ufs: validate maximum fast symlink size from superblock

The maximum fast symlink size is set in the superblock of certain types
of UFS filesystem. Before using it we need to check that it isn't longer
than the available space we have in the inode.
Signed-off-by: Duane Griffin <duaneg@dghda.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ufs/super.c

@@ -636,6 +636,7 @@ static int ufs_fill_super(struct super_block *sb, void *data, int silent)
 	unsigned block_size, super_block_size;
 	unsigned flags;
 	unsigned super_block_offset;
+	unsigned maxsymlen;
 	int ret = -EINVAL;
 
 	uspi = NULL;
@@ -1069,6 +1070,16 @@ static int ufs_fill_super(struct super_block *sb, void *data, int silent)
 		uspi->s_maxsymlinklen =
 		    fs32_to_cpu(sb, usb3->fs_un2.fs_44.fs_maxsymlinklen);
 
+	if (uspi->fs_magic == UFS2_MAGIC)
+		maxsymlen = 2 * 4 * (UFS_NDADDR + UFS_NINDIR);
+	else
+		maxsymlen = 4 * (UFS_NDADDR + UFS_NINDIR);
+	if (uspi->s_maxsymlinklen > maxsymlen) {
+		ufs_warning(sb, __func__, "ufs_read_super: excessive maximum "
+			    "fast symlink size (%u)\n", uspi->s_maxsymlinklen);
+		uspi->s_maxsymlinklen = maxsymlen;
+	}
+
 	inode = ufs_iget(sb, UFS_ROOTINO);
 	if (IS_ERR(inode)) {
 		ret = PTR_ERR(inode);