[PATCH 6.18.y] mm/mseal: update VMA end correctly on merge

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

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 <antonius@bluedragonsec.com>
Closes: https://lore.kernel.org/linux-mm/CAK8a0jwWGj9-SgFk0yKFh7i8jMkwKm5b0ao9=kmXWjO54veX2g@mail.gmail.com/
Suggested-by: David Hildenbrand (ARM) <david@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Fixes: 6c2da14ae1e0 ("mm/mseal: rework mseal apply logic")
Cc: <stable@vger.kernel.org>
(cherry picked from commit 88995f43fdc2045ff0b030ca054898483004de36)
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
---
 mm/mseal.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/mseal.c b/mm/mseal.c
index e5b205562d2e..c561f0ea93e8 100644
--- a/mm/mseal.c
+++ b/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. */
@@ -66,7 +65,8 @@ static int mseal_apply(struct mm_struct *mm,
 		prev = vma;
 
 	for_each_vma_range(vmi, vma, end) {
-		unsigned long curr_end = MIN(vma->vm_end, 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)) {
 			vma = vma_modify_flags(&vmi, prev, vma,
@@ -78,7 +78,6 @@ static int mseal_apply(struct mm_struct *mm,
 		}
 
 		prev = vma;
-		curr_start = curr_end;
 	}
 
 	return 0;
-- 
2.53.0