[PATCH] mempolicy: migration attempt to match interleave nodes: fix

[Hugh Dickins <hughd@google.com>]

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@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-mm/000000000000c05f1b0608657fde@google.com/
Fixes: edd33b8807a1 ("mempolicy: migration attempt to match interleave nodes")
Signed-off-by: Hugh Dickins <hughd@google.com>
---
 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