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Date:   Thu, 8 Apr 2021 15:56:03 +0200
From:   Jan Kara <jack@suse.cz>
To:     Zhang Yi <yi.zhang@huawei.com>
Cc:     linux-ext4@vger.kernel.org, tytso@mit.edu,
        adilger.kernel@dilger.ca, jack@suse.cz, yukuai3@huawei.com
Subject: Re: [PATCH 3/3] ext4: add rcu to prevent use after free when umount
 filesystem
Message-ID: <20210408135603.GD3271@quack2.suse.cz>
References: <20210408113618.1033785-1-yi.zhang@huawei.com>
 <20210408113618.1033785-4-yi.zhang@huawei.com>
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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@huawei.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@suse.com>
SUSE Labs, CR