linux-kernel - [PATCH v4 09/10] KVM: x86/MMU: Require reboot permission to disable NX hugepages

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Message-Id: <20220411211015.3091615-10-bgardon@google.com>
Date:   Mon, 11 Apr 2022 14:10:14 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: [PATCH v4 09/10] KVM: x86/MMU: Require reboot permission to disable
 NX hugepages

Ensure that the userspace actor attempting to disable NX hugepages has
permission to reboot the system. Since disabling NX hugepages would
allow a guest to crash the system, it is similar to reboot permissions.

This approach is the simplest permission gating, but passing a file
descriptor opened for write for the module parameter would also work
well and be more precise.
The latter approach was suggested by Sean Christopherson.

Suggested-by: Jim Mattson <jmattson@...gle.com>
Signed-off-by: Ben Gardon <bgardon@...gle.com>
---
 Documentation/virt/kvm/api.rst | 2 ++
 arch/x86/kvm/x86.c             | 9 +++++++++
 2 files changed, 11 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 31fb002632bb..021452a9fa91 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7861,6 +7861,8 @@ should adjust CPUID leaf 0xA to reflect that the PMU is disabled.
 :Capability KVM_CAP_PMU_CAPABILITY
 :Architectures: x86
 :Type: vm
+:Returns 0 on success, -EPERM if the userspace process does not
+	 have CAP_SYS_BOOT
 
 This capability disables the NX huge pages mitigation for iTLB MULTIHIT.
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index de1d211f8aa3..8d3d6c48c5ec 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6081,6 +6081,15 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		mutex_unlock(&kvm->lock);
 		break;
 	case KVM_CAP_VM_DISABLE_NX_HUGE_PAGES:
+		/*
+		 * Since the risk of disabling NX hugepages is a guest crashing
+		 * the system, ensure the userspace process has permission to
+		 * reboot the system.
+		 */
+		if (!capable(CAP_SYS_BOOT)) {
+			r = -EPERM;
+			break;
+		}
 		kvm->arch.disable_nx_huge_pages = true;
 		kvm_update_nx_huge_pages(kvm);
 		r = 0;
-- 
2.35.1.1178.g4f1659d476-goog