Am 13.06.21 um 11:51 schrieb Oliver Hartkopp:

On 12.06.21 23:09, Norbert Slusarek wrote:

quoted

From: Norbert Slusarek <redacted>
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 <redacted>

Acked-by: Oliver Hartkopp <socketcan@hartkopp.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