[PATCH] mm/mremap: populate calculated fields in vrm state correctly

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

Commit d5c8aec0542e ("mm/mremap: initial refactor of move_vma()")
introduced the vma_remap_struct helper to pass state sanely through
mremap() logic.

Part of this state is vrm->delta - equal to the absolute difference between
vrm->old_len and vrm->new_len (the old and new lengths of the VMA,
respectively).

This value is used to correctly charge accounted VMAs.

When shrinking a VMA, mremap_to() sets vrm->old_len equal to vrm->new_len
having performed the shrink of the VMA first, prior to any potential
move (e.g. MREMAP_FIXED | MREMAP_MAYMOVE specified).

However it does not recalculate vrm->delta, which results in incorrectly
overcharging the VMA.

The charge leaks to Committed_AS globally, meaning a mischievous user can
cause allocations to fail across the system in strict overcommit mode.

Resolve the issue by abstracting the update of vrm->delta to
vrm_populate_calculated() and invoking this from mremap_to() when
vrm->old_len is changed.

Additionally, update vrm->remap_type whose value depends on
vrm->delta. The field is not read again after the shrink, but it is the
logically correct thing to do and protects against future changes which
break this.

Fixes: d5c8aec0542e ("mm/mremap: initial refactor of move_vma()")
Cc: stable@vger.kernel.org
Reported-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Closes: https://lore.kernel.org/all/20261017233527.797191-1-nickolai@csail.mit.edu/
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/mremap.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 6970783fc445..24827e13e617 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -887,12 +887,6 @@ unsigned long move_page_tables(struct pagetable_move_control *pmc)
 	return pmc_progress(pmc);
 }
 
-/* Set vrm->delta to the difference in VMA size specified by user. */
-static void vrm_set_delta(struct vma_remap_struct *vrm)
-{
-	vrm->delta = abs_diff(vrm->old_len, vrm->new_len);
-}
-
 /* Determine what kind of remap this is - shrink, expand or no resize at all. */
 static enum mremap_type vrm_remap_type(struct vma_remap_struct *vrm)
 {
@@ -905,6 +899,13 @@ static enum mremap_type vrm_remap_type(struct vma_remap_struct *vrm)
 	return MREMAP_EXPAND;
 }
 
+/* Populate calculated fields. */
+static void vrm_populate_calculated(struct vma_remap_struct *vrm)
+{
+	vrm->delta = abs_diff(vrm->old_len, vrm->new_len);
+	vrm->remap_type = vrm_remap_type(vrm);
+}
+
 /*
  * When moving a VMA to vrm->new_adr, does this result in the new and old VMAs
  * overlapping?
@@ -1523,6 +1524,7 @@ static unsigned long mremap_to(struct vma_remap_struct *vrm)
 
 		/* Set up for the move now shrink has been executed. */
 		vrm->old_len = vrm->new_len;
+		vrm_populate_calculated(vrm);
 	}
 
 	/* MREMAP_DONTUNMAP expands by old_len since old_len == new_len */
@@ -1795,8 +1797,7 @@ static int check_prep_vma(struct vma_remap_struct *vrm)
 	if (vma_is_hugetlb(vma) && !align_hugetlb(vrm))
 		return -EINVAL;
 
-	vrm_set_delta(vrm);
-	vrm->remap_type = vrm_remap_type(vrm);
+	vrm_populate_calculated(vrm);
 	/* For convenience, we set new_addr even if VMA won't move. */
 	if (!vrm_implies_new_addr(vrm))
 		vrm->new_addr = addr;

---
base-commit: 4dbe24720eb3811563cd56b35d87ddf3f9beed2a
change-id: 20261019-mremap-fix-shrink-charge-be490a2311d7

Cheers,
-- 
Lorenzo Stoakes (ARM) <ljs@kernel.org>