Commit 829b37b8 authored by Theodore Ts'o's avatar Theodore Ts'o
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ext4: avoid race conditions when remounting with options that change dax 



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: default avatar <syzbot+bca9799bf129256190da@syzkaller.appspotmail.com>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 68cd4492

fs/ext4/super.c

+24 −18
Original line number Diff line number Diff line
@@ -2104,16 +2104,40 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token, 
		switch (token) {

		case Opt_dax:

		case Opt_dax_always:

			if (is_remount &&

			    (!(sbi->s_mount_opt & EXT4_MOUNT_DAX_ALWAYS) ||

			     (sbi->s_mount_opt2 & EXT4_MOUNT2_DAX_NEVER))) {

			fail_dax_change_remount:

				ext4_msg(sb, KERN_ERR, "can't change "

					 "dax mount option while remounting");

				return -1;

			}

			if (is_remount &&

			    (test_opt(sb, DATA_FLAGS) ==

			     EXT4_MOUNT_JOURNAL_DATA)) {

				    ext4_msg(sb, KERN_ERR, "can't mount with "

					     "both data=journal and dax");

				    return -1;

			}

			ext4_msg(sb, KERN_WARNING,

				"DAX enabled. Warning: EXPERIMENTAL, use at your own risk");

			sbi->s_mount_opt |= EXT4_MOUNT_DAX_ALWAYS;

			sbi->s_mount_opt2 &= ~EXT4_MOUNT2_DAX_NEVER;

			break;

		case Opt_dax_never:

			if (is_remount &&

			    (!(sbi->s_mount_opt2 & EXT4_MOUNT2_DAX_NEVER) ||

			     (sbi->s_mount_opt & EXT4_MOUNT_DAX_ALWAYS)))

				goto fail_dax_change_remount;

			sbi->s_mount_opt2 |= EXT4_MOUNT2_DAX_NEVER;

			sbi->s_mount_opt &= ~EXT4_MOUNT_DAX_ALWAYS;

			break;

		case Opt_dax_inode:

			if (is_remount &&

			    ((sbi->s_mount_opt & EXT4_MOUNT_DAX_ALWAYS) ||

			     (sbi->s_mount_opt2 & EXT4_MOUNT2_DAX_NEVER) ||

			     !(sbi->s_mount_opt2 & EXT4_MOUNT2_DAX_INODE)))

				goto fail_dax_change_remount;

			sbi->s_mount_opt &= ~EXT4_MOUNT_DAX_ALWAYS;

			sbi->s_mount_opt2 &= ~EXT4_MOUNT2_DAX_NEVER;

			/* Strictly for printing options */
@@ -5454,12 +5478,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data) 
			err = -EINVAL;

			goto restore_opts;

		}

		if (test_opt(sb, DAX_ALWAYS)) {

			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 "
@@ -5475,18 +5493,6 @@ 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_ALWAYS ||

	    (sbi->s_mount_opt2 ^ old_opts.s_mount_opt2) & EXT4_MOUNT2_DAX_NEVER ||

	    (sbi->s_mount_opt2 ^ old_opts.s_mount_opt2) & EXT4_MOUNT2_DAX_INODE) {

		ext4_msg(sb, KERN_WARNING, "warning: refusing change of "

			"dax mount option with busy inodes while remounting");

		sbi->s_mount_opt &= ~EXT4_MOUNT_DAX_ALWAYS;

		sbi->s_mount_opt |= old_opts.s_mount_opt & EXT4_MOUNT_DAX_ALWAYS;

		sbi->s_mount_opt2 &= ~(EXT4_MOUNT2_DAX_NEVER | EXT4_MOUNT2_DAX_INODE);

		sbi->s_mount_opt2 |= old_opts.s_mount_opt2 &

				     (EXT4_MOUNT2_DAX_NEVER | EXT4_MOUNT2_DAX_INODE);

	}



	if (sbi->s_mount_flags & EXT4_MF_FS_ABORTED)

		ext4_abort(sb, EXT4_ERR_ESHUTDOWN, "Abort forced by user");

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