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KVM: SVM: Disallow guest from changing userspace's MSR_AMD64_DE_CFG value

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Inject a #GP if the guest attempts to change MSR_AMD64_DE_CFG from its
*current* value, not if the guest attempts to write a value other than
KVM's set of supported bits.  As per the comment and the changelog of the
original code, the intent is to effectively make MSR_AMD64_DE_CFG read-
only for the guest.

Opportunistically use a more conventional equality check instead of an
exclusive-OR check to detect attempts to change bits.

Fixes: d1d93fa90f ("KVM: SVM: Add MSR-based feature support for serializing LFENCE")
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240802181935.292540-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2024-08-02 11:19:26 -07:00
parent e0183a42e3
commit 74a0e79df6
1 changed files with 7 additions and 2 deletions
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9
arch/x86/kvm/svm/svm.c
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@ -3189,8 +3189,13 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
if (data & ~msr_entry.data)
return 1;
/* Don't allow the guest to change a bit, #GP */
if (!msr->host_initiated && (data ^ msr_entry.data))
/*
* Don't let the guest change the host-programmed value. The
* MSR is very model specific, i.e. contains multiple bits that
* are completely unknown to KVM, and the one bit known to KVM
* is simply a reflection of hardware capabilities.
*/
if (!msr->host_initiated && data != svm->msr_decfg)
return 1;
svm->msr_decfg = data;