Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

H. Peter Anvin
2014-04-25T21:20:34Z

On 04/25/2014 05:02 AM, Pavel Machek wrote:
>
> Just to understand the consequences -- we leak 16 bit of kernel data
> to the userspace, right? Because it is %esp, we know that we leak
> stack address, which is not too sensitive, but will make kernel
> address randomization less useful...?
>

It is rather sensitive, in fact.

>> The 64-bit implementation works like this:
>>
>> Set up a ministack for each CPU, which is then mapped 65536 times
>> using the page tables. This implementation uses the second-to-last
>> PGD slot for this; with a 64-byte espfix stack this is sufficient for
>> 2^18 CPUs (currently we support a max of 2^13 CPUs.)
>
> 16-bit stack segments on 64-bit machine. Who still uses it? Dosemu?
> Wine? Would the solution be to disallow that?

Welcome to the show. We do, in fact disallow it now in the 3.15-rc
series. The Wine guys are complaining.

-hpa