Re: [PATCH 6.12.y 4/5] KVM: x86: model canonical checks more precisely

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 23, 2025, Sasha Levin wrote:
> From: Maxim Levitsky <mlevitsk@redhat.com>
> 
> [ Upstream commit 9245fd6b8531497d129a7a6e3eef258042862f85 ]
> 
> As a result of a recent investigation, it was determined that x86 CPUs
> which support 5-level paging, don't always respect CR4.LA57 when doing
> canonical checks.
> 
> In particular:
> 
> 1. MSRs which contain a linear address, allow full 57-bitcanonical address
> regardless of CR4.LA57 state. For example: MSR_KERNEL_GS_BASE.
> 
> 2. All hidden segment bases and GDT/IDT bases also behave like MSRs.
> This means that full 57-bit canonical address can be loaded to them
> regardless of CR4.LA57, both using MSRS (e.g GS_BASE) and instructions
> (e.g LGDT).
> 
> 3. TLB invalidation instructions also allow the user to use full 57-bit
> address regardless of the CR4.LA57.
> 
> Finally, it must be noted that the CPU doesn't prevent the user from
> disabling 5-level paging, even when the full 57-bit canonical address is
> present in one of the registers mentioned above (e.g GDT base).
> 
> In fact, this can happen without any userspace help, when the CPU enters
> SMM mode - some MSRs, for example MSR_KERNEL_GS_BASE are left to contain
> a non-canonical address in regard to the new mode.
> 
> Since most of the affected MSRs and all segment bases can be read and
> written freely by the guest without any KVM intervention, this patch makes
> the emulator closely follow hardware behavior, which means that the
> emulator doesn't take in the account the guest CPUID support for 5-level
> paging, and only takes in the account the host CPU support.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> Link: https://lore.kernel.org/r/20240906221824.491834-4-mlevitsk@redhat.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Stable-dep-of: fa787ac07b3c ("KVM: x86/hyper-v: Skip non-canonical addresses during PV TLB flush")
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---

Acked-by: Sean Christopherson <seanjc@google.com>