netdev - Re: [PATCH net] net: filter: x86: fix JIT address randomization

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Date:	Tue, 13 May 2014 17:38:28 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	hpa@...or.com
Cc:	ast@...mgrid.com, eric.dumazet@...il.com, edumazet@...gle.com,
	dborkman@...hat.com, heiko.carstens@...ibm.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net] net: filter: x86: fix JIT address randomization

From: "H. Peter Anvin" <hpa@...or.com>
Date: Tue, 13 May 2014 14:28:55 -0700

> On 05/13/2014 01:34 PM, Alexei Starovoitov wrote:
>>>
>>> The argument of not having code ending on (or being very close of) page
>>> boundary seems orthogonal to this bug fix.
>> 
>> Gotta pick some number... page/2 seems good enough to have
>> large range for prandom() to choose and better performance.
>> Another alternative is to do min(…, PAGE_SIZE - sizeof(*header)),
>> but that is harder to understand.
>> 
> 
> The latter is correct by construction, and thus doesn't end up with the
> question "what is going on here" or has hidden failure conditions.

Agreed.

>> Also just realized that I miscalculated the breaking point:
>> "If prandom_u32() % hole selects a number >= 4096, then kernel will crash"
>> it should read: "… >= 4092 ..."
>> since sizeof(*header) needs to be accounted for.
> 
> No, it should read PAGE_SIZE - sizeof(*header) if anything.

Also agreed.
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