linux-kernel - Re: [PATCH 6/7] lkdtm: crash on overwriting protected pmalloc var

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Message-ID: <6378e63e-174f-642e-d319-1d121b74d3d7@huawei.com>
Date:   Wed, 7 Mar 2018 15:18:16 +0200
From:   Igor Stoppa <igor.stoppa@...wei.com>
To:     J Freyensee <why2jjj.linux@...il.com>, <david@...morbit.com>,
        <willy@...radead.org>, <keescook@...omium.org>, <mhocko@...nel.org>
CC:     <labbott@...hat.com>, <linux-security-module@...r.kernel.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 6/7] lkdtm: crash on overwriting protected pmalloc var

On 06/03/18 19:20, J Freyensee wrote:

> On 2/28/18 12:06 PM, Igor Stoppa wrote:

[...]

>>   void __init lkdtm_perms_init(void);
>>   void lkdtm_WRITE_RO(void);
>>   void lkdtm_WRITE_RO_AFTER_INIT(void);
>> +void lkdtm_WRITE_RO_PMALLOC(void);
> 
> Does this need some sort of #ifdef too?

Not strictly. It's just a function declaration.
As long as it is not used, the linker will not complain.
The #ifdef placed around the use and definition is sufficient, from a
correctness perspective.

But it's a different question if there is any standard in linux about
hiding also the declaration.

I am not very fond of #ifdefs, so when I can I try to avoid them.

>> +	pr_info("attempting bad pmalloc write at %p\n", i);
>> +	*i = 0;
> 
> OK, now I'm on the right version of this patch series, same comment 
> applies.  I don't get the local *i assignment at the end of the 
> function, but seems harmless.

Because that's the whole point of the function: prove that pmalloc
protection works (see the message in the pr_info one line above).

The function is supposed to do:

* create a pool
* allocate memory from it
* protect it
* try to alter it (and crash)

*i = 0; performs the last step

--
igor