[PATCH] ext4: reject mount options not supported when remounting in handle_mount_opt()

["Theodore Ts'o" <tytso@xxxxxxx>]

Rejecting the mount options in ext4_remount() means that some mount
options would be enabled for a small amount of time, and then the
mount option change would be reverted.  In the case of "mount -o
remount,dax", this can cause a race where files would temporarily
treated as DAX --- and then not.

Cc: stable@xxxxxxxxxx
Reported-and-tested-by: syzbot+bca9799bf129256190da@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
---
 fs/ext4/super.c | 37 +++++++++++--------------------------
 1 file changed, 11 insertions(+), 26 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index bf5fcb477f66..6fe32f9aa889 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1726,6 +1726,7 @@ static int clear_qf_name(struct super_block *sb, int qtype)
 #define MOPT_NO_EXT3	0x0200
 #define MOPT_EXT4_ONLY	(MOPT_NO_EXT2 | MOPT_NO_EXT3)
 #define MOPT_STRING	0x0400
+#define MOPT_NO_REMOUNT	0x0800
 
 static const struct mount_opts {
 	int	token;
@@ -1775,12 +1776,12 @@ static const struct mount_opts {
 	{Opt_min_batch_time, 0, MOPT_GTE0},
 	{Opt_inode_readahead_blks, 0, MOPT_GTE0},
 	{Opt_init_itable, 0, MOPT_GTE0},
-	{Opt_dax, EXT4_MOUNT_DAX, MOPT_SET},
+	{Opt_dax, EXT4_MOUNT_DAX, MOPT_SET | MOPT_NO_REMOUNT},
 	{Opt_stripe, 0, MOPT_GTE0},
 	{Opt_resuid, 0, MOPT_GTE0},
 	{Opt_resgid, 0, MOPT_GTE0},
-	{Opt_journal_dev, 0, MOPT_NO_EXT2 | MOPT_GTE0},
-	{Opt_journal_path, 0, MOPT_NO_EXT2 | MOPT_STRING},
+	{Opt_journal_dev, 0, MOPT_NO_EXT2 | MOPT_GTE0 | MOPT_NO_REMOUNT},
+	{Opt_journal_path, 0, MOPT_NO_EXT2 | MOPT_STRING | MOPT_NO_REMOUNT},
 	{Opt_journal_ioprio, 0, MOPT_NO_EXT2 | MOPT_GTE0},
 	{Opt_data_journal, EXT4_MOUNT_JOURNAL_DATA, MOPT_NO_EXT2 | MOPT_DATAJ},
 	{Opt_data_ordered, EXT4_MOUNT_ORDERED_DATA, MOPT_NO_EXT2 | MOPT_DATAJ},
@@ -1817,7 +1818,7 @@ static const struct mount_opts {
 	{Opt_jqfmt_vfsv1, QFMT_VFS_V1, MOPT_QFMT},
 	{Opt_max_dir_size_kb, 0, MOPT_GTE0},
 	{Opt_test_dummy_encryption, 0, MOPT_GTE0},
-	{Opt_nombcache, EXT4_MOUNT_NO_MBCACHE, MOPT_SET},
+	{Opt_nombcache, EXT4_MOUNT_NO_MBCACHE, MOPT_SET | MOPT_NO_REMOUNT},
 	{Opt_err, 0, 0}
 };
 
@@ -1915,6 +1916,12 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
 			 "Mount option \"%s\" incompatible with ext3", opt);
 		return -1;
 	}
+	if ((m->flags & MOPT_NO_REMOUNT) && is_remount) {
+		ext4_msg(sb, KERN_ERR,
+			 "Mount option \"%s\" not supported when remounting",
+			 opt);
+		return -1;
+	}
 
 	if (args->from && !(m->flags & MOPT_STRING) && match_int(args, &arg))
 		return -1;
@@ -1994,11 +2001,6 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
 		}
 		sbi->s_resgid = gid;
 	} else if (token == Opt_journal_dev) {
-		if (is_remount) {
-			ext4_msg(sb, KERN_ERR,
-				 "Cannot specify journal on remount");
-			return -1;
-		}
 		*journal_devnum = arg;
 	} else if (token == Opt_journal_path) {
 		char *journal_path;
@@ -2006,11 +2008,6 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
 		struct path path;
 		int error;
 
-		if (is_remount) {
-			ext4_msg(sb, KERN_ERR,
-				 "Cannot specify journal on remount");
-			return -1;
-		}
 		journal_path = match_strdup(&args[0]);
 		if (!journal_path) {
 			ext4_msg(sb, KERN_ERR, "error: could not dup "
@@ -5427,18 +5424,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 		}
 	}
 
-	if ((sbi->s_mount_opt ^ old_opts.s_mount_opt) & EXT4_MOUNT_NO_MBCACHE) {
-		ext4_msg(sb, KERN_ERR, "can't enable nombcache during remount");
-		err = -EINVAL;
-		goto restore_opts;
-	}
-
-	if ((sbi->s_mount_opt ^ old_opts.s_mount_opt) & EXT4_MOUNT_DAX) {
-		ext4_msg(sb, KERN_WARNING, "warning: refusing change of "
-			"dax flag with busy inodes while remounting");
-		sbi->s_mount_opt ^= EXT4_MOUNT_DAX;
-	}
-
 	if (sbi->s_mount_flags & EXT4_MF_FS_ABORTED)
 		ext4_abort(sb, EXT4_ERR_ESHUTDOWN, "Abort forced by user");
 
-- 
2.24.1