[PATCH] block: add lock for safe nrpages access in invalidate_bdev()

[Yuan Tan <tanyuan@tinylab.org>]

Syzbot reported a data-race in __filemap_add_folio / invalidate_bdev[1]
due to concurrent access to mapping->nrpages.
Adds a lock around the access to nrpages.

[1] https://syzkaller.appspot.com/bug?extid=f2aaf773187f5cae54f3

Signed-off-by: Yuan Tan <tanyuan@tinylab.org>
Reported-by: syzbot+f2aaf773187f5cae54f3@syzkaller.appspotmail.com
---
 block/bdev.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

I had already completed and tested this patch before Matthew sent the
email. I'm not sure if this solution is correct. If it's not, please
ignore the patch :)

diff --git a/block/bdev.c b/block/bdev.c
index 9d73a8fbf7f9..934043d09068 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -96,7 +96,14 @@ void invalidate_bdev(struct block_device *bdev)
 {
 	struct address_space *mapping = bdev->bd_mapping;
 
-	if (mapping->nrpages) {
+	XA_STATE(xas, &mapping->i_pages, 0);  /* we don't care about the index */
+	unsigned long nrpages;
+
+	xas_lock_irq(&xas);
+	nrpages = mapping->nrpages;
+	xas_unlock_irq(&xas);
+
+	if (nrpages) {
 		invalidate_bh_lrus();
 		lru_add_drain_all();	/* make sure all lru add caches are flushed */
 		invalidate_mapping_pages(mapping, 0, -1);
-- 
2.25.1