[PATCH v2] mm/ksm: Fix race with VMA iteration and mm_struct teardown

[Liam R. Howlett]

exit_mmap() will tear down the VMAs and maple tree with the mmap_lock
held in write mode.  Ensure that the maple tree is still valid by
checking ksm_test_exit() after taking the mmap_lock in read mode, but
before the for_each_vma() iterator dereferences a destroyed maple tree.

Since the maple tree is destroyed, the flags telling lockdep to check an
external lock has been cleared.  Skip the for_each_vma() iterator to
avoid dereferencing a maple tree without the external lock flag, which
would create a lockdep warning.

Reported-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Link: https://lore.kernel.org/lkml/ZAdUUhSbaa6fHS36@xxxxxxxxxxxxxxxx/
Reported-by: syzbot+2ee18845e89ae76342c5@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://syzkaller.appspot.com/bug?id=64a3e95957cd3deab99df7cd7b5a9475af92c93e
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: heng.su@xxxxxxxxx
Cc: lkp@xxxxxxxxx
Cc: <Stable@xxxxxxxxxxxxxxx>
Fixes: a5f18ba07276 ("mm/ksm: use vma iterators instead of vma linked list")
Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
---

Andrew,

I've included this note because I've changed the subject between v1 and
v2.   This is for mm-hotfixes-unstable.

Replaces: b6d4338e8dd2 ("mm/ksm: fix race with ksm_exit() in VMA
iteration")

v1: https://lore.kernel.org/linux-mm/20230307205951.2465275-1-Liam.Howlett@xxxxxxxxxx/

Changes since v1:
 - Commit log & subject.
 - Added Acked-by


 mm/ksm.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 525c3306e78b..d6280b258ece 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1044,9 +1044,15 @@ static int unmerge_and_remove_all_rmap_items(void)
 
 		mm = mm_slot->slot.mm;
 		mmap_read_lock(mm);
+
+		/*
+		 * Exit right away if mm is exiting to avoid lockdep issue in
+		 * the maple tree
+		 */
+		if (ksm_test_exit(mm))
+			goto mm_exiting;
+
 		for_each_vma(vmi, vma) {
-			if (ksm_test_exit(mm))
-				break;
 			if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
 				continue;
 			err = unmerge_ksm_pages(vma,
@@ -1055,6 +1061,7 @@ static int unmerge_and_remove_all_rmap_items(void)
 				goto error;
 		}
 
+mm_exiting:
 		remove_trailing_rmap_items(&mm_slot->rmap_list);
 		mmap_read_unlock(mm);
 
-- 
2.39.2