linux-kernel - [PATCH 5.7 436/477] ext4: avoid race conditions when remounting with options that change dax

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Date:   Tue, 23 Jun 2020 21:57:13 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, stable@...nel.org,
        syzbot+bca9799bf129256190da@...kaller.appspotmail.com,
        Theodore Tso <tytso@....edu>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.7 436/477] ext4: avoid race conditions when remounting with options that change dax

From: Theodore Ts'o <tytso@....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@...nel.org
Reported-by: syzbot+bca9799bf129256190da@...kaller.appspotmail.com
Signed-off-by: Theodore Ts'o <tytso@....edu>
Signed-off-by: Sasha Levin <sashal@...nel.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 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");
 
-- 
2.25.1