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Message-ID: <20210408135603.GD3271@quack2.suse.cz>
Date: Thu, 8 Apr 2021 15:56:03 +0200
From: Jan Kara <jack@...e.cz>
To: Zhang Yi <yi.zhang@...wei.com>
Cc: linux-ext4@...r.kernel.org, tytso@....edu,
adilger.kernel@...ger.ca, jack@...e.cz, yukuai3@...wei.com
Subject: Re: [PATCH 3/3] ext4: add rcu to prevent use after free when umount
filesystem
On Thu 08-04-21 19:36:18, Zhang Yi wrote:
> There is a race between bdev_try_to_free_page() and
> jbd2_journal_destroy() that could end up triggering a use after free
> issue about journal.
>
> drop cache umount filesystem
> bdev_try_to_free_page()
> get journal
> jbd2_journal_try_to_free_buffers() ext4_put_super()
> kfree(journal)
> access journal <-- lead to UAF
>
> The above race also could happens between the bdev_try_to_free_page()
> and the error path of ext4_fill_super(). This patch avoid this race by
> add rcu protection around accessing sbi->s_journal in
> bdev_try_to_free_page() and destroy the journal after an rcu grace
> period.
>
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
OK, I see the problem. But cannot the use-after-free happen even for the
superblock itself (e.g., EXT4_SB(sb)->s_journal dereference)? I don't see
anything preventing that as blkdev_releasepage() just shamelessly does:
super = BDEV_I(page->mapping->host)->bdev.bd_super
without making sure the sb cannot go away the instant we load a pointer to
it. Or am I missing something Ted? If I'm right, we'd need some careful
sprinkling of RCU, READ_ONCE(), and careful superblock reference grabbing
to make bdev_try_to_free_page() safe against concurrent
kill_block_super()...
Honza
> ---
> fs/ext4/super.c | 33 ++++++++++++++++++++++++---------
> fs/jbd2/journal.c | 30 +++++++++++++++++++++++++++---
> include/linux/jbd2.h | 11 ++++++++++-
> 3 files changed, 61 insertions(+), 13 deletions(-)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 02ba47a5bc70..6bbaadc5357b 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1150,6 +1150,21 @@ static inline void ext4_quota_off_umount(struct super_block *sb)
> }
> #endif
>
> +static int ext4_journal_release(struct ext4_sb_info *sbi)
> +{
> + journal_t *journal = sbi->s_journal;
> + int ret;
> +
> + ret = jbd2_journal_release(journal);
> + sbi->s_journal = NULL;
> + /*
> + * Call rcu to prevent racing with bdev_try_to_free_page()
> + * accessing the journal at the same time.
> + */
> + call_rcu(&journal->j_rcu, jbd2_journal_release_rcu);
> + return ret;
> +}
> +
> static void ext4_put_super(struct super_block *sb)
> {
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -1174,11 +1189,9 @@ static void ext4_put_super(struct super_block *sb)
>
> if (sbi->s_journal) {
> aborted = is_journal_aborted(sbi->s_journal);
> - err = jbd2_journal_destroy(sbi->s_journal);
> - sbi->s_journal = NULL;
> - if ((err < 0) && !aborted) {
> + err = ext4_journal_release(sbi);
> + if ((err < 0) && !aborted)
> ext4_abort(sb, -err, "Couldn't clean up the journal");
> - }
> }
>
> ext4_es_unregister_shrinker(sbi);
> @@ -1449,14 +1462,18 @@ static int ext4_nfs_commit_metadata(struct inode *inode)
> static int bdev_try_to_free_page(struct super_block *sb, struct page *page,
> gfp_t wait)
> {
> - journal_t *journal = EXT4_SB(sb)->s_journal;
> + journal_t *journal;
> int ret = 0;
>
> WARN_ON(PageChecked(page));
> if (!page_has_buffers(page))
> return 0;
> +
> + rcu_read_lock();
> + journal = READ_ONCE(EXT4_SB(sb)->s_journal);
> if (journal)
> ret = jbd2_journal_try_to_free_buffers(journal, page);
> + rcu_read_unlock();
> if (!ret)
