Re: [PATCH] proc/task_mmu: handle special PMDs in clear_refs and pagemap

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

On 9/12/26 14:28, Gregory Price wrote:
> mshv_vtl_low can install a special PMD from a PFN supplied in the mmap file
> offset.  That PFN need not have a memmap entry.  Reading /proc/PID/pagemap
> or writing /proc/PID/clear_refs for such a mapping oopses the kernel.
> 
> With a test driver mapping the PFN at the 1 TiB mark:
> 
>   echo 1 > /proc/self/clear_refs
> 
>   BUG: unable to handle page fault for address: fffffb5b80000008
>   #PF: supervisor read access in kernel mode
>   #PF: error_code(0x0000) - not-present page
>   RIP: 0010:clear_refs_pte_range+0xb1/0x1e0
>   Call Trace:
>    walk_pgd_range+0x50a/0xad0
>    __walk_page_range+0x6a/0x1d0
>    walk_page_range_mm_unsafe+0x193/0x230
>    clear_refs_write+0x18e/0x3f0
> 
>   pread(pagemap_fd, buf, 512 * 8, addr / PAGE_SIZE * 8)
> 
>   BUG: unable to handle page fault for address: fffff57840000008
>   RIP: 0010:pagemap_pmd_range+0x3cf/0x6b0
>   Call Trace:
>    walk_pgd_range+0x50a/0xad0
>    __walk_page_range+0x6a/0x1d0
>    walk_page_range_mm_unsafe+0x193/0x230
>    pagemap_read+0x1dc/0x350
> 
> clear_refs_pte_range() passes the PMD to pmd_folio(), while
> pagemap_pmd_range_thp() uses pmd_page() followed by page_folio().
> 
> Both paths assume the PFN has a struct page and dereference bad address.
> 
> Use vm_normal_folio_pmd() and vm_normal_page_pmd() so clear_refs skips
> folio operations and pagemap skips folio-derived flags for special PMDs.
> This matches the PTE paths, which already use vm_normal_folio() and
> vm_normal_page().
> 
> User-facing notes:
>  - Pagemap still reports PM_PRESENT and the PFN to a CAP_SYS_ADMIN reader.
> 
>  - For a special PMD backed by a valid memmap entry and for the huge
>    zero PMD, PM_FILE changes from set to clear.
> 
>  - PM_MMAP_EXCLUSIVE is already clear in both cases.
> 
>  - The output for an anonymous THP is unchanged.
> 
> Tested in QEMU with the test driver on broken and fixed kernels.  Both proc
> operations oops before the fix and return 0 after it.
> 
> Reported-by: sashiko-bot <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/20260912034833.2952750-1-gourry%40gourry.net
> Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject")
> Cc: <stable@vger.kernel.org> # v6.19+
> Assisted-by: LLM
> Signed-off-by: Gregory Price <gourry@gourry.net>
> ---

Simple fix, let's CC-stable indeed (unless I am too late and this is already
upstream :) )

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David