[PATCH 2/7] KVM: x86/mmu: Conditionally resched when retrying gPTE walk on A/D update failure

[Paolo Bonzini]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

Conditionally reschedule when retrying a guest PTE walk because the CMPXCHG
to update A/D bits failed, e.g. because a different task/vCPU modified the
relevant gPTEs.  On kernels without involuntary preemption of some kind, a
misbehaving guest can "capture" a pCPU effectively indefinitely and trigger
soft lockups by continuously modifying its gPTEs such that the result of
the walk is remains valid.

Fixes: b3e4e63fd9ee ("KVM: MMU: Use cmpxchg for pte updates on walk_addr()")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Stefan Teodorescu <fane@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Message-ID: <20261022015903.282261-1-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/paging_tmpl.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 8e350095508c..18f7eb633127 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -340,6 +340,13 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 	gpa_t real_gpa;
 	gfn_t gfn;
 
+	/*
+	 * KVM might sleep/resched when retrying the walk if the updating A/D
+	 * fails because the gPTE changed between reading the gPTE and trying
+	 * the CMPXHG to set A/D bits.
+	 */
+	might_sleep();
+
 	trace_kvm_mmu_pagetable_walk(addr, access);
 retry_walk:
 	walker->level = w->cpu_role.base.level;
@@ -482,8 +489,10 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 							addr, write_fault);
 		if (unlikely(ret < 0))
 			goto error;
-		else if (ret)
+		else if (ret) {
+			cond_resched();
 			goto retry_walk;
+		}
 	}
 
 	return 1;
-- 
2.52.0