Re: BUG: KASAN: use-after-free in dec_rlimit_ucounts

[Qian Cai <quic_qiancai@quicinc.com>]

On Thu, Nov 18, 2021 at 01:46:05PM -0600, Eric W. Biederman wrote:
> Is it possible?  Yes it is possible.  That is one place where
> a use-after-free has shown up and I expect would show up in the
> future.
> 
> That said it is hard to believe there is still a user-after-free in the
> code.  We spent the last kernel development cycle pouring through and
> correcting everything we saw until we ultimately found one very subtle
> use-after-free.
> 
> If you have a reliable reproducer that you can share, we can look into
> this and see if we can track down where the reference count is going
> bad.
> 
> It tends to take instrumenting the entire life cycle every increment and
> every decrement and then pouring through the logs to track down a
> use-after-free.  Which is not something we can really do without a
> reproducer.

The reproducer is just to run trinity by an unprivileged user on defconfig
with KASAN enabled (On linux-next, you can do "make defconfig debug.conf"
[1], but dont think other debugging options are relevent here.)

$ trinity -C 31 -N 10000000

It is always reproduced on an arm64 server here within 5-minute so far.
Some debugging progress so far. BTW, this could happen on user_shm_unlock()
path as well.

 Call trace:
  dec_rlimit_ucounts
  user_shm_unlock
  (inlined by) user_shm_unlock at mm/mlock.c:854
  shmem_lock
  shmctl_do_lock
  ksys_shmctl.constprop.0
  __arm64_sys_shmctl
  invoke_syscall
  el0_svc_common.constprop.0
  do_el0_svc
  el0_svc
  el0t_64_sync_handler
  el0t_64_sync

I noticed in dec_rlimit_ucounts(), dec == 0 and type ==
UCOUNT_RLIMIT_MEMLOCK. 

[1] https://lore.kernel.org/lkml/20211115134754.7334-1-quic_qiancai@quicinc.com/