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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/20260327173104.322405-1-ljs@kernel.org
Fixes: 6c2da14ae1e0 ("mm/mseal: rework mseal apply logic")
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
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>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jeff Xu <jeffxu@chromium.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

authored by

Lorenzo Stoakes (Oracle) and committed by
Andrew Morton 5ac9c7c2 d2fd4225

+1 -2
+1 -2
mm/mseal.c
··· 56 56 unsigned long start, unsigned long end) 57 57 { 58 58 struct vm_area_struct *vma, *prev; 59 - unsigned long curr_start = start; 60 59 VMA_ITERATOR(vmi, mm, start); 61 60 62 61 /* We know there are no gaps so this will be non-NULL. */ ··· 65 66 prev = vma; 66 67 67 68 for_each_vma_range(vmi, vma, end) { 69 + const unsigned long curr_start = MAX(vma->vm_start, start); 68 70 const unsigned long curr_end = MIN(vma->vm_end, end); 69 71 70 72 if (!(vma->vm_flags & VM_SEALED)) { ··· 79 79 } 80 80 81 81 prev = vma; 82 - curr_start = curr_end; 83 82 } 84 83 85 84 return 0;