Re: [PATCH] fs/jfs: Add a mutex named txEnd_lmLogClose_mutex to prevent a race condition between txEnd and lmLogClose functions

[Zheng Hacker]

Hello Michal,


Michal Koutný <mkoutny@xxxxxxxx> 于2023年7月20日周四 17:56写道：
>
> Hello Zheng.
>
> On Mon, May 15, 2023 at 05:59:56PM +0800, Zheng Wang <zyytlz.wz@xxxxxxx> wrote:
> > ==================================================================
> > BUG: KASAN: slab-use-after-free in instrument_atomic_write include/linux/instrumented.h:87 [inline]
> > BUG: KASAN: slab-use-after-free in clear_bit include/asm-generic/bitops/instrumented-atomic.h:41 [inline]
> > BUG: KASAN: slab-use-after-free in txEnd+0x2a3/0x5a0 fs/jfs/jfs_txnmgr.c:535
> > Write of size 8 at addr ffff888021bee840 by task jfsCommit/130
> >
> > CPU: 3 PID: 130 Comm: jfsCommit Not tainted 6.3.0-rc7-pasta #1
>
> Is this still pertinent with the current mainline? (There were some
> changes to jfs.)

Thank you very much for your reply and suggestion. I thought that this
BUG still exists in the current mainline kernel version. Since I am
not very familiar with this part of the code, I am not sure if the
proposed fix is correct.

>
> > Through analysis, it was found that a race condition occurred between two
> > functions lmLogClose and txEnd, which were executed in different threads.
> > The possible sequence is as follows:
> >
> > -------------------------------------------------------------------------
> > cpu1(free thread)        |        cpu2(use thread)
> > -------------------------------------------------------------------------
> > lmLogClose               |        txEnd
> >                          |        log = JFS_SBI(tblk->sb)->log;
> > sbi->log = NULL;         |
> > kfree(log); [1] free log |
> >                          |        clear_bit(log_FLUSH, &log->flag); [2] UAF
>
> That looks sane to a by-passer.
>
> > Fix it by add a mutex lock between lmLogClose and txEnd:
>
> It doesn't feel right wrt "lock data, not code" heuristics.
> And when I apply that, it turns out there's already jfs_log_mutex.
> I'd suggest you explain more why a new lock is needed (if that's the
> preferred solutino).

You're right, I think my fix method is not a good solution. Hoping you
and other developers can help fix it.

>
> Thanks,
> Michal

Once again, I appreciate your help and will take your feedback into
consideration when working on a solution.

Best regards,
Zheng