Re: [PATCH v2 2/3] kfence: maximize allocation wait timeout duration

[Marco Elver]

On Sat, 18 Sept 2021 at 10:07, Liu Shixin <liushixin2@xxxxxxxxxx> wrote:
>
> On 2021/9/16 16:49, Marco Elver wrote:
> > On Thu, 16 Sept 2021 at 03:20, Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:
> >> Hi Marco,
> >>
> >> We found kfence_test will fails  on ARM64 with this patch with/without
> >> CONFIG_DETECT_HUNG_TASK,
> >>
> >> Any thought ?
> > Please share log and instructions to reproduce if possible. Also, if
> > possible, please share bisection log that led you to this patch.
> >
> > I currently do not see how this patch would cause that, it only
> > increases the timeout duration.
> >
> > I know that under QEMU TCG mode, there are occasionally timeouts in
> > the test simply due to QEMU being extremely slow or other weirdness.
> >
> >
> Hi Marco,
>
> There are some of the results of the current test:
> 1. Using qemu-kvm on arm64 machine, all testcase can pass.
> 2. Using qemu-system-aarch64 on x86_64 machine, randomly some testcases fail.
> 3. Using qemu-system-aarch64 on x86_64, but removing the judgment of kfence_allocation_key in kfence_alloc(), all testcase can pass.
>
> I add some printing to the kernel and get very strange results.
> I add a new variable kfence_allocation_key_gate to track the
> state of kfence_allocation_key. As shown in the following code, theoretically,
> if kfence_allocation_key_gate is zero, then kfence_allocation_key must be
> enabled, so the value of variable error in kfence_alloc() should always be
> zero. In fact, all the passed testcases fit this point. But as shown in the
> following failed log, although kfence_allocation_key has been enabled, it's
> still check failed here.
>
> So I think static_key might be problematic in my qemu environment.
> The change of timeout is not a problem but caused us to observe this problem.
> I tried changing the wait_event to a loop. I set timeout to HZ and re-enable/disabled
> in each loop, then the failed testcase disappears.

Nice analysis, thanks! What I gather is that static_keys/jump_labels
are somehow broken in QEMU.

This does remind me that I found a bug in QEMU that might be relevant:
https://bugs.launchpad.net/qemu/+bug/1920934
Looks like it was never fixed. :-/

The failures I encountered caused the kernel to crash, but never saw
the kfence test to fail due to that (never managed to get that far).
Though the bug I saw was on x86 TCG mode, and I never tried arm64. If
you can, try to build a QEMU with ASan and see if you also get the
same use-after-free bug.

Unless we observe the problem on a real machine, I think for now we
can conclude with fairly high confidence that QEMU TCG still has
issues and cannot be fully trusted here (see bug above).

Thanks,
-- Marco