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From:   Andreas Dilger <adilger@dilger.ca>
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Subject: Re: [PATCH] ext4: reject mount options not supported when remounting
 in handle_mount_opt()
Date:   Wed, 15 Apr 2020 12:12:39 -0600
In-Reply-To: <20200415174839.461347-1-tytso@mit.edu>
Cc:     Ext4 Developers List <linux-ext4@vger.kernel.org>,
        stable@kernel.org
To:     Theodore Ts'o <tytso@mit.edu>
References: <to=00000000000098a5d505a34d1e48@google.com>
 <20200415174839.461347-1-tytso@mit.edu>
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On Apr 15, 2020, at 11:48 AM, Theodore Ts'o <tytso@mit.edu> wrote:
>=20
> 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.
>=20
> Cc: stable@kernel.org
> Reported-and-tested-by: =
syzbot+bca9799bf129256190da@syzkaller.appspotmail.com
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

> ---
> fs/ext4/super.c | 37 +++++++++++--------------------------
> 1 file changed, 11 insertions(+), 26 deletions(-)
>=20
> 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
>=20
> 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}
> };
>=20
> @@ -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;
> +	}
>=20
> 	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 =3D gid;
> 	} else if (token =3D=3D Opt_journal_dev) {
> -		if (is_remount) {
> -			ext4_msg(sb, KERN_ERR,
> -				 "Cannot specify journal on remount");
> -			return -1;
> -		}
> 		*journal_devnum =3D arg;
> 	} else if (token =3D=3D 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;
>=20
> -		if (is_remount) {
> -			ext4_msg(sb, KERN_ERR,
> -				 "Cannot specify journal on remount");
> -			return -1;
> -		}
> 		journal_path =3D 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)
> 		}
> 	}
>=20
> -	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 =3D -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 ^=3D EXT4_MOUNT_DAX;
> -	}
> -
> 	if (sbi->s_mount_flags & EXT4_MF_FS_ABORTED)
> 		ext4_abort(sb, EXT4_ERR_ESHUTDOWN, "Abort forced by =
user");
>=20
> --
> 2.24.1
>=20


Cheers, Andreas


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