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Date: Sat, 14 Jun 2025 17:19:59 +0100
From: Simon Horman <horms@kernel.org>
To: Kuniyuki Iwashima <kuni1840@gmail.com>
Cc: Chas Williams <3chas3@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Kuniyuki Iwashima <kuniyu@google.com>, netdev@vger.kernel.org,
	linux-atm-general@lists.sourceforge.net,
	syzbot+1d3c235276f62963e93a@syzkaller.appspotmail.com
Subject: Re: [PATCH v1 net-next] atm: atmtcp: Free invalid length skb in
 atmtcp_c_send().
Message-ID: <20250614161959.GR414686@horms.kernel.org>
References: <20250613055700.415596-1-kuni1840@gmail.com>
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In-Reply-To: <20250613055700.415596-1-kuni1840@gmail.com>

On Thu, Jun 12, 2025 at 10:56:55PM -0700, Kuniyuki Iwashima wrote:
> From: Kuniyuki Iwashima <kuniyu@google.com>
> 
> syzbot reported the splat below. [0]
> 
> vcc_sendmsg() copies data passed from userspace to skb and passes
> it to vcc->dev->ops->send().
> 
> atmtcp_c_send() accesses skb->data as struct atmtcp_hdr after
> checking if skb->len is 0, but it's not enough.
> 
> Also, when skb->len == 0, skb and sk (vcc) were leaked because
> dev_kfree_skb() is not called and atm_return() is missing to
> revert atm_account_tx() in vcc_sendmsg().

Hi Iwashima-san,

I agree with the above and your patch.
But I am wondering if atm_return() also needs to be called when:

* atmtcp_c_send returns -ENOBUFS because atm_alloc_charge() fails.
* copy_from_iter_full returns false in vcc_sendmsg.

I ask because both occur after the call to atm_account_tx() in vcc_sendmsg().

> 
> Let's properly free skb with an invalid length in atmtcp_c_send().

...

> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: syzbot+1d3c235276f62963e93a@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=1d3c235276f62963e93a
> Tested-by: syzbot+1d3c235276f62963e93a@syzkaller.appspotmail.com
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>

My question above not withstanding, this looks good to me.

Reviewed-by: Simon Horman <horms@kernel.org>

...