Re: [PATCH] iommu/iova: Clear the slab cache pointers when destroying them

[Davidlohr Bueso]

On Mon, 17 Aug 2026, Robin Murphy wrote:

> On 16/08/2026 7:43 pm, Davidlohr Bueso wrote:
> > Both iova_cache_get() failure path and iova_cache_put() destroy the two
> > slab caches without clearing the pointers, leaving them dangling with
> > iova_cache_users at zero. The next iova_cache_get() then re-enters the
> > respective block, and if it fails early enough to reach 'out_err' before
> > re-creating both caches, it calls kmem_cache_destroy() a second time on
> > whichever cache is still stale:
> >
> >   BUG: KASAN: slab-use-after-free in iova_cache_get+0x216/0x280
> >   Read of size 1 at addr ffff888001b9fdc0 by task swapper/0/1
> >   CPU: 1 UID: 0 PID: 1 Comm: swapper/0 Not tainted 7.2.0-rc1 #2
> >   Call Trace:
> >    <TASK>
> >    dump_stack_lvl+0x53/0x70
> >    print_report+0xce/0x620
> >    kasan_report+0xce/0x100
> >    __kasan_check_byte+0x36/0x50
> >    kmem_cache_destroy+0x1b/0x1c0
> >    iova_cache_get+0x216/0x280
> >    ...
> >   Freed by task 1:
> >    kasan_save_stack+0x33/0x60
> >    kasan_save_track+0x14/0x30
> >    kasan_save_free_info+0x3b/0x60
> >    __kasan_slab_free+0x43/0x70
> >    kmem_cache_free+0xbe/0x3c0
> >    kobject_put+0x14d/0x280
> >    iova_cache_put+0x8e/0xd0
> >
> > Clear both pointers after destroying them, in both places.
>
> Out of curiosity, what architecture/kernel config have you found this
> with? I see the logic, but off the top of my head I'm somewhat
> struggling to imagine the scenario in which iova_cache_get() succeeds,
> the last user (so no iommu-dma) cleanly calls iova_cache_put() to be
> able to free the state, then another iova_cache_get() fails. Is
> everything else also falling apart in flames anyway at this point?

This was on x86-64, triggered through a dma fuzzer/stresser, but
could not reproduce this after leaving it running all week. What could
have happened, not sure, was that the program patched the code to
trigger the failure points, for which you can ignore this patch I guess.

Thanks,
Davidlohr