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

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Date:   Sun, 13 Jun 2021 15:35:48 +0200
From:   Norbert Slusarek <nslusarek@....net>
To:     Patrick Menschel <menschel.p@...teo.de>
Cc:     Oliver Hartkopp <socketcan@...tkopp.net>, 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

>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

>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

I didn't notice any syzbot report about this leak, nor did I find it with syzkaller.

Norbert