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Subject[PATCH v3 10/13 fix] mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock(): fix
madvise_collapse() setting "mmap_locked = true" after calling
collapse_pte_mapped_thp() looked good but was wrong. If the loop then
moves on to the next extent, mmap_locked assures it that "vma" has been
revalidated under mmap_lock, which was not the case: and led to UAFs,
crashes in __fput() or task_work_run(), even collapse_file()'s
VM_BUG_ON(start & (HPAGE_PMD_NR - 1)) - all detected by syzbot.

(collapse_pte_mapped_thp() does validate the vma that it works on:
but it's not passed in as an argument, collapse_pte_mapped_thp() finds
the vma for mm and addr by itself - which may by this time have changed
from the vma saved in madvise_collapse().)

Reported-by: syzbot+fe7b1487405295d29268@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/000000000000f9de430600ae05db@google.com/
Reported-by: syzbot+173cc8cfdfbbef6dd755@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-mm/000000000000e4b0f0060123ca40@google.com/
Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/khugepaged.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6bad69c0e4bd..1c773db26e88 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2747,7 +2747,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
BUG_ON(*prev);
mmap_read_lock(mm);
result = collapse_pte_mapped_thp(mm, addr, true);
- mmap_locked = true;
+ mmap_read_unlock(mm);
goto handle_result;
/* Whitelisted set of results where continuing OK */
case SCAN_PMD_NULL:
--
2.35.3
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