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

[Gregory Price <gourry@gourry.net>]

On Mon, Sep 14, 2026 at 09:49:10PM -0700, Andrew Morton wrote:
> 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?
> 

Technically reachable via mshv_vtl_low, so i tagged it stable.

It's hard to make a judgment call on some of these - so I err on the
side of backporting.  Feel free to drop stable if you think it's not
worth it.

~Gregory