[PATCH V2] fuse: Block access to folio overlimit

[Edward Adam Davis <eadavis@qq.com>]

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

When the number of bytes to be retrieved is truncated to the upper limit
by fc->max_pages and there is an offset, the oob is triggered.

Add a loop termination condition to prevent overruns.

Fixes: 3568a9569326 ("fuse: support large folios for retrieves")
Reported-by: syzbot+2d215d165f9354b9c4ea@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
Tested-by: syzbot+2d215d165f9354b9c4ea@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
---
V1 -> V2: update root cause

 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