[PATCH -v2] ext4: fix use-after-free race in ext4_remount()'s error path

["Theodore Ts'o" <tytso@xxxxxxx>]

It's possible for ext4_show_quota_options() to try reading
s_qf_names[i] while it is being modified by ext4_remount() --- most
notably, in ext4_remount's error path when the original values of the
quota file name gets restored.

Reported-by: syzbot+a2872d6feea6918008a9@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Cc: stable@xxxxxxxxxx
---

The -v1 patch had circular locking problems as reported by Lockdep.
So I've redone this change using RCU.

 fs/ext4/super.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index faf293ed8060..0843fd5ce449 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1578,6 +1578,7 @@ static int clear_qf_name(struct super_block *sb, int qtype)
 {
 
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
+	char *to_free;
 
 	if (sb_any_quota_loaded(sb) &&
 		sbi->s_qf_names[qtype]) {
@@ -1585,8 +1586,11 @@ static int clear_qf_name(struct super_block *sb, int qtype)
 			" when quota turned on");
 		return -1;
 	}
-	kfree(sbi->s_qf_names[qtype]);
-	sbi->s_qf_names[qtype] = NULL;
+	to_free = rcu_dereference_protected(sbi->s_qf_names[qtype],
+					    lockdep_is_held(&sb->s_umount));
+	rcu_assign_pointer(sbi->s_qf_names[qtype], NULL);
+	synchronize_rcu();
+	kfree(to_free);
 	return 1;
 }
 #endif
@@ -2048,11 +2052,15 @@ static inline void ext4_show_quota_options(struct seq_file *seq,
 		seq_printf(seq, ",jqfmt=%s", fmtname);
 	}
 
+	rcu_read_lock();
 	if (sbi->s_qf_names[USRQUOTA])
-		seq_show_option(seq, "usrjquota", sbi->s_qf_names[USRQUOTA]);
+		seq_show_option(seq, "usrjquota",
+				rcu_dereference(sbi->s_qf_names[USRQUOTA]));
 
 	if (sbi->s_qf_names[GRPQUOTA])
-		seq_show_option(seq, "grpjquota", sbi->s_qf_names[GRPQUOTA]);
+		seq_show_option(seq, "grpjquota",
+				rcu_dereference(sbi->s_qf_names[GRPQUOTA]));
+	rcu_read_unlock();
 #endif
 }
 
@@ -5104,6 +5112,7 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 	int err = 0;
 #ifdef CONFIG_QUOTA
 	int i, j;
+	char *to_free[EXT4_MAXQUOTAS];
 #endif
 	char *orig_data = kstrdup(data, GFP_KERNEL);
 
@@ -5353,9 +5362,13 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 #ifdef CONFIG_QUOTA
 	sbi->s_jquota_fmt = old_opts.s_jquota_fmt;
 	for (i = 0; i < EXT4_MAXQUOTAS; i++) {
-		kfree(sbi->s_qf_names[i]);
-		sbi->s_qf_names[i] = old_opts.s_qf_names[i];
+		to_free[i] = rcu_dereference_protected(sbi->s_qf_names[i],
+						       &sb->s_umount);
+		rcu_assign_pointer(sbi->s_qf_names[i], old_opts.s_qf_names[i]);
 	}
+	for (i = 0; i < EXT4_MAXQUOTAS; i++)
+		kfree(to_free[i]);
+	synchronize_rcu();
 #endif
 	kfree(orig_data);
 	return err;
-- 
2.18.0.rc0