Re: [PATCH kernel v3 2/3] KVM: SEV: Enable data breakpoints in SEV-ES

[Borislav Petkov]

Hey Sean,

On Fri, Jan 20, 2023 at 02:10:46PM +1100, Alexey Kardashevskiy wrote:
> Prior to SEV-ES, KVM stored/loaded host debug registers upon switching
> to/from a VM. Changing those registers inside a running SEV VM
> triggered #VC exit to KVM.
> 
> SEV-ES added the encrypted state (ES) which uses an encrypted guest page
> for the VM state (VMSA). The hardware saves/restores certain registers on
> VMRUN/VMEXIT according to a swap type (A, B, C), see
> "Table B-3. Swap Types" in the AMD Architecture Programmer’s Manual
> volume 2.
> 
> AMD Milan (Fam 19h) introduces support for the debug registers swapping.
> DR6 and DR7 are always swapped. DR[0-3] and DR[0-3]_ADDR_MASK are swapped
> a type B when SEV_FEATURES[5] ("DebugSwap") is set.
> 
> Enable DebugSwap in VMSA. But only do so if CPUID Fn80000021_EAX[0]
> ("NoNestedDataBp", "Processor ignores nested data breakpoints") is
> supported by the SOC as otherwise a malicious SEV-ES guest can set up
> data breakpoints on the #VC IDT entry/stack and cause an infinite loop.
> 
> Eliminate DR7 and #DB intercepts as:
> - they are not needed when DebugSwap is supported;
> - #VC for these intercepts is most likely not supported anyway and
> kills the VM.
> Keep DR7 intercepted unless DebugSwap enabled to prevent the infinite #DB
> loop DoS.

...

ok to take this through the tip tree?

Thx.

-- 
Regards/Gruss,
    Boris.

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