespaces" there are too many of them: https://groups.google.com/forum/#!searchin/syzkaller-bugs/%22exit_task_work$7Cexit_task_namespaces%22%7Csort:date I can only say that syzbot does not make up reports. That's something that actually happened and was provoked by userspace. >> Reported-by: syzbot <[email protected]> >> Cc: Ingo Molnar <[email protected]> >> Cc: Al Viro <[email protected]> >> Cc: Andrew Morton <[email protected]> >> Cc: Linus Torvalds <[email protected]> >> Cc: [email protected] >> Signed-off-by: Cong Wang <[email protected]> >> --- >> kernel/exit.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/kernel/exit.c b/kernel/exit.c >> index 6b4298a41167..909e43c45158 100644 >> --- a/kernel/exit.c >> +++ b/kernel/exit.c >> @@ -861,8 +861,8 @@ void __noreturn do_exit(long code) >> exit_fs(tsk); >> if (group_dead) >> disassociate_ctty(1); >> - exit_task_namespaces(tsk); >> exit_task_work(tsk); >> + exit_task_namespaces(tsk); >> exit_thread(tsk); >> >> /*
