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

[Eric W. Biederman]

Qian Cai <quic_qiancai@xxxxxxxxxxx> writes:

> On Wed, Nov 24, 2021 at 04:49:19PM -0500, Qian Cai wrote:
>> Hmm, I don't know if that or it is just this platfrom is lucky to trigger
>> the race condition quickly, but I can't reproduce it on x86 so far. I am
>> Cc'ing a few arm64 people to see if they have spot anything I might be
>> missing. The original bug report is here:
>> 
>> https://lore.kernel.org/lkml/YZV7Z+yXbsx9p3JN@xxxxxxxxxxxxx/
>
> Okay, I am finally able to reproduce this on x86_64 with the latest
> mainline as well by setting CONFIG_USER_NS and KASAN on the top of
> defconfig (I did not realize it did not select CONFIG_USER_NS in the first
> place). Anyway, it still took less than 5-minute by running:
>
> $ trinity -C 48

It took me a while to get to the point of reproducing this but I can
confirm I see this with 2 core VM, running 5.16.0-rc4.

Running trinity 2019.06 packaged in debian 11.

I didn't watch so I don't know if it was 5 minutes but I do know it took
less than an hour.

Now I am puzzled why there are not other reports of problems.

Now to start drilling down to figure out why the user namespace was
freed early.
----

The failure I got looked like:
> BUG: KASAN: use-after-free in dec_rlimit_ucounts+0x7b/0xb0
> Read of size 8 at addr ffff88800b7dd018 by task trinity-c3/67982
> 
> CPU: 1 PID: 67982 Comm: trinity-c3 Tainted: G  O 5.16.0-rc4 #1
> Hardware name: Xen HVM domU, BIOS 4.8.5-35.fc25 08/25/2021
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x48/0x5e
>  print_address_descrtion.constprop.0+0x1f/0x140
>  ? dec_rlimit_ucounts+0x7b/0xb0
>  ? dec_rlimit_ucounts+0x7b/0xb0
>  kasan_report.cold+0x7f/0xe0
>  ? _raw_spin_lock+0x7f/0x11b
>  ? dec_rlimit_ucounts+0x7b/0xb0
>  dec_rlimit_ucounts+0x7b/0xb0
>  mqueue_evict_inode+0x417/0x590
>  ? perf_trace_global_dirty_state+0x350/0x350
>  ? __x64_sys_mq_unlink+0x250/0x250
>  ? _raw_spin_lock_bh+0xe0/0xe0
>  ? _raw_spin_lock_bh+0xe0/0xe0
>  evict+0x155/0x2a0
>  __x64_sys_mq_unlink+0x1a7/0x250
>  do_syscall_64+0x3b/0x90
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7f0505ebc9b9
> Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 00 0f 1f 44 00 00 48 89 ....
> 
> Allocated by task 67717
> Freed by task 6027
> 
> The buggy address belongs to the object at ffff88800b7dce38
>  which belongs to the cache user_namespace of size 600
> The buggy address is located 480 bytes inside of
>  600-byte region [ffff88800b7dce38, ffff88800b7dd090]
> The buggy address belongs to the page:
> 
> trinity: Detected kernel tainting. Last seed was 1891442794

Eric