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: syzbot+bca9799bf129256190da@syzkaller.appspotmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index a22d67c5bc00bedd8344bf4dd71db56b5aff0c13..edf06c1bee9d40116ef7f43796a21d2a2a230467 100644 (file)
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -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");