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Date:   Wed, 18 Jul 2018 07:13:21 +0200
From:   Dominique Martinet <asmadeus@codewreck.org>
To:     Tomas Bortoli <tomasbortoli@gmail.com>
Cc:     ericvh@gmail.com, rminnich@sandia.gov, lucho@ionkov.net,
        viro@ZenIV.linux.org.uk, davem@davemloft.net,
        v9fs-developer@lists.sourceforge.net, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller@googlegroups.com
Subject: Re: [V9fs-developer] [PATCH] p9_parse_header() validate PDU length
Message-ID: <20180718051321.GB16605@nautica>
References: <20180712110211.25535-1-tomasbortoli@gmail.com>
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Tomas Bortoli wrote on Thu, Jul 12, 2018:
> This patch adds checks to the p9_parse_header() function to
> verify that the length found within the header coincides with the actual
> length of the PDU. Furthermore, it checks that the length stays within the
> acceptable range. To do this the patch brings the actual length of the PDU
> from the different transport layers (rdma and virtio). For TCP (trans_fd.c)
> the length is not know before, so we get it from the header but we check it
> anyway that it's within the valid range.
> 
> Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
> Reported-by: syzbot+65c6b72f284a39d416b4@syzkaller.appspotmail.com
> ---
> [..]
> @@ -498,6 +489,21 @@ p9_parse_header(struct p9_fcall *pdu, int32_t *size, int8_t *type, int16_t *tag,
>  	if (size)
>  		*size = r_size;
>  
> +	if (pdu->size != r_size) {
> +		err = -EINVAL;
> +		goto rewind_and_exit;
> +	}
> +	if (pdu->size >= pdu->capacity || pdu->size < 7) {
> +		p9_debug(P9_DEBUG_ERROR,
> +			 "requested packet size too big or too small: %d\n",
> +			 pdu->size);
> +		return -EIO;
> +	}

Actually, I've been bad advice - this breaks on virtio with zc packets -
a read or ls in a big directory fails with this in dmesg
[ 1006.853775] 9pnet: -- p9_parse_header (17123): requested packet size too big or too small: 4306
[ 1006.853780] 9pnet: -- p9_check_zc_errors (17123): couldn't parse header -5

I haven't given this any thought yet, but dropping the patch for now

-- 
Dominique