linux-kernel - Re: usercopy whitelist woe in scsi_sense

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Date:   Wed, 04 Apr 2018 23:34:56 +0200
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     Kees Cook <keescook@...omium.org>
Cc:     David Windsor <dave@...lcore.net>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        keescook@...gle.com
Subject: Re: usercopy whitelist woe in scsi_sense_cache

Hi.

04.04.2018 23:25, Kees Cook wrote:
>> Actually, I can trigger a BUG too:
>> 
>> [  129.259213] usercopy: Kernel memory exposure attempt detected from 
>> SLUB
>> object 'scsi_sense_cache' (offset 119, size 22)!
> 
> Wow, yeah, that's totally outside the slub object_size. How did you
> trigger this? Just luck or something specific?

Just luck, I suppose. It usually comes after the first warning if you 
wait long enough (maybe, a couple of extra minutes).

To give you an idea regarding variety of offsets, I've summarised kernel 
log from the server:

$ sudo journalctl -kb | grep "Kernel memory exposure attempt detected" | 
grep -oE 'offset [0-9]+, size [0-9]+' | sort | uniq -c
       9 offset 107, size 22
       6 offset 108, size 22
       8 offset 109, size 22
       7 offset 110, size 22
       5 offset 111, size 22
       5 offset 112, size 22
       2 offset 113, size 22
       2 offset 114, size 22
       1 offset 115, size 22
       1 offset 116, size 22
       1 offset 119, size 22
       1 offset 85, size 22

> I'd really like to understand how the buffer position can be
> changing... I'd expect that to break all kinds of things (i.e.
> free()ing the slab later would break too...)

I haven't checked the code yet, but the first thing that comes to my 
mind is some uninitialised variable. Just guessing here, though.

> Thanks for the report! I hope someone more familiar with sg_io() can
> help explain the changing buffer offset... :P

Hopefully, SCSI people are Cc'ed here properly…

Thanks!

Regards,
   Oleksandr