On Fri, Dec 15, 2017 at 8:35 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > On Fri, Dec 15, 2017 at 7:56 AM, Eric W. Biederman > <ebiederm@xxxxxxxxxxxx> wrote: >> Cong Wang <xiyou.wangcong@xxxxxxxxx> 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. Ah, found that bug: https://groups.google.com/d/msg/syzkaller-bugs/1XBaqnPSXzs/VF-eCSPuCQAJ >>> Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx> >>> Cc: Ingo Molnar <mingo@xxxxxxxxxx> >>> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> >>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> >>> Cc: stable@xxxxxxxxxxxxxxx >>> Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx> >>> --- >>> 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); >>> >>> /*