LKML Archive on
help / color / mirror / Atom feed
From: (Eric W. Biederman)
To: Qian Cai <>
Cc: Alexey Gladkov <>, Yu Zhao <>,
Subject: Re: BUG: KASAN: use-after-free in dec_rlimit_ucounts
Date: Thu, 18 Nov 2021 14:57:17 -0600	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <> (Qian Cai's message of "Thu, 18 Nov 2021 15:32:33 -0500")

Qian Cai <> writes:

> 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.

Does this only happen on a single architecture?  If so I wonder if
perhaps some of the architectures atomic primitives are implemented

Unfortunately I don't have any arm64 machines where I can easily test

The call path you posted from user_shm_unlock is another path where
a use-after-free has show up in the past.

My blind guess would be that I made an implementation mistake in
inc_rlimit_get_ucounts or dec_rlimit_put_ucounts but I can't see it
right now.


>  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 ==
> [1]

  reply	other threads:[~2021-11-18 20:59 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
2021-11-18 20:57     ` Eric W. Biederman [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \
    --subject='Re: BUG: KASAN: use-after-free in dec_rlimit_ucounts' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).