[PATCH] ext4: filter out EXT4_FC_REPLAY from on-disk superblock field s_state

["Theodore Ts'o" <tytso@mit.edu>]

The EXT4_FC_REPLAY bit in sbi->s_mount_state is used to indicate that
we are in the middle of replay the fast commit journal.  This was
actually a mistake, since the sbi->s_mount_info is initialized from
es->s_state.  Arguably s_mount_state is misleadingly named, but the
name is historical --- s_mount_state and s_state dates back to ext2.

What should have been used is the ext4_{set,clear,test}_mount_flag()
inline functions, which sets EXT4_MF_* bits in sbi->s_mount_flags.

The problem with using EXT4_FC_REPLAY is that a maliciously corrupted
superblock could result in EXT4_FC_REPLAY getting set in
s_mount_state.  This bypasses some sanity checks, and this can trigger
a BUG() in ext4_es_cache_extent().  As a easy-to-backport-fix, filter
out the EXT4_FC_REPLAY bit for now.  We should eventually transition
away from EXT4_FC_REPLAY to something like EXT4_MF_REPLAY.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20220420192312.1655305-1-phind.uet@gmail.com
Reported-by: syzbot+c7358a3cd05ee786eb31@syzkaller.appspotmail.com
---
 fs/ext4/super.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 4b0ea8df1f5c..f7ae53d986f1 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4889,7 +4889,7 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
 					sbi->s_inodes_per_block;
 	sbi->s_desc_per_block = blocksize / EXT4_DESC_SIZE(sb);
 	sbi->s_sbh = bh;
-	sbi->s_mount_state = le16_to_cpu(es->s_state);
+	sbi->s_mount_state = le16_to_cpu(es->s_state) & ~EXT4_FC_REPLAY;
 	sbi->s_addr_per_block_bits = ilog2(EXT4_ADDR_PER_BLOCK(sb));
 	sbi->s_desc_per_block_bits = ilog2(EXT4_DESC_PER_BLOCK(sb));
 
@@ -6452,7 +6452,8 @@ static int __ext4_remount(struct fs_context *fc, struct super_block *sb)
 				if (err)
 					goto restore_opts;
 			}
-			sbi->s_mount_state = le16_to_cpu(es->s_state);
+			sbi->s_mount_state = (le16_to_cpu(es->s_state) &
+					      ~EXT4_FC_REPLAY);
 
 			err = ext4_setup_super(sb, es, 0);
 			if (err)
-- 
2.31.0