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    Subject[PATCH 4.19 179/206] ext4: avoid race conditions when remounting with options that change dax
    Date
    From: Theodore Ts'o <tytso@mit.edu>

    [ Upstream commit 829b37b8cddb1db75c1b7905505b90e593b15db1 ]

    Trying to change dax mount options when remounting could allow mount
    options to 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@kernel.org
    Reported-by: syzbot+bca9799bf129256190da@syzkaller.appspotmail.com
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/ext4/super.c | 22 ++++++++++------------
    1 file changed, 10 insertions(+), 12 deletions(-)

    diff --git a/fs/ext4/super.c b/fs/ext4/super.c
    index affccf55294e9..1428dab2afff8 100644
    --- a/fs/ext4/super.c
    +++ b/fs/ext4/super.c
    @@ -1989,6 +1989,16 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
    #endif
    } else if (token == Opt_dax) {
    #ifdef CONFIG_FS_DAX
    + if (is_remount && test_opt(sb, DAX)) {
    + ext4_msg(sb, KERN_ERR, "can't mount with "
    + "both data=journal and dax");
    + return -1;
    + }
    + if (is_remount && !(sbi->s_mount_opt & EXT4_MOUNT_DAX)) {
    + ext4_msg(sb, KERN_ERR, "can't change "
    + "dax mount option while remounting");
    + return -1;
    + }
    ext4_msg(sb, KERN_WARNING,
    "DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
    sbi->s_mount_opt |= m->mount_opt;
    @@ -5274,12 +5284,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
    err = -EINVAL;
    goto restore_opts;
    }
    - if (test_opt(sb, DAX)) {
    - ext4_msg(sb, KERN_ERR, "can't mount with "
    - "both data=journal and dax");
    - err = -EINVAL;
    - goto restore_opts;
    - }
    } else if (test_opt(sb, DATA_FLAGS) == EXT4_MOUNT_ORDERED_DATA) {
    if (test_opt(sb, JOURNAL_ASYNC_COMMIT)) {
    ext4_msg(sb, KERN_ERR, "can't mount with "
    @@ -5295,12 +5299,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
    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, "Abort forced by user");

    --
    2.25.1


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