Messages in this thread | | | Subject | Re: [patch V2 21/28] x86/speculation: Prepare for conditional IBPB in switch_mm() | From | Andy Lutomirski <> | Date | Sun, 25 Nov 2018 15:04:33 -0800 |
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> On Nov 25, 2018, at 2:20 PM, Thomas Gleixner <tglx@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
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