Re: [patch V2 21/28] x86/speculation: Prepare for conditional IBPB in switch_mm()

[Andy Lutomirski]

> On Nov 25, 2018, at 2:20 PM, Thomas Gleixner <t...@linutronix.de> wrote:
> 
> On Sun, 25 Nov 2018, Andi Kleen wrote:
> 
>>> The current check whether two tasks belong to the same context is using the
>>> tasks context id. While correct, it's simpler to use the mm pointer because
>>> it allows to mangle the TIF_SPEC_IB bit into it. The context id based
>>> mechanism requires extra storage, which creates worse code.
>> 
>> [We tried similar in some really early versions, but it was replaced
>> with the context id later.]
>> 
>> One issue with using the pointer is that the pointer can be reused
>> when the original mm_struct is freed, and then gets reallocated
>> immediately to an attacker. Then the attacker may avoid the IBPB.
>> 
>> Given it's probably hard to generate any reasonable leak bandwidth with
>> such a complex scenario, but it still seemed better to close the hole.
> 
> Sorry, but that's really a purely academic exercise. 
> 
> 

I would guess that it’s actually very easy to force mm_struct* reuse.  Don’t 
the various allocators try to allocate hot memory?  There’s nothing hotter than 
a just-freed allocation of the same size.

Can someone explain the actual problem with ctx_id?  If you just need an extra 
bit, how about:

2*ctx_id vs 2*ctx_id+1

Or any of the many variants of approximately the same thing?

—Andy