[PATCH AUTOSEL 5.4 5/7] Squashfs: sanity check symbolic link size

Sasha Levin
2024-10-12T11:30:01Z

From: Phillip Lougher <phillip@squashfs.org.uk>

[ 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@squashfs.org.uk>
Link: https://lore.kernel.org/r/20240811232821.13903-1-phillip@squashfs.org.uk
Reported-by: Lizhi Xu <lizhi.xu@windriver.com>
Reported-by: syzbot+24ac24ff58dc5b0d26b9@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000a90e8c061e86a76b@google.com/
V2: fix spelling mistake.
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 24463145b3513..f31649080a881 100644
--- a/fs/squashfs/inode.c
+++ b/fs/squashfs/inode.c
@@ -276,8 +276,13 @@ 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