[merged mm-hotfixes-stable] mm-mempolicy-correctly-update-prev-when-policy-is-equal-on-mbind.patch removed from -mm tree

[merged mm-hotfixes-stable] mm-mempolicy-correctly-update-prev-when-policy-is-equal-on-mbind.patch removed from -mm tree

[Andrew Morton]

The quilt patch titled
     Subject: mm/mempolicy: correctly update prev when policy is equal on mbind
has been removed from the -mm tree.  Its filename was
     mm-mempolicy-correctly-update-prev-when-policy-is-equal-on-mbind.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Lorenzo Stoakes <lstoakes@gmail.com>
Subject: mm/mempolicy: correctly update prev when policy is equal on mbind
Date: Sun, 30 Apr 2023 16:07:07 +0100

The refactoring in commit f4e9e0e69468 ("mm/mempolicy: fix use-after-free
of VMA iterator") introduces a subtle bug which arises when attempting to
apply a new NUMA policy across a range of VMAs in mbind_range().

The refactoring passes a **prev pointer to keep track of the previous VMA
in order to reduce duplication, and in all but one case it keeps this
correctly updated.

The bug arises when a VMA within the specified range has an equivalent
policy as determined by mpol_equal() - which unlike other cases, does not
update prev.

This can result in a situation where, later in the iteration, a VMA is
found whose policy does need to change.  At this point, vma_merge() is
invoked with prev pointing to a VMA which is before the previous VMA.

Since vma_merge() discovers the curr VMA by looking for the one
immediately after prev, it will now be in a situation where this VMA is
incorrect and the merge will not proceed correctly.

This is checked in the VM_WARN_ON() invariant case with end >
curr->vm_end, which, if a merge is possible, results in a warning (if
CONFIG_DEBUG_VM is specified).

I note that vma_merge() performs these invariant checks only after
merge_prev/merge_next are checked, which is debatable as it hides this
issue if no merge is possible even though a buggy situation has arisen.

The solution is simply to update the prev pointer even when policies are
equal.

This caused a bug to arise in the 6.2.y stable tree, and this patch
resolves this bug.

Link: https://lkml.kernel.org/r/83f1d612acb519d777bebf7f3359317c4e7f4265.1682866629.git.lstoakes@gmail.com
Fixes: f4e9e0e69468 ("mm/mempolicy: fix use-after-free of VMA iterator")
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
  Link: https://lore.kernel.org/oe-lkp/202304292203.44ddeff6-oliver.sang@intel.com
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/mempolicy.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/mm/mempolicy.c~mm-mempolicy-correctly-update-prev-when-policy-is-equal-on-mbind
+++ a/mm/mempolicy.c
@@ -808,8 +808,10 @@ static int mbind_range(struct vma_iterat
 		vmstart = vma->vm_start;
 	}
 
-	if (mpol_equal(vma_policy(vma), new_pol))
+	if (mpol_equal(vma_policy(vma), new_pol)) {
+		*prev = vma;
 		return 0;
+	}
 
 	pgoff = vma->vm_pgoff + ((vmstart - vma->vm_start) >> PAGE_SHIFT);
 	merged = vma_merge(vmi, vma->vm_mm, *prev, vmstart, vmend, vma->vm_flags,
_

Patches currently in -mm which might be from lstoakes@gmail.com are

mm-mmap-vma_merge-always-check-invariants.patch
mm-mmap-separate-writenotify-and-dirty-tracking-logic.patch
mm-gup-disallow-foll_longterm-gup-nonfast-writing-to-file-backed-mappings.patch
mm-gup-disallow-foll_longterm-gup-fast-writing-to-file-backed-mappings.patch