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From: Qian Cai <quic_qiancai@quicinc.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alexey Gladkov <legion@kernel.org>, Yu Zhao <yuzhao@google.com>,
	<linux-kernel@vger.kernel.org>
Subject: Re: BUG: KASAN: use-after-free in dec_rlimit_ucounts
Date: Thu, 18 Nov 2021 15:32:33 -0500	[thread overview]
Message-ID: <YZa4YbcOyjtD3+pL@fixkernel.com> (raw)
In-Reply-To: <875ysptfgi.fsf@email.froward.int.ebiederm.org>

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/

  reply	other threads:[~2021-11-18 20:32 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-17 22:00 Qian Cai
2021-11-18 19:46 ` Eric W. Biederman
2021-11-18 20:32   ` Qian Cai [this message]
2021-11-18 20:57     ` Eric W. Biederman
2021-11-19 13:32       ` Qian Cai
2021-11-24 21:49       ` Qian Cai
2021-11-26  5:34         ` Qian Cai
2021-12-20  5:58           ` Eric W. Biederman
2021-12-21 13:09             ` Alexey Gladkov
2021-12-27 15:22               ` Eric W. Biederman

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