lkml.org 
[lkml]   [2022]   [Apr]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH] x86/speculation, KVM: respect user IBPB configuration
Date


> On Apr 18, 2022, at 12:28 PM, Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, Apr 15, 2022, Jon Kohler wrote:
>>
>>> On Apr 15, 2022, at 10:28 AM, Sean Christopherson <seanjc@google.com> wrote:
>>> But stepping back, why does KVM do its own IBPB in the first place? The goal is
>>> to prevent one vCPU from attacking the next vCPU run on the same pCPU. But unless
>>> userspace is running multiple VMs in the same process/mm_struct, switching vCPUs,
>>> i.e. switching tasks, will also switch mm_structs and thus do IPBP via cond_mitigation.
>>
>> Good question, I couldn’t figure out the answer to this by walking the code and looking
>> at git history/blame for this area. Are there VMMs that even run multiple VMs within
>> the same process? The only case I could think of is a nested situation?
>
> Selftests? :-)

Ah! I’ll take a mulligan, I was only thinking about run of the mill user stuff, not the tests, thx.

>
>>> If userspace runs multiple VMs in the same process, enables cond_ipbp, _and_ sets
>>> TIF_SPEC_IB, then it's being stupid and isn't getting full protection in any case,
>>> e.g. if userspace is handling an exit-to-userspace condition for two vCPUs from
>>> different VMs, then the kernel could switch between those two vCPUs' tasks without
>>> bouncing through KVM and thus without doing KVM's IBPB.
>>
>> Exactly, so meaning that the only time this would make sense is for some sort of nested
>> situation or some other funky VMM tomfoolery, but that nested hypervisor might not be
>> KVM, so it's a farce, yea? Meaning that even in that case, there is zero guarantee
>> from the host kernel perspective that barriers within that process are being issued on
>> switch, which would make this security posture just window dressing?
>>
>>>
>>> I can kinda see doing this for always_ibpb, e.g. if userspace is unaware of spectre
>>> and is naively running multiple VMs in the same process.
>>
>> Agreed. I’ve thought of always_ibpb as "paranoid mode" and if a user signs up for that,
>> they rarely care about the fast path / performance implications, even if some of the
>> security surface area is just complete window dressing :(
>>
>> Looking forward, what if we simplified this to have KVM issue barriers IFF always_ibpb?
>>
>> And drop the cond’s, since the switching mm_structs should take care of that?
>>
>> The nice part is that then the cond_mitigation() path handles the going to thread
>> with flag or going from a thread with flag situation gracefully, and we don’t need to
>> try to duplicate that smarts in kvm code or somewhere else.
>
> Unless there's an edge case we're overlooking, that has my vote. And if the
> above is captured in a comment, then there shouldn't be any confusion as to why
> the kernel/KVM is consuming a flag named "switch_mm" when switching vCPUs, i.e.
> when there may or may not have been a change in mm structs.

Ok great. I’ll work up a v2 and send it out.

\
 
 \ /
  Last update: 2022-04-18 21:36    [W:0.052 / U:0.848 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site