Re: [syzbot] [mm?] KASAN: slab-use-after-free Read in __vma_reservation_common

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 15, 2024 at 03:05:44PM -0700, Vishal Moola wrote:
> Commit 9acad7ba3e25 ("hugetlb: use vmf_anon_prepare() instead of
> anon_vma_prepare()") may bailout after allocating a folio if we do not
> hold the mmap lock. When this occurs, vmf_anon_prepare() will release the
> vma lock. Hugetlb then attempts to call restore_reserve_on_error(),
> which depends on the vma lock being held.
> 
> We can move vmf_anon_prepare() prior to the folio allocation in order to
> avoid calling restore_reserve_on_error() without the vma lock.

But now you're calling vmf_anon_prepare() in the wrong place -- before
we've determined that we need an anon folio.  So we'll create an
anon_vma even when we don't need one for this vma.

This is definitely a pre-existing bug which you've exposed by making it
happen more easily.  Needs a different fix though.