linux-kernel - [PATCH 3/3] KVM: Explicitly disallow activatating a gfn_to_pfn_cache with INVALID

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Date: Tue, 19 Mar 2024 17:15:42 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Sean Christopherson <seanjc@...gle.com>, syzbot+106a4f72b0474e1d1b33@...kaller.appspotmail.com, 
	David Woodhouse <dwmw2@...radead.org>, Paul Durrant <paul@....org>
Subject: [PATCH 3/3] KVM: Explicitly disallow activatating a gfn_to_pfn_cache
 with INVALID_GPA

Explicit disallow activating a gfn_to_pfn_cache with an error gpa, i.e.
INVALID_GPA, to ensure that KVM doesn't mistake a GPA-based cache for an
HVA-based cache (KVM uses INVALID_GPA as a magic value to differentiate
between GPA-based and HVA-based caches).

WARN if KVM attempts to activate a cache with INVALID_GPA, purely so that
new caches need to at least consider what to do with a "bad" GPA, as all
existing usage of kvm_gpc_activate() guarantees gpa != INVALID_GPA.  I.e.
removing the WARN in the future is completely reasonable if doing so would
yield cleaner/better code overall.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 virt/kvm/pfncache.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 91b0e329006b..f618719644e0 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -418,6 +418,13 @@ static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned

 int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
 {
+	/*
+	 * Explicitly disallow INVALID_GPA so that the magic value can be used
+	 * by KVM to differentiate between GPA-based and HVA-based caches.
+	 */
+	if (WARN_ON_ONCE(kvm_is_error_gpa(gpa)))
+		return -EINVAL;
+
 	return __kvm_gpc_activate(gpc, gpa, KVM_HVA_ERR_BAD, len);
 }

-- 
2.44.0.291.gc1ea87d7ee-goog