udf: Prevent write-unsupported filesystem to be remounted read-write

There are certain filesystem features which we support for reading but
not for writing. We properly refuse to mount such filesystems read-write
however for some features (such as read-only partitions), we don't check
for these features when remounting the filesystem from read-only to
read-write. Thus such filesystems could be remounted read-write leading
to strange behavior (most likely crashes).

Fix the problem by marking in superblock whether the filesystem has some
features that are supported in read-only mode and check this flag during
remount.
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/super.c

@@ -613,14 +613,11 @@ static int udf_remount_fs(struct super_block *sb, int *flags, char *options)
 	struct udf_options uopt;
 	struct udf_sb_info *sbi = UDF_SB(sb);
 	int error = 0;
-	struct logicalVolIntegrityDescImpUse *lvidiu = udf_sb_lvidiu(sb);
+
+	if (!(*flags & SB_RDONLY) && UDF_QUERY_FLAG(sb, UDF_FLAG_RW_INCOMPAT))
+		return -EACCES;
 
 	sync_filesystem(sb);
-	if (lvidiu) {
-		int write_rev = le16_to_cpu(lvidiu->minUDFWriteRev);
-		if (write_rev > UDF_MAX_WRITE_VERSION && !(*flags & SB_RDONLY))
-			return -EACCES;
-	}
 
 	uopt.flags = sbi->s_flags;
 	uopt.uid   = sbi->s_uid;
@@ -1257,6 +1254,7 @@ static int udf_load_partdesc(struct super_block *sb, sector_t block)
 			ret = -EACCES;
 			goto out_bh;
 		}
+		UDF_SET_FLAG(sb, UDF_FLAG_RW_INCOMPAT);
 		ret = udf_load_vat(sb, i, type1_idx);
 		if (ret < 0)
 			goto out_bh;
@@ -2155,10 +2153,12 @@ static int udf_fill_super(struct super_block *sb, void *options, int silent)
 				UDF_MAX_READ_VERSION);
 			ret = -EINVAL;
 			goto error_out;
-		} else if (minUDFWriteRev > UDF_MAX_WRITE_VERSION &&
-			   !sb_rdonly(sb)) {
-			ret = -EACCES;
-			goto error_out;
+		} else if (minUDFWriteRev > UDF_MAX_WRITE_VERSION) {
+			if (!sb_rdonly(sb)) {
+				ret = -EACCES;
+				goto error_out;
+			}
+			UDF_SET_FLAG(sb, UDF_FLAG_RW_INCOMPAT);
 		}
 
 		sbi->s_udfrev = minUDFWriteRev;
@@ -2176,10 +2176,12 @@ static int udf_fill_super(struct super_block *sb, void *options, int silent)
 	}
 
 	if (sbi->s_partmaps[sbi->s_partition].s_partition_flags &
-			UDF_PART_FLAG_READ_ONLY &&
-	    !sb_rdonly(sb)) {
-		ret = -EACCES;
-		goto error_out;
+			UDF_PART_FLAG_READ_ONLY) {
+		if (!sb_rdonly(sb)) {
+			ret = -EACCES;
+			goto error_out;
+		}
+		UDF_SET_FLAG(sb, UDF_FLAG_RW_INCOMPAT);
 	}
 
 	if (udf_find_fileset(sb, &fileset, &rootdir)) {

fs/udf/udf_sb.h

@@ -30,6 +30,8 @@
 #define UDF_FLAG_LASTBLOCK_SET	16
 #define UDF_FLAG_BLOCKSIZE_SET	17
 #define UDF_FLAG_INCONSISTENT	18
+#define UDF_FLAG_RW_INCOMPAT	19	/* Set when we find RW incompatible
+					 * feature */
 
 #define UDF_PART_FLAG_UNALLOC_BITMAP	0x0001
 #define UDF_PART_FLAG_UNALLOC_TABLE	0x0002