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ext4: fix use-after-free race in ext4_remount()'s error path
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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>
Cc: stable@kernel.org # 3.2+
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Theodore Ts'o committed Oct 12, 2018
1 parent 6fd9417 commit 33458ea
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Showing 2 changed files with 50 additions and 26 deletions.
3 changes: 2 additions & 1 deletion fs/ext4/ext4.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1405,7 +1405,8 @@ struct ext4_sb_info {
u32 s_min_batch_time;
struct block_device *journal_bdev;
#ifdef CONFIG_QUOTA
char *s_qf_names[EXT4_MAXQUOTAS]; /* Names of quota files with journalled quota */
/* Names of quota files with journalled quota */
char __rcu *s_qf_names[EXT4_MAXQUOTAS];
int s_jquota_fmt; /* Format of quota to use */
#endif
unsigned int s_want_extra_isize; /* New inodes should reserve # bytes */
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73 changes: 48 additions & 25 deletions fs/ext4/super.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -914,6 +914,18 @@ static inline void ext4_quota_off_umount(struct super_block *sb)
for (type = 0; type < EXT4_MAXQUOTAS; type++)
ext4_quota_off(sb, type);
}

/*
* 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));
}
#else
static inline void ext4_quota_off_umount(struct super_block *sb)
{
Expand Down Expand Up @@ -965,7 +977,7 @@ static void ext4_put_super(struct super_block *sb)
percpu_free_rwsem(&sbi->s_journal_flag_rwsem);
#ifdef CONFIG_QUOTA
for (i = 0; i < EXT4_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
Expand Down Expand Up @@ -1531,11 +1543,10 @@ static const char deprecated_msg[] =
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");
Expand All @@ -1552,8 +1563,8 @@ static int set_qf_name(struct super_block *sb, int qtype, substring_t *args)
"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,
Expand All @@ -1566,7 +1577,7 @@ static int set_qf_name(struct super_block *sb, int qtype, substring_t *args)
"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:
Expand All @@ -1578,15 +1589,16 @@ static int clear_qf_name(struct super_block *sb, int qtype)
{

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
Expand Down Expand Up @@ -1961,7 +1973,7 @@ static int parse_options(char *options, struct super_block *sb,
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;

Expand Down Expand Up @@ -1992,11 +2004,13 @@ static int parse_options(char *options, struct super_block *sb,
"Cannot enable project quota enforcement.");
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)) {
Expand Down Expand Up @@ -2030,6 +2044,7 @@ static inline void ext4_show_quota_options(struct seq_file *seq,
{
#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 = "";
Expand All @@ -2048,11 +2063,14 @@ static inline void ext4_show_quota_options(struct seq_file *seq,
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
}

Expand Down Expand Up @@ -5104,6 +5122,7 @@ 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);

Expand All @@ -5123,8 +5142,9 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
old_opts.s_jquota_fmt = sbi->s_jquota_fmt;
for (i = 0; i < EXT4_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]);
Expand Down Expand Up @@ -5353,9 +5373,12 @@ 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] = 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 < EXT4_MAXQUOTAS; i++)
kfree(to_free[i]);
#endif
kfree(orig_data);
return err;
Expand Down Expand Up @@ -5546,7 +5569,7 @@ static int ext4_write_info(struct super_block *sb, int type)
*/
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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