ext4: don't clear SB_RDONLY when remounting r/w until quota is re-enabled

When a file system currently mounted read/only is remounted
read/write, if we clear the SB_RDONLY flag too early, before the quota
is initialized, and there is another process/thread constantly
attempting to create a directory, it's possible to trigger the

	WARN_ON_ONCE(dquot_initialize_needed(inode));

in ext4_xattr_block_set(), with the following stack trace:

   WARNING: CPU: 0 PID: 5338 at fs/ext4/xattr.c:2141 ext4_xattr_block_set+0x2ef2/0x3680
   RIP: 0010:ext4_xattr_block_set+0x2ef2/0x3680 fs/ext4/xattr.c:2141
   Call Trace:
    ext4_xattr_set_handle+0xcd4/0x15c0 fs/ext4/xattr.c:2458
    ext4_initxattrs+0xa3/0x110 fs/ext4/xattr_security.c:44
    security_inode_init_security+0x2df/0x3f0 security/security.c:1147
    __ext4_new_inode+0x347e/0x43d0 fs/ext4/ialloc.c:1324
    ext4_mkdir+0x425/0xce0 fs/ext4/namei.c:2992
    vfs_mkdir+0x29d/0x450 fs/namei.c:4038
    do_mkdirat+0x264/0x520 fs/namei.c:4061
    __do_sys_mkdirat fs/namei.c:4076 [inline]
    __se_sys_mkdirat fs/namei.c:4074 [inline]
    __x64_sys_mkdirat+0x89/0xa0 fs/namei.c:4074

Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Reported-by: [email protected]
Link: https://syzkaller.appspot.com/bug?id=6513f6cb5cd6b5fc9f37e3bb70d273b94be9c34c
Signed-off-by: Theodore Ts'o <[email protected]>

Author: tytso

fs/ext4/super.c

@@ -6387,6 +6387,7 @@ static int __ext4_remount(struct fs_context *fc, struct super_block *sb)
 	struct ext4_mount_options old_opts;
 	ext4_group_t g;
 	int err = 0;
+	int enable_rw = 0;
 #ifdef CONFIG_QUOTA
 	int enable_quota = 0;
 	int i, j;
@@ -6573,7 +6574,7 @@ static int __ext4_remount(struct fs_context *fc, struct super_block *sb)
 			if (err)
 				goto restore_opts;
 
-			sb->s_flags &= ~SB_RDONLY;
+			enable_rw = 1;
 			if (ext4_has_feature_mmp(sb)) {
 				err = ext4_multi_mount_protect(sb,
 						le64_to_cpu(es->s_mmp_block));
@@ -6632,6 +6633,9 @@ static int __ext4_remount(struct fs_context *fc, struct super_block *sb)
 	if (!test_opt(sb, BLOCK_VALIDITY) && sbi->s_system_blks)
 		ext4_release_system_zone(sb);
 
+	if (enable_rw)
+		sb->s_flags &= ~SB_RDONLY;
+
 	if (!ext4_has_feature_mmp(sb) || sb_rdonly(sb))
 		ext4_stop_mmpd(sbi);