linux-kernel - [PATCH] fuse: Block access to folio overlimit

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Message-ID: <tencent_F0CF4B761BAA2549BAA0BB1E33D09E561B08@qq.com>
Date: Sat, 23 Aug 2025 09:17:13 +0800
From: Edward Adam Davis <eadavis@...com>
To: syzbot+2d215d165f9354b9c4ea@...kaller.appspotmail.com
Cc: linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	miklos@...redi.hu,
	syzkaller-bugs@...glegroups.com
Subject: [PATCH] fuse: Block access to folio overlimit

syz reported a slab-out-of-bounds Write in fuse_dev_do_write.

Using the number of bytes alone as the termination condition in a loop
can prematurely exhaust the allocated memory if the incremented byte count
is less than PAGE_SIZE.

Add a loop termination condition to prevent overruns.

Fixes: 3568a9569326 ("fuse: support large folios for retrieves")
Reported-by: syzbot+2d215d165f9354b9c4ea@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
Tested-by: syzbot+2d215d165f9354b9c4ea@...kaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@...com>
---
 fs/fuse/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index e80cd8f2c049..5150aa25e64b 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1893,7 +1893,7 @@ static int fuse_retrieve(struct fuse_mount *fm, struct inode *inode,
 
 	index = outarg->offset >> PAGE_SHIFT;
 
-	while (num) {
+	while (num && ap->num_folios < num_pages) {
 		struct folio *folio;
 		unsigned int folio_offset;
 		unsigned int nr_bytes;
-- 
2.43.0