> return try_to_free_buffers(page);
> return 0;
> @@ -5146,10 +5163,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> ext4_xattr_destroy_cache(sbi->s_ea_block_cache);
> sbi->s_ea_block_cache = NULL;
>
> - if (sbi->s_journal) {
> - jbd2_journal_destroy(sbi->s_journal);
> - sbi->s_journal = NULL;
> - }
> + if (sbi->s_journal)
> + ext4_journal_release(sbi);
> failed_mount3a:
> ext4_es_unregister_shrinker(sbi);
> failed_mount3:
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index 2dc944442802..071caaaa9de1 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -76,6 +76,8 @@ EXPORT_SYMBOL(jbd2_journal_check_available_features);
> EXPORT_SYMBOL(jbd2_journal_set_features);
> EXPORT_SYMBOL(jbd2_journal_load);
> EXPORT_SYMBOL(jbd2_journal_destroy);
> +EXPORT_SYMBOL(jbd2_journal_release);
> +EXPORT_SYMBOL(jbd2_journal_release_rcu);
> EXPORT_SYMBOL(jbd2_journal_abort);
> EXPORT_SYMBOL(jbd2_journal_errno);
> EXPORT_SYMBOL(jbd2_journal_ack_err);
> @@ -1951,14 +1953,14 @@ int jbd2_journal_load(journal_t *journal)
> }
>
> /**
> - * jbd2_journal_destroy() - Release a journal_t structure.
> + * jbd2_journal_release() - Release a journal_t structure.
> * @journal: Journal to act on.
> *
> * Release a journal_t structure once it is no longer in use by the
> * journaled object.
> * Return <0 if we couldn't clean up the journal.
> */
> -int jbd2_journal_destroy(journal_t *journal)
> +int jbd2_journal_release(journal_t *journal)
> {
> int err = 0;
>
> @@ -2021,11 +2023,33 @@ int jbd2_journal_destroy(journal_t *journal)
> crypto_free_shash(journal->j_chksum_driver);
> kfree(journal->j_fc_wbuf);
> kfree(journal->j_wbuf);
> - kfree(journal);
>
> return err;
> }
>
> +/**
> + * jbd2_journal_release_rcu() - Free a journal_t structure.
> + * @rcu: rcu list node relate to the journal want to free.
> + *
> + * Freeing a journal_t structure after a rcu grace period.
> + */
> +void jbd2_journal_release_rcu(struct rcu_head *rcu)
> +{
> + kfree(container_of(rcu, journal_t, j_rcu));
> +}
> +
> +/**
> + * jbd2_journal_destroy() - Release and free a journal_t structure.
> + * @journal: Journal to act on.
> + *
> + * Release and free a journal_t structure once it is no longer in use
> + * by the journaled object.
> + */
> +void jbd2_journal_destroy(journal_t *journal)
> +{
> + jbd2_journal_release(journal);
> + kfree(journal);
> +}
>
> /**
> * jbd2_journal_check_used_features() - Check if features specified are used.
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index 99d3cd051ac3..39a8d04596a2 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -1238,6 +1238,13 @@ struct journal_s
> */
> __u32 j_csum_seed;
>
> + /**
> + * @j_rcu:
> + *
> + * Prevent racing between accessing and destroy at the same time.
> + */
> + struct rcu_head j_rcu;
> +
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
> /**
> * @j_trans_commit_map:
> @@ -1509,7 +1516,9 @@ extern int jbd2_journal_set_features
> extern void jbd2_journal_clear_features
> (journal_t *, unsigned long, unsigned long, unsigned long);
> extern int jbd2_journal_load (journal_t *journal);
> -extern int jbd2_journal_destroy (journal_t *);
> +extern void jbd2_journal_destroy (journal_t *);
> +extern int jbd2_journal_release (journal_t *);
> +extern void jbd2_journal_release_rcu (struct rcu_head *rcu);
> extern int jbd2_journal_recover (journal_t *journal);
> extern int jbd2_journal_wipe (journal_t *, int);
> extern int jbd2_journal_skip_recovery (journal_t *);
> --
> 2.25.4
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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