Re: [PATCH 1/2] userfaultfd: Fix pmd_trans_huge() recheck race

[David Hildenbrand]

On 12.08.24 18:42, Jann Horn wrote:
> The following race can occur:
>
>    mfill_atomic                other thread
>    ============                ============
>                                <zap PMD>
>    pmdp_get_lockless() [reads none pmd]
>    <bail if trans_huge>
>    <if none:>
>                                <pagefault creates transhuge zeropage>
>      __pte_alloc [no-op]
>                                <zap PMD>
>    <bail if pmd_trans_huge(*dst_pmd)>
>    BUG_ON(pmd_none(*dst_pmd))
>
> I have experimentally verified this in a kernel with extra mdelay() calls;
> the BUG_ON(pmd_none(*dst_pmd)) triggers.
>
> On kernels newer than commit 0d940a9b270b ("mm/pgtable: allow
> pte_offset_map[_lock]() to fail"), this can't lead to anything worse than
> a BUG_ON(), since the page table access helpers are actually designed to
> deal with page tables concurrently disappearing; but on older kernels
> (<=6.4), I think we could probably theoretically race past the two BUG_ON()
> checks and end up treating a hugepage as a page table.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation")
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb