Re: FAILED: patch "[PATCH] KVM: x86/mmu: Treat NX as used (not reserved) for all !TDP" failed to apply to 5.4-stable tree

[Sudip Mukherjee <sudipm.mukherjee@gmail.com>]

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Hi Greg,

On Sun, Jul 11, 2021 at 02:40:49PM +0200, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 5.4-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 <stable@vger.kernel.org>.

Here is the backport.

--
Regards
Sudip

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From 925625f45cb571d12230c9254261a375250bb806 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Tue, 22 Jun 2021 10:56:47 -0700
Subject: [PATCH] KVM: x86/mmu: Treat NX as used (not reserved) for all !TDP
 shadow MMUs

commit 112022bdb5bc372e00e6e43cb88ee38ea67b97bd upstream

Mark NX as being used for all non-nested shadow MMUs, as KVM will set the
NX bit for huge SPTEs if the iTLB mutli-hit mitigation is enabled.
Checking the mitigation itself is not sufficient as it can be toggled on
at any time and KVM doesn't reset MMU contexts when that happens.  KVM
could reset the contexts, but that would require purging all SPTEs in all
MMUs, for no real benefit.  And, KVM already forces EFER.NX=1 when TDP is
disabled (for WP=0, SMEP=1, NX=0), so technically NX is never reserved
for shadow MMUs.

Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[sudip: use old path]
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
---
 arch/x86/kvm/mmu.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 260c64c205b8..d3877dd713ae 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4666,7 +4666,15 @@ static void reset_rsvds_bits_mask_ept(struct kvm_vcpu *vcpu,
 void
 reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
 {
-	bool uses_nx = context->nx ||
+	/*
+	 * KVM uses NX when TDP is disabled to handle a variety of scenarios,
+	 * notably for huge SPTEs if iTLB multi-hit mitigation is enabled and
+	 * to generate correct permissions for CR0.WP=0/CR4.SMEP=1/EFER.NX=0.
+	 * The iTLB multi-hit workaround can be toggled at any time, so assume
+	 * NX can be used by any non-nested shadow MMU to avoid having to reset
+	 * MMU contexts.  Note, KVM forces EFER.NX=1 when TDP is disabled.
+	 */
+	bool uses_nx = context->nx || !tdp_enabled ||
 		context->mmu_role.base.smep_andnot_wp;
 	struct rsvd_bits_validate *shadow_zero_check;
 	int i;
-- 
2.30.2