From: Sean Christopherson <seanjc@google.com>
To: amirmohammad.eftekhar@cispa.de
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
rossow@cispa.de
Subject: Re: [PATCH] KVM: SVM: use wrmsrq_safe() when writing the nested TSC ratio MSR
Date: Wed, 22 Jul 2026 06:51:35 -0700 [thread overview]
Message-ID: <amDK50Z9WhMQZwYD@google.com> (raw)
In-Reply-To: <20260722084702.1028983-2-amirmohammad.eftekhar@cispa.de>
On Wed, Jul 22, 2026, amirmohammad.eftekhar@cispa.de wrote:
> From: Amirmohammad Eftekhar <amirmohammad.eftekhar@cispa.de>
>
> __svm_write_tsc_multiplier() writes vcpu->arch.tsc_scaling_ratio to the
> physical MSR_AMD64_TSC_RATIO with a plain wrmsrq(). Under nested TSC
> scaling the ratio is a composed value (see the companion x86.c change),
> and a composed value can set bits above the MSR's 40-bit width; those
> are reserved bits that architecturally fault (#GP) the WRMSR. Today
> only the EX_TYPE_WRMSR extable stands between a guest-influenced ratio
> and a host #GP oops in ring 0 -- on a build or path without it the host
> faults.
>
> Do not depend on the extable for a guest-influenced MSR write: use
> wrmsrq_safe(), and on failure keep the previous ratio (don't update the
> current_tsc_ratio cache) and warn. Defense in depth alongside bounding
> the composed ratio at its source.
NAK. As I said off-list, the EX_TYPE_WRMSR exception fixup _is_ the defense in
depth we want. If KVM does have a bug, I want a WARN.
next prev parent reply other threads:[~2026-07-22 13:51 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-07-22 8:47 [PATCH] KVM: x86: bound the composed nested TSC multiplier to the max ratio amirmohammad.eftekhar
2026-07-22 8:47 ` [PATCH] KVM: SVM: use wrmsrq_safe() when writing the nested TSC ratio MSR amirmohammad.eftekhar
2026-07-22 9:14 ` sashiko-bot
2026-07-22 13:51 ` Sean Christopherson [this message]
2026-07-22 9:16 ` [PATCH] KVM: x86: bound the composed nested TSC multiplier to the max ratio sashiko-bot
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