mm-unstable commit edd33b8807a1 ("mempolicy: migration attempt to match
interleave nodes") added a second vma_iter search to do_mbind(), to
determine the interleave index to be used in the MPOL_INTERLEAVE case.
But sadly it added it just after the mmap_write_unlock(), leaving this
new VMA search unprotected: and so syzbot reports suspicious RCU usage
from lib/maple_tree.c:856.
This could be fixed with an rcu_read_lock/unlock() pair (per Liam);
but since we have been relying on the mmap_lock up to this point, it's
slightly better to extend it over the new search too, for a well-defined
result consistent with the policy this mbind() is establishing (rather
than whatever might follow once the mmap_lock is dropped).
Reported-by: syzbot+79fcba037b6df73756d3@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/linux-mm/000000000000c05f1b0608657fde@xxxxxxxxxx/
Fixes: edd33b8807a1 ("mempolicy: migration attempt to match interleave nodes")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
mm/mempolicy.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 989293180eb6..5e472e6e0507 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1291,8 +1291,6 @@ static long do_mbind(unsigned long start, unsigned long len,
}
}
- mmap_write_unlock(mm);
-
if (!err && !list_empty(&pagelist)) {
/* Convert MPOL_DEFAULT's NULL to task or default policy */
if (!new) {
@@ -1334,7 +1332,11 @@ static long do_mbind(unsigned long start, unsigned long len,
mmpol.ilx -= page->index >> order;
}
}
+ }
+ mmap_write_unlock(mm);
+
+ if (!err && !list_empty(&pagelist)) {
nr_failed |= migrate_pages(&pagelist,
alloc_migration_target_by_mpol, NULL,
(unsigned long)&mmpol, MIGRATE_SYNC,
--
2.35.3
