mm/mseal: update VMA end correctly on merge

Previously we stored the end of the current VMA in curr_end, and then upon iterating to the next VMA updated curr_start to curr_end to advance to the next VMA.

However, this doesn't take into account the fact that a VMA might be updated due to a merge by vma_modify_flags(), which can result in curr_end being stale and thus, upon setting curr_start to curr_end, ending up with an incorrect curr_start on the next iteration.

Resolve the issue by setting curr_end to vma->vm_end unconditionally to ensure this value remains updated should this occur.

While we're here, eliminate this entire class of bug by simply setting const curr_[start/end] to be clamped to the input range and VMAs, which also happens to simplify the logic.

Reported-by Antonius <[email protected]>

Closes https://lore.kernel.org/linux-mm/CAK8a0jwWGj9-SgFk0yKFh7i8jMkwKm5b0ao9=kmXWjO54veX2g@mail.gmail.com/ open here

Suggested-by David Hildenbrand (ARM) <[email protected]>

Acked-by Vlastimil Babka (SUSE) <[email protected]>

Reviewed-by Pedro Falcato <[email protected]>

Signed-off-by Lorenzo Stoakes (Oracle) <[email protected]>

Fixes 6c2da14ae1e0 ("mm/mseal: rework mseal apply logic")

Cc <[email protected]>

---
v2:
* Correct Closes: tag
* Use David's excellent idea to improve the patch

v1:
https://lore.kernel.org/all/[email protected]/

 mm/mseal.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

mm/mseal.c +1 -3

index 316b5e1dec78..ac58643181f7 100644
@@ -56,7 +56,6 @@ static int mseal_apply(struct mm_struct *mm,
 		unsigned long start, unsigned long end)
 {
 	struct vm_area_struct *vma, *prev;
-	unsigned long curr_start = start;
 	VMA_ITERATOR(vmi, mm, start);
 
 	/* We know there are no gaps so this will be non-NULL. */

@@ -66,6 +65,7 @@ static int mseal_apply(struct mm_struct *mm,
 		prev = vma;
 
 	for_each_vma_range(vmi, vma, end) {
+		const unsigned long curr_start = MAX(vma->vm_start, start);
 		const unsigned long curr_end = MIN(vma->vm_end, end);
 
 		if (!(vma->vm_flags & VM_SEALED)) {

@@ -79,7 +79,6 @@ static int mseal_apply(struct mm_struct *mm,
 		}
 
 		prev = vma;
-		curr_start = curr_end;
 	}
 
 	return 0;
--

2.53.0

Previously we stored the end of the current VMA in curr_end, and then upon iterating to the next VMA updated curr_start to curr_end to advance to the next VMA.

However, this doesn't take into account the fact that a VMA might be updated due to a merge by vma_modify_flags(), which can result in curr_end being stale and thus, upon setting curr_start to curr_end, ending up with an incorrect curr_start on the next iteration.

Resolve the issue by setting curr_end to vma->vm_end unconditionally to ensure this value remains updated should this occur.

While we're here, eliminate this entire class of bug by simply setting const curr_[start/end] to be clamped to the input range and VMAs, which also happens to simplify the logic.

Reported-by Antonius <[email protected]>

Closes https://lore.kernel.org/linux-mm/CAK8a0jwWGj9-SgFk0yKFh7i8jMkwKm5b0ao9=kmXWjO54veX2g@mail.gmail.com/ open here

Suggested-by David Hildenbrand (ARM) <[email protected]>

Acked-by Vlastimil Babka (SUSE) <[email protected]>

Reviewed-by Pedro Falcato <[email protected]>

Signed-off-by Lorenzo Stoakes (Oracle) <[email protected]>

Fixes 6c2da14ae1e0 ("mm/mseal: rework mseal apply logic")

Cc <[email protected]>

---