Re: [RFC 0/5] vmalloc_exec for modules and BPF programs

[Christophe Leroy]

Le 23/08/2022 à 07:42, Peter Zijlstra a écrit :
> On Mon, Aug 22, 2022 at 04:56:47PM +0000, Song Liu wrote:
>>
>>
>>> On Aug 22, 2022, at 9:34 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>>
>>> On Mon, Aug 22, 2022 at 03:46:38PM +0000, Song Liu wrote:
>>>> Could you please share your feedback on this?
>>>
>>> I've looked at it all of 5 minutes, so perhaps I've missed something.
>>>
>>> However, I'm a little surprised you went with a second tree instead of
>>> doing the top-down thing for data. The way you did it makes it hard to
>>> have guard pages between text and data.
>>
>> I didn't realize the importance of the guard pages. But it is not too
> 
> I'm not sure how important it is, just seems like a good idea to trap
> anybody trying to cross that divide. Also, to me it seems like a good
> idea to have a single large contiguous text region instead of splintered
> 2M pages.
> 
>> hard to do it with this approach. For each 2MB text page, we can reserve
>> 4kB on the beginning and end of it. Would this work?
> 
> Typically a guard page has different protections (as in none what so
> ever) so that every access goes *splat*. >

Text is RO-X, on some architectures even only X. So the only real thing 
to protect against is bad execution, isn't it ?. So I guess having some 
areas with invalid or trap instructions would be enough ?