Re: [PATCH v4 3/4] mm/huge_memory: read a special PMD under the lock in copy_huge_pmd()

["David Hildenbrand (Arm)" <david@kernel.org>]

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 <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/20261014-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v1-1-d225c60bc316@virtuozzo.com?part=1
> Suggested-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
> ---
>  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) <david@kernel.org>

-- 
Cheers,

David