FAILED: patch "[PATCH] KVM: VMX: Handle bad values on proxied writes to LBR MSRs" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x ca674df13b195eb6d124ab059799d4e03fa40624
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026071631-unblock-ambiguous-5bd2@gregkh' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From ca674df13b195eb6d124ab059799d4e03fa40624 Mon Sep 17 00:00:00 2001
From: Xuanqing Shi <[email protected]>
Date: Tue, 26 May 2026 19:26:17 -0700
Subject: [PATCH] KVM: VMX: Handle bad values on proxied writes to LBR MSRs

Use the "safe" WRMSR API when writing LBRs on behalf of the guest (or host userspace), and propagate any errors back to the instigator, as the value being written is untrusted. E.g. if the guest (or host userspace) attempts to set reserved bits in LBR_SELECT, then KVM needs to return an error, and not WARN on the bad value.

Continue using the "unsafe" version of RDMSR, as it should be impossible to reach the helper with a completely bogus MSR, i.e. WARNing on RDMSR failure is very desirable, e.g. to make KVM bugs more visible.

unchecked MSR access error: WRMSR to 0x1c8 (tried to write 0x0000000000004000)

  Call Trace:
   intel_pmu_set_msr+0x4e0/0x7f0 [kvm_intel]
   kvm_pmu_set_msr+0x17e/0x1c0 [kvm]
   kvm_set_msr_common+0xc76/0x1440 [kvm]
   vmx_set_msr+0x5e6/0x1570 [kvm_intel]
   kvm_emulate_wrmsr+0x54/0x1d0 [kvm]
   vmx_handle_exit+0x7fc/0x970 [kvm_intel]

index 27eb76e6b6a0..22138c487216 100644
@@ -308,13 +308,15 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
 	 */
 	local_irq_disable();
 	if (lbr_desc->event->state == PERF_EVENT_STATE_ACTIVE) {
+		int err = 0;
+
 		if (read)
 			rdmsrq(index, msr_info->data);
 		else
-			wrmsrq(index, msr_info->data);
+			err = wrmsrq_safe(index, msr_info->data);
 		__set_bit(INTEL_PMC_IDX_FIXED_VLBR, vcpu_to_pmu(vcpu)->pmc_in_use);
 		local_irq_enable();
-		return true;
+		return !err;
 	}
 	clear_bit(INTEL_PMC_IDX_FIXED_VLBR, vcpu_to_pmu(vcpu)->pmc_in_use);
 	local_irq_enable();