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

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 31, 2023, Borislav Petkov wrote:
> 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?

I would prefer to take this through KVM, there's enough subtle complexity in this
code that it'd be nice to have it close by.

If you're happy with patch 1, maybe ack that one and take it through KVM, and
route patch 3 through tip?