+ mm-filemap-stop-fault-around-when-the-walk-steps-back.patch added to mm-hotfixes-unstable branch

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

The patch titled
     Subject: mm/filemap: stop fault-around when the walk steps back
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-filemap-stop-fault-around-when-the-walk-steps-back.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-filemap-stop-fault-around-when-the-walk-steps-back.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: "Jose Fernandez (Anthropic)" <jose.fernandez@linux.dev>
Subject: mm/filemap: stop fault-around when the walk steps back
Date: Sat, 31 Oct 2026 04:33:59 +0000

filemap_map_pages() can dereference a PTE pointer outside the page table
it locked.  KASAN has reported this as a use-after-free read [1].  The
same path can also write out of bounds.  When the stray entry reads as
none, the fault-around loop maps a page there with set_pte_range().

Its fault-around loop walks the page cache and moves vmf->pte by the
change in index.  A lower index moves it back.  xas_find() can step back
to a lower index when a multi-index entry is split during the walk.

[1] fixes xas_find(), but the loop still relies on the walk never stepping
back.  Stop the fault-around when the walk steps back.  ->fault() maps the
faulting page if the loop did not.

Link: https://lore.kernel.org/20261031-jf-mm-filemap-map-pages-lower-bound-v2-1-21029744e589@linux.dev
Link: https://lore.kernel.org/20260904121301.200049-1-krystianmkaniewski@gmail.com [1]
Fixes: 6b24ca4a1a8d ("mm: Use multi-index entries in the page cache")
Signed-off-by: Jose Fernandez (Anthropic) <jose.fernandez@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Krystian Kaniewski <krystianmkaniewski@gmail.com>
Cc: <stable@vger.kernel.org>
---

 mm/filemap.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/mm/filemap.c~mm-filemap-stop-fault-around-when-the-walk-steps-back
+++ a/mm/filemap.c
@@ -3968,6 +3968,13 @@ vm_fault_t filemap_map_pages(struct vm_f
 		unsigned long end;
 		vm_fault_t map_ret;
 
+		/* A lower index could move vmf->pte out of its page table. */
+		if (unlikely(xas.xa_index < last_pgoff)) {
+			folio_unlock(folio);
+			folio_put(folio);
+			break;
+		}
+
 		addr += (xas.xa_index - last_pgoff) << PAGE_SHIFT;
 		vmf->pte += xas.xa_index - last_pgoff;
 		last_pgoff = xas.xa_index;
_

Patches currently in -mm which might be from jose.fernandez@linux.dev are

mm-filemap-stop-fault-around-when-the-walk-steps-back.patch