On 10/21/26 22:36, Konstantin Khorenko wrote:
> copy_huge_pmd() reads a special PMD without the page table lock and
> copies that value to the child. The mmap write lock held by fork does
> not stop unmap_mapping_range(): it zaps under i_mmap_rwsem and the PMD
> lock.
>
> The zap visits the parent VMA before the child's, as dup_mmap() links
> the child right after the parent in i_mmap. If it clears the parent PMD
> after fork has read it, and passes the child before fork copies it, the
> child keeps a mapping the driver has revoked, for example:
>
> - vfio-pci: the child reaches the BAR after the device memory was
> disabled or the device was reset, instead of getting SIGBUS;
> - panthor: the child maps the pages of an evicted buffer object after
> the driver has released them.
>
> The same branch deposits no page table under the copy. ppc64 in hash
> mode keeps the hash slot array of every huge PMD in that table, so the
> child dereferences a NULL table on the first hash fault, in
> __hash_page_thp(), and on the zap, in hash__pmdp_huge_get_and_clear().
>
> Take the locks first and let vm_normal_page_pmd() tell a PFN mapping
> from a folio under them, as copy_present_ptes() does with
> vm_normal_page(). The zap clears the parent PMD under the same lock, so
> either fork finds it none and skips it, or the copy is in place before
> the zap reaches the child.
>
> The copy of a PFN mapping then goes through the same tail as a folio:
>
> - it gets a page table where the architecture needs one;
> - in a shared mapping it is marked clean in the child, as a PTE is;
> - in a CoW mapping, which only VM_MIXEDMAP allows, both sides are write
> protected instead of a warning being printed.
>
> Fixes: bc02afbd4d73 ("mm/fork: accept huge pfnmap entries")
> Fixes: d1503aa9ab80 ("powerpc/64s: Add support for huge pfnmaps")
> Reported-by: Sashiko <[email protected]>
> Closes:
> https://sashiko.dev/#/patchset/20261014-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v1-1-d225c60bc...@virtuozzo.com?part=1
> Suggested-by: David Hildenbrand <[email protected]>
> Signed-off-by: Konstantin Khorenko <[email protected]>
> ---
> mm/huge_memory.c | 24 +++---------------------
> 1 file changed, 3 insertions(+), 21 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 13e3674fb6115..da5918a0ade87 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1961,25 +1961,6 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct
> mm_struct *src_mm,
> int ret = -ENOMEM;
> bool writable;
>
> - pmd = pmdp_get_lockless(src_pmd);
> - if (unlikely(pmd_present(pmd) && pmd_special(pmd) &&
> - !is_huge_zero_pmd(pmd))) {
> - dst_ptl = pmd_lock(dst_mm, dst_pmd);
> - src_ptl = pmd_lockptr(src_mm, src_pmd);
> - spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
> - /*
> - * No need to recheck the pmd, it can't change with write
> - * mmap lock held here.
> - *
> - * Meanwhile, making sure it's not a CoW VMA with writable
> - * mapping, otherwise it means either the anon page wrongly
> - * applied special bit, or we made the PRIVATE mapping be
> - * able to wrongly write to the backend MMIO.
> - */
> - VM_WARN_ON_ONCE(vma_is_cow_mapping(src_vma) && pmd_write(pmd));
> - goto set_pmd;
> - }
> -
> if (vma_is_anonymous(dst_vma) || arch_needs_pgtable_deposit()) {
> pgtable = pte_alloc_one(dst_mm);
> if (unlikely(!pgtable))
> @@ -2021,7 +2002,9 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct
> mm_struct *src_mm,
> goto copy_pmd;
> }
>
> - src_page = pmd_page(pmd);
> + src_page = vm_normal_page_pmd(src_vma, addr, pmd);
> + if (!src_page)
> + goto copy_pmd;
> VM_BUG_ON_PAGE(!PageHead(src_page), src_page);
> src_folio = page_folio(src_page);
>
> @@ -2061,7 +2044,6 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct
> mm_struct *src_mm,
> }
> if (src_vma->vm_flags & VM_SHARED)
> pmd = pmd_mkclean(pmd);
> -set_pmd:
> pmd = pmd_mkold(pmd);
> set_pmd_at(dst_mm, addr, dst_pmd, pmd);
>
>
Looks correct. Reading the above, I wonder about Cc: stable. It would affect
v6.12, so not too crazy to backport.
We'd have to backport patch #2 as well in that case (or have a simpler fix, but
I am not sure that's really worth it).
Acked-by: David Hildenbrand (Arm) <[email protected]>
--
Cheers,
David
