Re: [RFC PATCH v3] UML: add support for KASAN under x86_64

[Dmitry Vyukov]

On Fri, 27 May 2022 at 15:27, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
>
> On Fri, 2022-05-27 at 15:18 +0200, Dmitry Vyukov wrote:
> > On Fri, 27 May 2022 at 15:15, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Fri, 2022-05-27 at 15:09 +0200, Dmitry Vyukov wrote:
> > > > > I did note (this is more for kasan-dev@) that the "freed by" is fairly
> > > > > much useless when using kfree_rcu(), it might be worthwhile to annotate
> > > > > that somehow, so the stack trace is recorded by kfree_rcu() already,
> > > > > rather than just showing the RCU callback used for that.
> > > >
> > > > KASAN is doing it for several years now, see e.g.:
> > > > https://groups.google.com/g/syzkaller-bugs/c/eTW9zom4O2o/m/_v7cOo2RFwAJ
> > > >
> > >
> > > Hm. It didn't for me:
> >
> > Please post a full report with line numbers and kernel version.
>
> That was basically it, apart from a few lines snipped from the stack
> traces. Kernel version was admittedly a little older - 5.18.0-rc1 + a
> few UML fixes + this KASAN patch (+ the fixes I pointed out earlier)
>
> I guess it doesn't really matter that much, just had to dig a bit to
> understand why it was freed.

Humm... I don't have any explanation based only on this info.
Generally call_rcu stacks are memorized and I see the call is still there:
https://elixir.bootlin.com/linux/v5.18/source/kernel/rcu/tree.c#L3595
It may be caused by some narrow races, depleted reserve memory in
stackdepot, or race with quarantine eviction.