From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	 Will Deacon <will@kernel.org>, Michal Luczaj <mhal@rbox.co>,
	Sean Christopherson <seanjc@google.com>,
	 Alexander Potapenko <glider@google.com>,
	Marc Zyngier <maz@kernel.org>,
	 Oliver Upton <oliver.upton@linux.dev>
Subject: [PATCH 2/6] KVM: Verify there's at least one online vCPU when iterating over all vCPUs
Date: Wed,  9 Oct 2024 08:04:51 -0700	[thread overview]
Message-ID: <20241009150455.1057573-3-seanjc@google.com> (raw)
In-Reply-To: <20241009150455.1057573-1-seanjc@google.com>

Explicitly check that there is at least online vCPU before iterating over
all vCPUs.  Because the max index is an unsigned long, passing "0 - 1" in
the online_vcpus==0 case results in xa_for_each_range() using an unlimited
max, i.e. allows it to access vCPU0 when it shouldn't.  This will allow
KVM to safely _erase_ from vcpu_array if the last stages of vCPU creation
fail, i.e. without generating a use-after-free if a different task happens
to be concurrently iterating over all vCPUs.

Note, because xa_for_each_range() is a macro, kvm_for_each_vcpu() subtly
reloads online_vcpus after each iteration, i.e. adding an extra load
doesn't meaningfully impact the total cost of iterating over all vCPUs.
And because online_vcpus is never decremented, there is no risk of a
reload triggering a walk of the entire xarray.

Cc: Will Deacon <will@kernel.org>
Cc: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 include/linux/kvm_host.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 450dd0444a92..5fe3b0c28fb3 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -985,9 +985,10 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
 	return xa_load(&kvm->vcpu_array, i);
 }
 
-#define kvm_for_each_vcpu(idx, vcpup, kvm)		   \
-	xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0, \
-			  (atomic_read(&kvm->online_vcpus) - 1))
+#define kvm_for_each_vcpu(idx, vcpup, kvm)				\
+	if (atomic_read(&kvm->online_vcpus))				\
+		xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0,	\
+				  (atomic_read(&kvm->online_vcpus) - 1))
 
 static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
 {
-- 
2.47.0.rc0.187.ge670bccf7e-goog

next prev parent reply	other threads:[~2024-10-09 15:05 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-10-09 15:04 [PATCH 0/6] KVM: Fix bugs in vCPUs xarray usage Sean Christopherson
2024-10-09 15:04 ` [PATCH 1/6] KVM: Explicitly verify target vCPU is online in kvm_get_vcpu() Sean Christopherson
2024-10-10  5:31   ` Gupta, Pankaj
2024-10-10 15:54     ` Sean Christopherson
2024-10-10 16:33       ` Gupta, Pankaj
2024-10-09 15:04 ` Sean Christopherson [this message]
2024-10-09 15:04 ` [PATCH 3/6] KVM: Grab vcpu->mutex across installing the vCPU's fd and bumping online_vcpus Sean Christopherson
2024-10-09 15:04 ` [PATCH 4/6] Revert "KVM: Fix vcpu_array[0] races" Sean Christopherson
2024-10-10 12:46   ` Paolo Bonzini
2024-10-10 17:48     ` Sean Christopherson
2024-10-20 11:23       ` Paolo Bonzini
2024-12-16 23:05         ` Sean Christopherson
2024-10-09 15:04 ` [PATCH 5/6] KVM: Don't BUG() the kernel if xa_insert() fails with -EBUSY Sean Christopherson
2024-10-10  5:33   ` Gupta, Pankaj
2024-10-09 15:04 ` [PATCH 6/6] KVM: Drop hack that "manually" informs lockdep of kvm->lock vs. vcpu->mutex Sean Christopherson
2024-10-29 14:18 ` [PATCH 0/6] KVM: Fix bugs in vCPUs xarray usage Will Deacon
2024-12-19  2:40 ` Sean Christopherson
2024-12-19 14:18   ` Paolo Bonzini

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