linux-kernel - [PATCH] mm/mempolicy: Correctly update prev when policy is equal on mbind

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Message-Id: <db42467a692d78c654ec5c1953329401bd8a9c34.1682859234.git.lstoakes@gmail.com>
Date:   Sun, 30 Apr 2023 14:00:15 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        oliver.sang@...el.com, Mel Gorman <mgorman@...e.de>,
        stable@...r.kernel.org, Lorenzo Stoakes <lstoakes@...il.com>
Subject: [PATCH] mm/mempolicy: Correctly update prev when policy is equal on mbind

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.

Reported-by: kernel test robot <oliver.sang@...el.com>
Link: https://lore.kernel.org/oe-lkp/202304292203.44ddeff6-oliver.sang@intel.com
Fixes: f4e9e0e69468 ("mm/mempolicy: fix use-after-free of VMA iterator")
Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
---
 mm/mempolicy.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 2068b594dc88..1756389a0609 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -808,8 +808,10 @@ static int mbind_range(struct vma_iterator *vmi, struct vm_area_struct *vma,
 		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,
-- 
2.40.1