netdev - Re: [PATCH net] net/xen-netback: prevent UAF in xenvif_flush

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Message-ID: <CAO9qdTGHJw-SUFH9D16N5wSn4KmaMUcX+GVFuEFu+jqMb3HU1g@mail.gmail.com>
Date: Wed, 28 Aug 2024 21:52:12 +0900
From: Jeongjun Park <aha310510@...il.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: wei.liu@...nel.org, paul@....org, davem@...emloft.net, edumazet@...gle.com, 
	kuba@...nel.org, madhuparnabhowmik04@...il.com, 
	xen-devel@...ts.xenproject.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net/xen-netback: prevent UAF in xenvif_flush_hash()

On Tue, 27 Aug 2024 13:19:59 +0200 Paolo Abeni wrote:
> On 8/22/24 20:11, Jeongjun Park wrote:
> > During the list_for_each_entry_rcu iteration call of xenvif_flush_hash,
> > kfree_rcu does not exist inside the rcu read critical section, so if
>
> The above wording is confusing, do you mean "kfree_rcu does not exit
> from "...?
>
> > kfree_rcu is called when the rcu grace period ends during the iteration,
> > UAF occurs when accessing head->next after the entry becomes free.
>
> The loop runs with irq disabled, the RCU critical section extends over
> it, uninterrupted.

Basically, list_for_each_entry_rcu is specified to be used under the protection
of rcu_read_lock(), but this is not the case with xenvif_new_hash(). If it is
used without the protection of rcu_read_lock(), kfree is called immediately
after the grace period ends after the call to kfree_rcu() inside
list_for_each_entry_rcu, so the entry is released, and a UAF occurs when
fetching with ->next thereafter.

Regards,
Jeongjun Park