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From: Patrick Menschel <menschel.p@posteo.de>
To: Norbert Slusarek <nslusarek@gmx.net>
Cc: Oliver Hartkopp <socketcan@hartkopp.net>,
	mkl@pengutronix.de, davem@davemloft.net, kuba@kernel.org,
	linux-can@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [PATCH] can: bcm: fix infoleak in struct bcm_msg_head
Date: Sun, 13 Jun 2021 15:36:43 +0000	[thread overview]
Message-ID: <a44df2a0-a403-40ba-3312-c6eb53ddf291@posteo.de> (raw)
In-Reply-To: <trinity-0d8be729-1e3c-452c-8171-962963abed0d-1623591348277@3c-app-gmx-bap71>


Am 13.06.21 um 15:35 schrieb Norbert Slusarek:
>> 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)
> 
> Hey Patrick,
> 
> having reproduced this leak I could only observe the issue on 64-bit systems.
> I've just tested it on a 32-bit OS running on a raspberry pi and I couldn't observe
> any leak. The offset difference on 32-bit between count and ival1 is 4.
> On 64-bit systems, it's 8:
> 
> (gdb) ptype struct bcm_msg_head
> type = struct bcm_msg_head {
>     __u32 opcode;
>     __u32 flags;
>     __u32 count;
>     struct bcm_timeval ival1;
>     struct bcm_timeval ival2;
>     canid_t can_id;
>     __u32 nframes;
>     struct can_frame frames[0];
> }
> (gdb) p/x &((struct bcm_msg_head *)0x0)->count
> $1 = 0x8
> (gdb) p/x &((struct bcm_msg_head *)0x0)->ival1
> $2 = 0x10
> (gdb) p sizeof(((struct bcm_msg_head *)0x0)->count)
> $3 = 4
> 

Ouch,

I should not skip lines while reading.
We're talking about different gaps as it seems. I didn't realize the gap
in front of ival1 before.

There is also a gap in between nframes and frames[0].
That one is caused by align(8) of data in struct can_frame.
It propagates upwards into that gap on 32bit arch.
You can find it if you actually fill frames[] with a frame.

I found it while concatenating bcm_msg_head and a can frame into a
python bytearray which was too short for the raspberry pi as I forgot
the alignment.

I came up with a format string "IIIllllII0q" for bcm_msg_head.

Kind Regards,
Patrick

  reply	other threads:[~2021-06-13 15:36 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-12 21:09 Norbert Slusarek
2021-06-13  9:51 ` Oliver Hartkopp
2021-06-13 11:18   ` Patrick Menschel
2021-06-13 13:35     ` Norbert Slusarek
2021-06-13 15:36       ` Patrick Menschel [this message]
2021-06-13 18:33         ` Norbert Slusarek
2021-06-14  7:20 ` Marc Kleine-Budde
2021-06-15 20:40   ` Norbert Slusarek

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