ext4: avoid race conditions when remounting with options that change dax

[ 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>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 830160a..f7c20bb 100644 (file)
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2034,6 +2034,16 @@ 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;
@@ -5367,12 +5377,6 @@ 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 "
@@ -5388,12 +5392,6 @@ 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");