Re: [PATCH] mm: fix racing of vb->va when kasan enabled

[Zhaoyang Huang]

On Tue, Jun 21, 2022 at 5:27 PM Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
>
> > On Mon, Jun 20, 2022 at 6:44 PM Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
> > >
> > > > > >
> > > > > Is it easy to reproduce? If so could you please describe the steps? As i see
> > > > > the freeing of the "vb" is RCU safe whereas vb->va is not. But from the first
> > > > > glance i do not see how it can accessed twice. Hm..
> > > > It was raised from a monkey test on A13_k515 system and got 1/20 pcs
> > > > failed. IMO, vb->va which out of vmap_purge_lock protection could race
> > > > with a concurrent ra freeing within __purge_vmap_area_lazy.
> > > >
> > > Do you have exact steps how you run "monkey" test?
> > There are about 30+ kos inserted during startup which could be a
> > specific criteria for reproduction. Do you have doubts about the test
> > result or the solution?
> > >
> I do not have any doubt about your test results, so if you can trigger it
> then there is an issue at least on the 5.4.161-android12 kernel.
>
> 1. With your fix we get expanded mutex range, thus the worst case of vmalloc
> allocation can be increased when it fails and repeat. Because it also invokes
> the purge_vmap_area_lazy() that access the same mutex.
I am not sure I get your point. _vm_unmap_aliases calls
_purge_vmap_area_lazy instead of purge_vmap_area_lazy. Do you have any
other solutions? I really don't think my patch is the best way as I
don't have a full view of vmalloc mechanism.
>
> 2. You run 5.4.161-android12 kernel what is quite old. Could you please
> retest with latest kernel? I am asking because on the latest kernel with
> CONFIG_KASAN i am not able to reproduce it.
>
> I do a lot of: vm_map_ram()/vm_unmap_ram()/vmalloc()/vfree() in parallel
> by 64 kthreads on my 64 CPUs test system.
The failure generates at 20s from starting up, I think it is a rare timing.
>
> Could you please confirm that you can trigger an issue on the latest kernel?
Sorry, I don't have an available latest kernel for now.
>
> Thanks!
>
> --
> Uladzislau Rezki