netdev - Re: [PATCH] X.25: Fix address field length calculation

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Date:	Thu, 17 Oct 2013 12:09:33 +0000
From:	Kelleter, Günther <GKelleter@...us.com>
To:	Andrew Hendry <andrew.hendry@...il.com>,
	David Laight <David.Laight@...lab.com>
CC:	Joe Perches <joe@...ches.com>, David Miller <davem@...emloft.net>,
	"linux-x25@...r.kernel.org" <linux-x25@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] X.25: Fix address field length calculation


E.g. called address 7 digits and caller address 3 digits. Called DCE
answering without facilities
gives us this packet (hex):

37 12 34 56 71 23 00

then x25_parse_address_block()  tries to pull 1+7+3 = 11 bytes from the
packet (with pskb_may_pull())
which only has 7 bytes.
When facilities are included the wrong calculated length has no effect
since the facilities make this packet long enough to make pskb_may_pull
with wrong number
of bytes succeed. later x25_addr_ntoa() correctly pulls 6 bytes for
addresses from the packet.


Am 17.10.2013 13:02, schrieb Andrew Hendry:
> Sorry for the previous html mail.
> This appears to be correct, what length addresses are you getting back
> in the call accept when this happens?
>
> On Wed, Oct 16, 2013 at 7:56 PM, David Laight <David.Laight@...lab.com> wrote:
>>> On Tue, 2013-10-15 at 14:29 +0000, Kelleter, Günther wrote:
>>>> Addresses are BCD encoded, not ASCII. x25_addr_ntoa got it right.
>>> []
>>>> Wrong length calculation leads to rejection of CALL ACCEPT packets.
>>> []
>>>> diff --git a/net/x25/af_x25.c b/net/x25/af_x25.c
>>> []
>>>> @@ -98,7 +98,7 @@ int x25_parse_address_block(struct sk_buff *skb,
>>>>     }
>>>>     len = *skb->data;
>>>> -   needed = 1 + (len >> 4) + (len & 0x0f);
>>>> +   needed = 1 + ((len >> 4) + (len & 0x0f) + 1) / 2;
>>> This calculation looks odd.
>> Looks correct to me...
>> In X.25 the lengths (in digits) of the called and calling addresses
>> are encoded in the high and low nibbles of one byte and then
>> followed by both addresses with a digit in each nibble.
>> If the length of the first address is odd, the second one
>> isn't byte aligned.
>>
>>         David
>>
>>
>>


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