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

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 12 Sep 2026 08:28:22 -0400 Gregory Price <gourry@gourry.net> 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().
> 
> Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject")
> Cc: <stable@vger.kernel.org> # v6.19+

Why cc:stable?  Is a crafted test driver the only way of hitting this?


> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1671,7 +1671,9 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
>  		if (!pmd_present(*pmd))
>  			goto out;
>  
> -		folio = pmd_folio(*pmd);
> +		folio = vm_normal_folio_pmd(vma, addr, *pmd);
> +		if (!folio)
> +			goto out;
>  
>  		/* Clear accessed and referenced bits. */
>  		pmdp_test_and_clear_young(vma, addr, pmd);
> @@ -1991,7 +1993,7 @@ static int pagemap_pmd_range_thp(pmd_t *pmdp, unsigned long addr,
>  		goto populate_pagemap;
>  
>  	if (pmd_present(pmd)) {
> -		page = pmd_page(pmd);
> +		page = vm_normal_page_pmd(vma, addr, pmd);
>  
>  		flags |= PM_PRESENT;
>  		if (pmd_soft_dirty(pmd))