Squashfs: sanity check symbolic link size

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.ukReported-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>

fs/squashfs/inode.c

@@ -279,8 +279,13 @@ 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;