Cong Wang <xiyou.wangc...@gmail.com> 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 > > Reported-by: syzbot <syzkal...@googlegroups.com> > Cc: Ingo Molnar <mi...@kernel.org> > Cc: Al Viro <v...@zeniv.linux.org.uk> > Cc: Andrew Morton <a...@linux-foundation.org> > Cc: Linus Torvalds <torva...@linux-foundation.org> > Cc: sta...@vger.kernel.org > Signed-off-by: Cong Wang <xiyou.wangc...@gmail.com> > --- > 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); > > /*
