jbd2: add fast_commit space check

If JBD2_FEATURE_INCOMPAT_FAST_COMMIT bit is set, it means the journal
have fast commit records need to recover, so the fast commit size
should not be too large, and the leftover normal journal size should
never less than JBD2_MIN_JOURNAL_BLOCKS. If it happens, the
journal->j_last is likely to be wrong and will probably lead to
incorrect journal recovery. So add a check into the
journal_check_superblock(), and drop the pointless check when
initializing the fastcommit parameters.
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230811063610.2980059-8-yi.zhang@huaweicloud.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/jbd2/journal.c

@@ -1347,6 +1347,7 @@ static void journal_fail_superblock(journal_t *journal)
 static int journal_check_superblock(journal_t *journal)
 {
 	journal_superblock_t *sb = journal->j_superblock;
+	int num_fc_blks;
 	int err = -EINVAL;
 
 	if (sb->s_header.h_magic != cpu_to_be32(JBD2_MAGIC_NUMBER) ||
@@ -1389,6 +1390,15 @@ static int journal_check_superblock(journal_t *journal)
 		return err;
 	}
 
+	num_fc_blks = jbd2_has_feature_fast_commit(journal) ?
+				jbd2_journal_get_num_fc_blks(sb) : 0;
+	if (be32_to_cpu(sb->s_maxlen) < JBD2_MIN_JOURNAL_BLOCKS ||
+	    be32_to_cpu(sb->s_maxlen) - JBD2_MIN_JOURNAL_BLOCKS < num_fc_blks) {
+		printk(KERN_ERR "JBD2: journal file too short %u,%d\n",
+		       be32_to_cpu(sb->s_maxlen), num_fc_blks);
+		return err;
+	}
+
 	if (jbd2_has_feature_csum2(journal) &&
 	    jbd2_has_feature_csum3(journal)) {
 		/* Can't have checksum v2 and v3 at the same time! */
@@ -1454,7 +1464,6 @@ static int journal_load_superblock(journal_t *journal)
 	int err;
 	struct buffer_head *bh;
 	journal_superblock_t *sb;
-	int num_fc_blocks;
 
 	bh = getblk_unmovable(journal->j_dev, journal->j_blk_offset,
 			      journal->j_blocksize);
@@ -1492,9 +1501,8 @@ static int journal_load_superblock(journal_t *journal)
 
 	if (jbd2_has_feature_fast_commit(journal)) {
 		journal->j_fc_last = be32_to_cpu(sb->s_maxlen);
-		num_fc_blocks = jbd2_journal_get_num_fc_blks(sb);
-		if (journal->j_last - num_fc_blocks >= JBD2_MIN_JOURNAL_BLOCKS)
-			journal->j_last = journal->j_fc_last - num_fc_blocks;
+		journal->j_last = journal->j_fc_last -
+				  jbd2_journal_get_num_fc_blks(sb);
 		journal->j_fc_first = journal->j_last + 1;
 		journal->j_fc_off = 0;
 	}