linux-kernel - Re: [patch 136/147] nilfs2: use refcount_dec_and

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Date:   Fri, 24 Sep 2021 12:35:33 +0200
From:   Pavel Machek <pavel@...x.de>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     akpm@...ux-foundation.org, konishi.ryusuke@...il.com,
        linux-mm@...ck.org, mm-commits@...r.kernel.org,
        thunder.leizhen@...wei.com, torvalds@...ux-foundation.org
Subject: Re: [patch 136/147] nilfs2: use refcount_dec_and_lock() to fix
 potential UAF

Hi!

> From: Zhen Lei <thunder.leizhen@...wei.com>
> Subject: nilfs2: use refcount_dec_and_lock() to fix potential UAF
> 
> When the refcount is decreased to 0, the resource reclamation branch is
> entered.  Before CPU0 reaches the race point (1), CPU1 may obtain the
> spinlock and traverse the rbtree to find 'root', see nilfs_lookup_root(). 
> Although CPU1 will call refcount_inc() to increase the refcount, it is
> obviously too late.  CPU0 will release 'root' directly, CPU1 then accesses
> 'root' and triggers UAF.
> 
> Use refcount_dec_and_lock() to ensure that both the operations of decrease
> refcount to 0 and link deletion are lock protected eliminates this risk.
> 
>      CPU0                      CPU1
> nilfs_put_root():
> 			    <-------- (1)
> spin_lock(&nilfs->ns_cptree_lock);
> rb_erase(&root->rb_node, &nilfs->ns_cptree);
> spin_unlock(&nilfs->ns_cptree_lock);
> 
> kfree(root);
> 			    <-------- use-after-free

> There is no reproduction program, and the above is only theoretical
> analysis.

Ok, so we have a theoretical bug, and fix already on its way to
stable. But ... is it correct?

> +++ a/fs/nilfs2/the_nilfs.c
> @@ -792,14 +792,13 @@ nilfs_find_or_create_root(struct the_nil
>  
>  void nilfs_put_root(struct nilfs_root *root)
>  {
> -	if (refcount_dec_and_test(&root->count)) {
> -		struct the_nilfs *nilfs = root->nilfs;
> +	struct the_nilfs *nilfs = root->nilfs;
>  
> -		nilfs_sysfs_delete_snapshot_group(root);
> -
> -		spin_lock(&nilfs->ns_cptree_lock);
> +	if (refcount_dec_and_lock(&root->count, &nilfs->ns_cptree_lock)) {
>  		rb_erase(&root->rb_node, &nilfs->ns_cptree);
>  		spin_unlock(&nilfs->ns_cptree_lock);
> +
> +		nilfs_sysfs_delete_snapshot_group(root);
>  		iput(root->ifile);
>  
>  		kfree(root);

spin_lock() is deleted, but spin_unlock() is not affected. This means
unbalanced locking, right?

Best regards,
								Pavel
--
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany


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