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Subject[PATCH 3.16 077/305] ext4: fix use-after-free race in ext4_remount()'s error path
3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Theodore Ts'o <tytso@mit.edu>

commit 33458eaba4dfe778a426df6a19b7aad2ff9f7eec upstream.

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@syzkaller.appspotmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.16:
- s/EXT4_MAXQUOTAS/MAXQUOTAS/
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1300,7 +1300,8 @@ struct ext4_sb_info {
u32 s_min_batch_time;
struct block_device *journal_bdev;
#ifdef CONFIG_QUOTA
- char *s_qf_names[MAXQUOTAS]; /* Names of quota files with journalled quota */
+ /* Names of quota files with journalled quota */
+ char __rcu *s_qf_names[MAXQUOTAS];
int s_jquota_fmt; /* Format of quota to use */
#endif
unsigned int s_want_extra_isize; /* New inodes should reserve # bytes */
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -780,6 +780,20 @@ static void dump_orphan_list(struct supe
}
}

+#ifdef CONFIG_QUOTA
+/*
+ * This is a helper function which is used in the mount/remount
+ * codepaths (which holds s_umount) to fetch the quota file name.
+ */
+static inline char *get_qf_name(struct super_block *sb,
+ struct ext4_sb_info *sbi,
+ int type)
+{
+ return rcu_dereference_protected(sbi->s_qf_names[type],
+ lockdep_is_held(&sb->s_umount));
+}
+#endif
+
static void ext4_put_super(struct super_block *sb)
{
struct ext4_sb_info *sbi = EXT4_SB(sb);
@@ -833,7 +847,7 @@ static void ext4_put_super(struct super_
brelse(sbi->s_sbh);
#ifdef CONFIG_QUOTA
for (i = 0; i < MAXQUOTAS; i++)
- kfree(sbi->s_qf_names[i]);
+ kfree(get_qf_name(sb, sbi, i));
#endif

/* Debugging code just in case the in-memory inode orphan list
@@ -1293,11 +1307,10 @@ static char deprecated_msg[] = "Mount op
static int set_qf_name(struct super_block *sb, int qtype, substring_t *args)
{
struct ext4_sb_info *sbi = EXT4_SB(sb);
- char *qname;
+ char *qname, *old_qname = get_qf_name(sb, sbi, qtype);
int ret = -1;

- if (sb_any_quota_loaded(sb) &&
- !sbi->s_qf_names[qtype]) {
+ if (sb_any_quota_loaded(sb) && !old_qname) {
ext4_msg(sb, KERN_ERR,
"Cannot change journaled "
"quota options when quota turned on");
@@ -1314,8 +1327,8 @@ static int set_qf_name(struct super_bloc
"Not enough memory for storing quotafile name");
return -1;
}
- if (sbi->s_qf_names[qtype]) {
- if (strcmp(sbi->s_qf_names[qtype], qname) == 0)
+ if (old_qname) {
+ if (strcmp(old_qname, qname) == 0)
ret = 1;
else
ext4_msg(sb, KERN_ERR,
@@ -1328,7 +1341,7 @@ static int set_qf_name(struct super_bloc
"quotafile must be on filesystem root");
goto errout;
}
- sbi->s_qf_names[qtype] = qname;
+ rcu_assign_pointer(sbi->s_qf_names[qtype], qname);
set_opt(sb, QUOTA);
return 1;
errout:
@@ -1340,15 +1353,16 @@ static int clear_qf_name(struct super_bl
{

struct ext4_sb_info *sbi = EXT4_SB(sb);
+ char *old_qname = get_qf_name(sb, sbi, qtype);

- if (sb_any_quota_loaded(sb) &&
- sbi->s_qf_names[qtype]) {
+ if (sb_any_quota_loaded(sb) && old_qname) {
ext4_msg(sb, KERN_ERR, "Cannot change journaled quota options"
" when quota turned on");
return -1;
}
- kfree(sbi->s_qf_names[qtype]);
- sbi->s_qf_names[qtype] = NULL;
+ rcu_assign_pointer(sbi->s_qf_names[qtype], NULL);
+ synchronize_rcu();
+ kfree(old_qname);
return 1;
}
#endif
@@ -1677,7 +1691,7 @@ static int parse_options(char *options,
int is_remount)
{
struct ext4_sb_info *sbi = EXT4_SB(sb);
- char *p;
+ char *p, __maybe_unused *usr_qf_name, __maybe_unused *grp_qf_name;
substring_t args[MAX_OPT_ARGS];
int token;

@@ -1704,11 +1718,13 @@ static int parse_options(char *options,
"feature is enabled");
return 0;
}
- if (sbi->s_qf_names[USRQUOTA] || sbi->s_qf_names[GRPQUOTA]) {
- if (test_opt(sb, USRQUOTA) && sbi->s_qf_names[USRQUOTA])
+ usr_qf_name = get_qf_name(sb, sbi, USRQUOTA);
+ grp_qf_name = get_qf_name(sb, sbi, GRPQUOTA);
+ if (usr_qf_name || grp_qf_name) {
+ if (test_opt(sb, USRQUOTA) && usr_qf_name)
clear_opt(sb, USRQUOTA);

- if (test_opt(sb, GRPQUOTA) && sbi->s_qf_names[GRPQUOTA])
+ if (test_opt(sb, GRPQUOTA) && grp_qf_name)
clear_opt(sb, GRPQUOTA);

if (test_opt(sb, GRPQUOTA) || test_opt(sb, USRQUOTA)) {
@@ -1742,6 +1758,7 @@ static inline void ext4_show_quota_optio
{
#if defined(CONFIG_QUOTA)
struct ext4_sb_info *sbi = EXT4_SB(sb);
+ char *usr_qf_name, *grp_qf_name;

if (sbi->s_jquota_fmt) {
char *fmtname = "";
@@ -1760,11 +1777,14 @@ static inline void ext4_show_quota_optio
seq_printf(seq, ",jqfmt=%s", fmtname);
}

- if (sbi->s_qf_names[USRQUOTA])
- seq_show_option(seq, "usrjquota", sbi->s_qf_names[USRQUOTA]);
-
- if (sbi->s_qf_names[GRPQUOTA])
- seq_show_option(seq, "grpjquota", sbi->s_qf_names[GRPQUOTA]);
+ rcu_read_lock();
+ usr_qf_name = rcu_dereference(sbi->s_qf_names[USRQUOTA]);
+ grp_qf_name = rcu_dereference(sbi->s_qf_names[GRPQUOTA]);
+ if (usr_qf_name)
+ seq_show_option(seq, "usrjquota", usr_qf_name);
+ if (grp_qf_name)
+ seq_show_option(seq, "grpjquota", grp_qf_name);
+ rcu_read_unlock();
#endif
}

@@ -4936,6 +4956,7 @@ static int ext4_remount(struct super_blo
int err = 0;
#ifdef CONFIG_QUOTA
int i, j;
+ char *to_free[MAXQUOTAS];
#endif
char *orig_data = kstrdup(data, GFP_KERNEL);

@@ -4952,8 +4973,9 @@ static int ext4_remount(struct super_blo
old_opts.s_jquota_fmt = sbi->s_jquota_fmt;
for (i = 0; i < MAXQUOTAS; i++)
if (sbi->s_qf_names[i]) {
- old_opts.s_qf_names[i] = kstrdup(sbi->s_qf_names[i],
- GFP_KERNEL);
+ char *qf_name = get_qf_name(sb, sbi, i);
+
+ old_opts.s_qf_names[i] = kstrdup(qf_name, GFP_KERNEL);
if (!old_opts.s_qf_names[i]) {
for (j = 0; j < i; j++)
kfree(old_opts.s_qf_names[j]);
@@ -5141,9 +5163,12 @@ restore_opts:
#ifdef CONFIG_QUOTA
sbi->s_jquota_fmt = old_opts.s_jquota_fmt;
for (i = 0; i < MAXQUOTAS; i++) {
- kfree(sbi->s_qf_names[i]);
- sbi->s_qf_names[i] = old_opts.s_qf_names[i];
+ to_free[i] = get_qf_name(sb, sbi, i);
+ rcu_assign_pointer(sbi->s_qf_names[i], old_opts.s_qf_names[i]);
}
+ synchronize_rcu();
+ for (i = 0; i < MAXQUOTAS; i++)
+ kfree(to_free[i]);
#endif
kfree(orig_data);
return err;
@@ -5291,7 +5316,7 @@ static int ext4_write_info(struct super_
*/
static int ext4_quota_on_mount(struct super_block *sb, int type)
{
- return dquot_quota_on_mount(sb, EXT4_SB(sb)->s_qf_names[type],
+ return dquot_quota_on_mount(sb, get_qf_name(sb, EXT4_SB(sb), type),
EXT4_SB(sb)->s_jquota_fmt, type);
}

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