linux-kernel - Re: [ghak-trim PATCH v1] audit: move put_tree() to avoid trim

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Date:   Tue, 24 Aug 2021 10:51:32 +0200
From:   Jan Kara <jack@...e.cz>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     Linux-Audit Mailing List <linux-audit@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>,
        Steve Grubb <sgrubb@...hat.com>, Jan Kara <jack@...e.cz>,
        Will Deacon <will@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Seiji Nishikawa <snishika@...hat.com>
Subject: Re: [ghak-trim PATCH v1] audit: move put_tree() to avoid trim_trees
 refcount underflow and UAF

On Mon 23-08-21 22:04:09, Richard Guy Briggs wrote:
> AUDIT_TRIM is expected to be idempotent, but multiple executions resulted in a
> refcount underflow and use-after-free.
> 
> git bisect fingered commit fb041bb7c0a918b95c6889fc965cdc4a75b4c0ca (2019-11)
> 	("locking/refcount: Consolidate implementations of refcount_t")
> but this patch with its more thorough checking that wasn't in the x86 assembly
> code merely exposed a previously existing tree refcount imbalance in the case
> of tree trimming code that was refactored with prune_one() to remove a tree
> introduced in commit 8432c70062978d9a57bde6715496d585ec520c3e (2018-11)
> 	("audit: Simplify locking around untag_chunk()")
> 
> Move the put_tree() to cover only the prune_one() case.
> 
> Passes audit-testsuite and 3 passes of "auditctl -t" with at least one
> directory watch.
> 
> Fixes: 8432c7006297 ("audit: Simplify locking around untag_chunk()")
> Signed-off-by: Richard Guy Briggs <rgb@...hat.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Will Deacon <will@...nel.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Seiji Nishikawa <snishika@...hat.com>

Ah, that was indeed a stupid mistake. Thanks for debugging this! The patch
looks good to me. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza


> ---
>  kernel/audit_tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> index b2be4e978ba3..2cd7b5694422 100644
> --- a/kernel/audit_tree.c
> +++ b/kernel/audit_tree.c
> @@ -593,7 +593,6 @@ static void prune_tree_chunks(struct audit_tree *victim, bool tagged)
>  		spin_lock(&hash_lock);
>  	}
>  	spin_unlock(&hash_lock);
> -	put_tree(victim);
>  }
>  
>  /*
> @@ -602,6 +601,7 @@ static void prune_tree_chunks(struct audit_tree *victim, bool tagged)
>  static void prune_one(struct audit_tree *victim)
>  {
>  	prune_tree_chunks(victim, false);
> +	put_tree(victim);
>  }
>  
>  /* trim the uncommitted chunks from tree */
> -- 
> 2.27.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR