netdev - Re: [PATCH] can: bcm: fix infoleak in struct bcm_msg

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Date:   Sun, 13 Jun 2021 11:18:32 +0000
From:   Patrick Menschel <menschel.p@...teo.de>
To:     Oliver Hartkopp <socketcan@...tkopp.net>,
        Norbert Slusarek <nslusarek@....net>
Cc:     mkl@...gutronix.de, davem@...emloft.net, kuba@...nel.org,
        linux-can@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] can: bcm: fix infoleak in struct bcm_msg_head

Am 13.06.21 um 11:51 schrieb Oliver Hartkopp:
> 
> 
> On 12.06.21 23:09, Norbert Slusarek wrote:
>> From: Norbert Slusarek <nslusarek@....net>
>> Date: Sat, 12 Jun 2021 22:18:54 +0200
>> Subject: [PATCH] can: bcm: fix infoleak in struct bcm_msg_head
>>
>> On 64-bit systems, struct bcm_msg_head has an added padding of 4 bytes
>> between
>> struct members count and ival1. Even though all struct members are
>> initialized,
>> the 4-byte hole will contain data from the kernel stack. This patch
>> zeroes out
>> struct bcm_msg_head before usage, preventing infoleaks to userspace.
>>
>> Fixes: ffd980f976e7 ("[CAN]: Add broadcast manager (bcm) protocol")
>> Signed-off-by: Norbert Slusarek <nslusarek@....net>
> 
> Acked-by: Oliver Hartkopp <socketcan@...tkopp.net>
> 
> Thanks Norbert!
> 
> Yes, when this data structure was created in 2003 either 64 bit machines
> were far away for me and infoleaks were not a hot topic like today.
> 
> Would be interesting to check where data structures are used in the
> Linux UAPI that became an infoleak in the 32-to-64-bit compilation
> transistion.
>
Hi,

1.
Are you sure this leak really happens on 64-bit and not on 32-bit instead?

I remember I got the problems with bcm msg head on the 32bit raspberry
pi because I missed the alignment by accident.

When I calculate the size of msg head on a Ryzen 1800X with Python
3.9.5, I get:

struct.calcsize("IIIllllII"),struct.calcsize("IIIllllII0q")
(56, 56)

First Value is raw, the second value is the alignment hack with the zero
length quad word "0q".

On the 32bit raspberry pi, same op results in the gap.

struct.calcsize("IIIllllII"),struct.calcsize("IIIllllII0q")
(36, 40)

2.
Finding stucts with non-zero-ed gaps should be easy with a skript or
even better with a GCC directive. I believe Syzbot does such a thing too.

Kind Regards,
Patrick Menschel