[PATCH v2 04/21] buffer: read the folio's mapping directly in buffer_set_crypto_ctx()

folio_mapping() was doing two jobs here.  One was to turn a slab folio into
NULL, which is what made this safe for jbd2's shadow buffers; the previous
patch removed the need for that by giving those buffers no folio at all.

The other is a hazard.  folio_mapping() maps a folio in the swap cache to
its swap_address_space, so if a buffer_head were ever attached to such a
folio this would hand fscrypt a swap mapping and dereference ->host on it.
There is no reason to want that here: this path wants the file's mapping or
nothing.

Read ->mapping directly.  Buffers with no folio are already handled above.

Suggested-by: Matthew Wilcox (Oracle) <[email protected]>
Acked-by: Weidong Zhu <[email protected]>
Signed-off-by: Chao Shi <[email protected]>
---
 fs/buffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 04fcc34e4fa6..2851830995d8 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1108,7 +1108,7 @@ static void buffer_set_crypto_ctx(struct bio *bio, const struct buffer_head *bh,
 	 */
 	if (!bh->b_folio)
 		return;
-	mapping = folio_mapping(bh->b_folio);
+	mapping = bh->b_folio->mapping;
 	if (!mapping)
 		return;
 	fscrypt_set_bio_crypt_ctx(bio, mapping->host,
-- 
2.43.0