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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 6c2da14 ("mm/mseal: rework mseal apply logic")
Signed-off-by: Lorenzo Stoakes (Oracle) <[email protected]>
Reported-by: Antonius <[email protected]>
Closes: https://lore.kernel.org/linux-mm/CAK8a0jwWGj9-SgFk0yKFh7i8jMkwKm5b0ao9=kmXWjO54veX2g@mail.gmail.com/
Suggested-by: David Hildenbrand (ARM) <[email protected]>
Acked-by: Vlastimil Babka (SUSE) <[email protected]>
Reviewed-by: Pedro Falcato <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Jeff Xu <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Lorenzo Stoakes (Oracle)

mm/mseal.c

@@ -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;