[PATCH AUTOSEL 4.19 5/6] Squashfs: sanity check symbolic link size

[Sasha Levin]

From: Phillip Lougher <phillip@xxxxxxxxxxxxxxx>

[ Upstream commit 810ee43d9cd245d138a2733d87a24858a23f577d ]

Syzkiller reports a "KMSAN: uninit-value in pick_link" bug.

This is caused by an uninitialised page, which is ultimately caused
by a corrupted symbolic link size read from disk.

The reason why the corrupted symlink size causes an uninitialised
page is due to the following sequence of events:

1. squashfs_read_inode() is called to read the symbolic
   link from disk.  This assigns the corrupted value
   3875536935 to inode->i_size.

2. Later squashfs_symlink_read_folio() is called, which assigns
   this corrupted value to the length variable, which being a
   signed int, overflows producing a negative number.

3. The following loop that fills in the page contents checks that
   the copied bytes is less than length, which being negative means
   the loop is skipped, producing an uninitialised page.

This patch adds a sanity check which checks that the symbolic
link size is not larger than expected.

--

Signed-off-by: Phillip Lougher <phillip@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240811232821.13903-1-phillip@xxxxxxxxxxxxxxx
Reported-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
Reported-by: syzbot+24ac24ff58dc5b0d26b9@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/000000000000a90e8c061e86a76b@xxxxxxxxxx/
V2: fix spelling mistake.
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/squashfs/inode.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/squashfs/inode.c b/fs/squashfs/inode.c
index e9793b1e49a56..89ac1c6de97bf 100644
--- a/fs/squashfs/inode.c
+++ b/fs/squashfs/inode.c
@@ -289,8 +289,13 @@ int squashfs_read_inode(struct inode *inode, long long ino)
 		if (err < 0)
 			goto failed_read;
 
-		set_nlink(inode, le32_to_cpu(sqsh_ino->nlink));
 		inode->i_size = le32_to_cpu(sqsh_ino->symlink_size);
+		if (inode->i_size > PAGE_SIZE) {
+			ERROR("Corrupted symlink\n");
+			return -EINVAL;
+		}
+
+		set_nlink(inode, le32_to_cpu(sqsh_ino->nlink));
 		inode->i_op = &squashfs_symlink_inode_ops;
 		inode_nohighmem(inode);
 		inode->i_data.a_ops = &squashfs_symlink_aops;
-- 
2.43.0