On Mon, Jun 22, 2020 at 07:50:56PM +0200, gregkh@linuxfoundation.org wrote:
From 829b37b8cddb1db75c1b7905505b90e593b15db1 Mon Sep 17 00:00:00 2001 From: Theodore Ts'o tytso@mit.edu Date: Wed, 10 Jun 2020 11:16:37 -0400 Subject: [PATCH] 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
We didn't have the conversion of the DAX mount option to a tri-state back in 5.7 and older:
9cb20f94afcd ("fs/ext4: Make DAX mount option a tri-state") a8ab6d3885ef ("fs/ext4: Update ext4_should_use_dax()") fc626fe322f1 ("fs/ext4: Change EXT4_MOUNT_DAX to EXT4_MOUNT_DAX_ALWAYS")
I think that something like this (untested) patch should work:
diff --git a/fs/ext4/super.c b/fs/ext4/super.c index d23afd6c909de..7318ca71b69eb 100644 --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -2080,6 +2080,16 @@ 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; @@ -5407,12 +5417,6 @@ 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 " @@ -5428,12 +5432,6 @@ 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, EXT4_ERR_ESHUTDOWN, "Abort forced by user");