From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:57506 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726135AbeJKRym (ORCPT ); Thu, 11 Oct 2018 13:54:42 -0400 Date: Thu, 11 Oct 2018 12:28:00 +0200 From: Jan Kara To: Theodore Ts'o Cc: Ext4 Developers List , joel@joelfernandes.org, paulmck@linux.ibm.com, Al Viro Subject: Re: [PATCH -v3] ext4: fix use-after-free race in ext4_remount()'s error path Message-ID: <20181011102800.GB9467@quack2.suse.cz> References: <20181007030706.9094-1-tytso@mit.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181007030706.9094-1-tytso@mit.edu> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Sat 06-10-18 23:07:06, Theodore Ts'o wrote: > 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 > Cc: stable@kernel.org Well, honestly the fact that ->show_options can be called while remount is changing stuff under you looks problematic to me and I bet ext4 is not the only one that would have issues with that. So I believe we might be better off with just synchronizing ->show_options with umount / remount properly. What were the lock dependency problems that made you switch to use RCU? Honza > --- > > v3: RCU clean ups and fix ups > v2: Use RCU to avoid lock dependency problems > > fs/ext4/ext4.h | 3 +- > fs/ext4/super.c | 73 ++++++++++++++++++++++++++++++++----------------- > 2 files changed, 50 insertions(+), 26 deletions(-) > > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h > index 86e1bacac757..12f90d48ba61 100644 > --- a/fs/ext4/ext4.h > +++ b/fs/ext4/ext4.h > @@ -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 */ > diff --git a/fs/ext4/super.c b/fs/ext4/super.c > index faf293ed8060..1909fbb28583 100644 > --- a/fs/ext4/super.c > +++ b/fs/ext4/super.c > @@ -920,6 +920,18 @@ static inline void ext4_quota_off_umount(struct super_block *sb) > } > #endif > > +/* > + * 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)); > +} > + > static void ext4_put_super(struct super_block *sb) > { > struct ext4_sb_info *sbi = EXT4_SB(sb); > @@ -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 > @@ -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"); > @@ -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, > @@ -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: > @@ -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 > @@ -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, *usr_qf_name, *grp_qf_name; > substring_t args[MAX_OPT_ARGS]; > int token; > > @@ -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)) { > @@ -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 = ""; > @@ -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 > } > > @@ -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); > > @@ -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]); > @@ -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; > @@ -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); > } > > -- > 2.18.0.rc0 > -- Jan Kara SUSE Labs, CR