linux-kernel - [PATCH] block: Fix iterating over an empty bio with bio_for_each_folio

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Message-Id: <20240116212959.3413014-1-willy@infradead.org>
Date: Tue, 16 Jan 2024 21:29:59 +0000
From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
	linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	syzbot+8b23309d5788a79d3eea@...kaller.appspotmail.com,
	syzbot+004c1e0fced2b4bc3dcc@...kaller.appspotmail.com,
	stable@...r.kernel.org
Subject: [PATCH] block: Fix iterating over an empty bio with bio_for_each_folio_all

If the bio contains no data, bio_first_folio() calls page_folio() on a
NULL pointer and oopses.  Move the test that we've reached the end of
the bio from bio_next_folio() to bio_first_folio().

Reported-by: syzbot+8b23309d5788a79d3eea@...kaller.appspotmail.com
Reported-by: syzbot+004c1e0fced2b4bc3dcc@...kaller.appspotmail.com
Fixes: 640d1930bef4 ("block: Add bio_for_each_folio_all()")
Cc: stable@...r.kernel.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 include/linux/bio.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/bio.h b/include/linux/bio.h
index ec4db73e5f4e..1518f1201ddd 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -286,6 +286,11 @@ static inline void bio_first_folio(struct folio_iter *fi, struct bio *bio,
 {
 	struct bio_vec *bvec = bio_first_bvec_all(bio) + i;
 
+	if (i >= bio->bi_vcnt) {
+		fi->folio = NULL;
+		return;
+	}
+
 	fi->folio = page_folio(bvec->bv_page);
 	fi->offset = bvec->bv_offset +
 			PAGE_SIZE * (bvec->bv_page - &fi->folio->page);
@@ -303,10 +308,8 @@ static inline void bio_next_folio(struct folio_iter *fi, struct bio *bio)
 		fi->offset = 0;
 		fi->length = min(folio_size(fi->folio), fi->_seg_count);
 		fi->_next = folio_next(fi->folio);
-	} else if (fi->_i + 1 < bio->bi_vcnt) {
-		bio_first_folio(fi, bio, fi->_i + 1);
 	} else {
-		fi->folio = NULL;
+		bio_first_folio(fi, bio, fi->_i + 1);
 	}
 }
 
-- 
2.43.0