Re: [syzbot] KASAN: use-after-free Read in xfs_qm_dqfree_one

[Dmitry Vyukov]

On Tue, 6 Dec 2022 at 16:32, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Tue, Dec 06, 2022 at 12:06:10PM +0100, Dmitry Vyukov wrote:
> > On Tue, 6 Dec 2022 at 04:34, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Dec 05, 2022 at 07:12:15PM -0800, syzbot wrote:
> > > > Hello,
> > > >
> > > > syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> > > > INFO: rcu detected stall in corrupted
> > > >
> > > > rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: { P4122 } 2641 jiffies s: 2877 root: 0x0/T
> > > > rcu: blocking rcu_node structures (internal RCU debug):
> > >
> > > I'm pretty sure this has nothing to do with the reproducer - the
> > > console log here:
> > >
> > > > Tested on:
> > > >
> > > > commit:         bce93322 proc: proc_skip_spaces() shouldn't think it i..
> > > > git tree:       https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1566216b880000
> > >
> > > indicates that syzbot is screwing around with bluetooth, HCI,
> > > netdevsim, bridging, bonding, etc.
> > >
> > > There's no evidence that it actually ran the reproducer for the bug
> > > reported in this thread - there's no record of a single XFS
> > > filesystem being mounted in the log....
> > >
> > > It look slike someone else also tried a private patch to fix this
> > > problem (which was obviously broken) and it failed with exactly the
> > > same RCU warnings. That was run from the same commit id as the
> > > original reproducer, so this looks like either syzbot is broken or
> > > there's some other completely unrelated problem that syzbot is
> > > tripping over here.
> > >
> > > Over to the syzbot people to debug the syzbot failure....
> >
> > Hi Dave,
> >
> > It's not uncommon for a single program to trigger multiple bugs.
> > That's what happens here. The rcu stall issue is reproducible with
> > this test program.
> > In such cases you can either submit more test requests, or test manually.
> >
> > I think there is an RCU expedited stall detection.
> > For some reason CONFIG_RCU_EXP_CPU_STALL_TIMEOUT is limited to 21
> > seconds, and that's not enough for reliable flake-free stress testing.
> > We bump other timeouts to 100+ seconds.
> > +RCU maintainers, do you mind removing the overly restrictive limit on
> > CONFIG_RCU_EXP_CPU_STALL_TIMEOUT?
> > Or you think there is something to fix in the kernel to not stall? I
> > see the test writes to
> > /proc/sys/vm/drop_caches, maybe there is some issue in that code.
>
> Like this?
>
> If so, I don't see why not.  And in that case, may I please have
> your Tested-by or similar?

I've tried with this patch and RCU_EXP_CPU_STALL_TIMEOUT=80000.
Running the test program I got some kernel BUG in XFS and no RCU
errors/warnings.

Tested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

Thanks

> At the same time, I am sure that there are things in the kernel that
> should be adjusted to avoid stalls, but I recognize that different
> developers in different situations will have different issues that they
> choose to focus on.  ;-)
>
>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
> index 49da904df6aa6..2984de629f749 100644
> --- a/kernel/rcu/Kconfig.debug
> +++ b/kernel/rcu/Kconfig.debug
> @@ -82,7 +82,7 @@ config RCU_CPU_STALL_TIMEOUT
>  config RCU_EXP_CPU_STALL_TIMEOUT
>         int "Expedited RCU CPU stall timeout in milliseconds"
>         depends on RCU_STALL_COMMON
> -       range 0 21000
> +       range 0 300000
>         default 0
>         help
>           If a given expedited RCU grace period extends more than the