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

    * Thomas Gleixner <tglx@linutronix.de> wrote:

    > On Sun, 25 Nov 2018, Andy Lutomirski wrote:
    > > > 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.
    >
    > Sure, but this is about a indirect branch predictor attack against
    > something which reuses the mm.
    >
    > So you'd need to pull off:
    >
    > P1 poisons branch predictor
    > P1 exit
    >
    > P2 starts and resuses mm(P1) and uses the poisoned branch predictor
    >
    > the only thing between P1 and P2 is either idle or some other kernel
    > thread, but no other user task. If that happens then the code would not
    > issue IBPB as it assumes to switch back to the same process.
    >
    > Even if you can pull that off the speculation would hit the startup code of
    > P2, which is truly a source of secret information. Creating a valuable
    > attack based on mm reuse is really less proabable than a lottery jackpot.
    >
    > So using mm is really good enough and results in better assembly code which
    > is surely more valuable than addressing some hypothetical hole.

    OTOH we could probably close even this with very little cost if we added
    an IBPB to non-threaded fork() and vfork()+exec() paths? Those are really
    slow paths compared to all the context switch paths we are trying to
    optimize here.

    Alternatively we could IBPB on the post-exit() final task struct freeing,
    which too is a relative slow path compared to the context switch paths.

    But no strong opinion.

    Thanks,

    Ingo

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