Re: [PATCH v3 3/3] virtio_pci_modern: move avq cleanup from reset to del_vqs

[Karl Mehltretter <kmehltretter@gmail.com>]

On Fri, Sep 11, 2026 at 05:20:15PM +0100, Michael S. Tsirkin wrote:
> New in v3. v2 dropped sync from modern vp_reset in one combined
> patch, leaving avq_cleanup in vp_reset. v3 moves avq_cleanup out
> of vp_reset entirely into vp_del_vqs, fixing the race. Adds NULL
> check for admin_vq.info for find_vqs error paths.
> 

I think patch 3 introduces a use-after-free in the MSI-X setup error
path.

With VP_VQ_VECTOR_POLICY_EACH, vp_setup_vq() stores the admin queue's
info pointer before request_irq(). If request_irq() fails, vp_del_vq()
frees info but leaves admin_vq.info set.

vp_find_vqs_msix() then calls vp_del_vqs(). vp_del_vqs() now calls
vp_modern_avq_cleanup() first and dereferences the freed pointer:

    vq = vp_dev->admin_vq.info->vq;

No interrupt needs to fire.

I reproduced this on v3 with KASAN by returning -ENOMEM only for the
admin queue's request_irq():

    BUG: KASAN: slab-use-after-free in vp_modern_avq_cleanup+0xf4/0x110

KASAN shows the allocation in vp_setup_vq(), the free in
vp_find_one_vq_msix(), and the access in vp_modern_avq_cleanup(). Adding
*p_info = NULL after vp_del_vq() made the same test fall back and boot
without a KASAN report.

Thanks,
Karl