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Subject: Re: [V9fs-developer] [PATCH] Integer underflow in pdu_read()
To:     Tomas Bortoli <tomasbortoli@gmail.com>, <ericvh@gmail.com>,
        <rminnich@sandia.gov>, <lucho@ionkov.net>
References: <20180709192651.28095-1-tomasbortoli@gmail.com>
CC:     <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <syzkaller@googlegroups.com>,
        <v9fs-developer@lists.sourceforge.net>, <davem@davemloft.net>
From:   piaojun <piaojun@huawei.com>
Message-ID: <5B440B6A.9090000@huawei.com>
Date:   Tue, 10 Jul 2018 09:27:06 +0800
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Hi Tomas,

It looks like pdu->size should always be greater than pdu->offset, right?
My question may be very easy for you, please help explaining.

Thanks,
Jun

On 2018/7/10 3:26, Tomas Bortoli wrote:
> The pdu_read() function suffers from an integer underflow.
> When pdu->offset is greater than pdu->size, the length calculation will have
> a wrong result, resulting in an out-of-bound read.
> This patch modifies also pdu_write() in the same way to prevent the same
> issue from happening there and for consistency.
> 
> Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
> Reported-by: syzbot+65c6b72f284a39d416b4@syzkaller.appspotmail.com
> ---
>  net/9p/protocol.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/net/9p/protocol.c b/net/9p/protocol.c
> index 931ea00c4fed..f1e2425f920b 100644
> --- a/net/9p/protocol.c
> +++ b/net/9p/protocol.c
> @@ -55,16 +55,20 @@ EXPORT_SYMBOL(p9stat_free);
>  
>  size_t pdu_read(struct p9_fcall *pdu, void *data, size_t size)
>  {
> -	size_t len = min(pdu->size - pdu->offset, size);
> -	memcpy(data, &pdu->sdata[pdu->offset], len);
> +	size_t len = pdu->offset > pdu->size ? 0 :
> +	 min(pdu->size - pdu->offset, size);
> +	if (len != 0)
> +		memcpy(data, &pdu->sdata[pdu->offset], len);
>  	pdu->offset += len;
>  	return size - len;
>  }
>  
>  static size_t pdu_write(struct p9_fcall *pdu, const void *data, size_t size)
>  {
> -	size_t len = min(pdu->capacity - pdu->size, size);
> -	memcpy(&pdu->sdata[pdu->size], data, len);
> +	size_t len = pdu->size > pdu->capacity ? 0 :
> +	 min(pdu->capacity - pdu->size, size);
> +	if (len != 0)
> +		memcpy(&pdu->sdata[pdu->size], data, len);
>  	pdu->size += len;
>  	return size - len;
>  }
>