linux-kernel - Re: [PATCH] KVM: x86: never trap MSR_KERNEL_GS

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Date:   Mon, 24 Sep 2018 09:04:57 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: never trap MSR_KERNEL_GS_BASE

On Mon, 2018-09-24 at 17:38 +0200, Paolo Bonzini wrote:
> KVM has an old optimization whereby accesses to the kernel GS base MSR
> are trapped when the guest is in 32-bit and not when it is in 64-bit mode.
> The idea is that swapgs is not available in 32-bit mode and thus the
> guest has no reason to access the MSR unless in 64-bit mode.  Therefore
> 32-bit applications need not pay the price of switching the kernel GS
> base between the host and the guest values, 64-bit applications.
> 
> However, this optimization adds complexity to the code for little
> benefit (these days most guests are going to be 64-bit anyway) and in fact
> broke after commit 678e315e78a7 ("KVM: vmx: add dedicated utility to
> access guest's kernel_gs_base", 2018-08-06); the guest kernel GS base
> can be corrupted across SMIs and UEFI Secure Boot is therefore broken
> (a secure boot Linux guest, for example, fails to reach the login prompt
> about half the time).  This patch just removes the optimization; the
> kernel GS base MSR is now never trapped by KVM, similarly to the FS and
> GS base MSRs.
> 
> Fixes: 678e315e78a780dbef384b92339c8414309dbc11
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Reviewed-by: Sean Christopherson <sean.j.christopherson@...el.com>