Re: [PATCH v2] KVM: nVMX: Tweak handling of failure code for nested VM-Enter failure

[Jim Mattson]

On Tue, Apr 28, 2020 at 10:32 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> Use an enum for passing around the failure code for a failed VM-Enter
> that results in VM-Exit to provide a level of indirection from the final
> resting place of the failure code, vmcs.EXIT_QUALIFICATION.  The exit
> qualification field is an unsigned long, e.g. passing around
> 'u32 exit_qual' throws up red flags as it suggests KVM may be dropping
> bits when reporting errors to L1.  This is a red herring because the
> only defined failure codes are 0, 2, 3, and 4, i.e. don't come remotely
> close to overflowing a u32.
>
> Setting vmcs.EXIT_QUALIFICATION on entry failure is further complicated
> by the MSR load list, which returns the (1-based) entry that failed, and
> the number of MSRs to load is a 32-bit VMCS field.  At first blush, it
> would appear that overflowing a u32 is possible, but the number of MSRs
> that can be loaded is hardcapped at 4096 (limited by MSR_IA32_VMX_MISC).
>
> In other words, there are two completely disparate types of data that
> eventually get stuffed into vmcs.EXIT_QUALIFICATION, neither of which is
> an 'unsigned long' in nature.  This was presumably the reasoning for
> switching to 'u32' when the related code was refactored in commit
> ca0bde28f2ed6 ("kvm: nVMX: Split VMCS checks from nested_vmx_run()").
>
> Using an enum for the failure code addresses the technically-possible-
> but-will-never-happen scenario where Intel defines a failure code that
> doesn't fit in a 32-bit integer.  The enum variables and values will
> either be automatically sized (gcc 5.4 behavior) or be subjected to some
> combination of truncation.  The former case will simply work, while the
> latter will trigger a compile-time warning unless the compiler is being
> particularly unhelpful.
>
> Separating the failure code from the failed MSR entry allows for
> disassociating both from vmcs.EXIT_QUALIFICATION, which avoids the
> conundrum where KVM has to choose between 'u32 exit_qual' and tracking
> values as 'unsigned long' that have no business being tracked as such.
> To cement the split, set vmcs12->exit_qualification directly from the
> entry error code or failed MSR index instead of bouncing through a local
> variable.
>
> Opportunistically rename the variables in load_vmcs12_host_state() and
> vmx_set_nested_state() to call out that they're ignored, set exit_reason
> on demand on nested VM-Enter failure, and add a comment in
> nested_vmx_load_msr() to call out that returning 'i + 1' can't wrap.
>
> No functional change intended.
>
> Reported-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>