MIPS: KVM: Fix mapped fault broken commpage handling

James Hogan · rkrcmar · commit c604cffa9347 · 2016-08-12T12:01:28.000+02:00
kvm_mips_handle_mapped_seg_tlb_fault() appears to map the guest page at virtual address 0 to PFN 0 if the guest has created its own mapping there. The intention is unclear, but it may have been an attempt to protect the zero page from being mapped to anything but the comm page in code paths you wouldn't expect from genuine commpage accesses (guest kernel mode cache instructions on that address, hitting trapping instructions when executing from that address with a coincidental TLB eviction during the KVM handling, and guest user mode accesses to that address). Fix this to check for mappings exactly at KVM_GUEST_COMMPAGE_ADDR (it may not be at address 0 since commit 42aa12e ("MIPS: KVM: Move commpage so 0x0 is unmapped")), and set the corresponding EntryLo to be interpreted as 0 (invalid). Fixes: 858dd5d ("KVM/MIPS32: MMU/TLB operations for the Guest.") Signed-off-by: James Hogan <james.hogan@imgtec.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: "Radim Krčmář" <rkrcmar@redhat.com> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: linux-mips@linux-mips.org Cc: kvm@vger.kernel.org Cc: <stable@vger.kernel.org> # 3.10.x- Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
diff --git a/arch/mips/kvm/mmu.c b/arch/mips/kvm/mmu.c
@@ -138,35 +138,42 @@ int kvm_mips_handle_mapped_seg_tlb_fault(struct kvm_vcpu *vcpu,
 	unsigned long entryhi = 0, entrylo0 = 0, entrylo1 = 0;
 	struct kvm *kvm = vcpu->kvm;
 	kvm_pfn_t pfn0, pfn1;
+	long tlb_lo[2];
 	int ret;
 
-	if ((tlb->tlb_hi & VPN2_MASK) == 0) {
-		pfn0 = 0;
-		pfn1 = 0;
-	} else {
-		if (kvm_mips_map_page(kvm, mips3_tlbpfn_to_paddr(tlb->tlb_lo[0])
-					   >> PAGE_SHIFT) < 0)
-			return -1;
-
-		if (kvm_mips_map_page(kvm, mips3_tlbpfn_to_paddr(tlb->tlb_lo[1])
-					   >> PAGE_SHIFT) < 0)
-			return -1;
-
-		pfn0 = kvm->arch.guest_pmap[
-			mips3_tlbpfn_to_paddr(tlb->tlb_lo[0]) >> PAGE_SHIFT];
-		pfn1 = kvm->arch.guest_pmap[
-			mips3_tlbpfn_to_paddr(tlb->tlb_lo[1]) >> PAGE_SHIFT];
-	}
+	tlb_lo[0] = tlb->tlb_lo[0];
+	tlb_lo[1] = tlb->tlb_lo[1];
+
+	/*
+	 * The commpage address must not be mapped to anything else if the guest
+	 * TLB contains entries nearby, or commpage accesses will break.
+	 */
+	if (!((tlb->tlb_hi ^ KVM_GUEST_COMMPAGE_ADDR) &
+			VPN2_MASK & (PAGE_MASK << 1)))
+		tlb_lo[(KVM_GUEST_COMMPAGE_ADDR >> PAGE_SHIFT) & 1] = 0;
+
+	if (kvm_mips_map_page(kvm, mips3_tlbpfn_to_paddr(tlb_lo[0])
+				   >> PAGE_SHIFT) < 0)
+		return -1;
+
+	if (kvm_mips_map_page(kvm, mips3_tlbpfn_to_paddr(tlb_lo[1])
+				   >> PAGE_SHIFT) < 0)
+		return -1;
+
+	pfn0 = kvm->arch.guest_pmap[
+		mips3_tlbpfn_to_paddr(tlb_lo[0]) >> PAGE_SHIFT];
+	pfn1 = kvm->arch.guest_pmap[
+		mips3_tlbpfn_to_paddr(tlb_lo[1]) >> PAGE_SHIFT];
 
 	/* Get attributes from the Guest TLB */
 	entrylo0 = mips3_paddr_to_tlbpfn(pfn0 << PAGE_SHIFT) |
 		((_page_cachable_default >> _CACHE_SHIFT) << ENTRYLO_C_SHIFT) |
-		(tlb->tlb_lo[0] & ENTRYLO_D) |
-		(tlb->tlb_lo[0] & ENTRYLO_V);
+		(tlb_lo[0] & ENTRYLO_D) |
+		(tlb_lo[0] & ENTRYLO_V);
 	entrylo1 = mips3_paddr_to_tlbpfn(pfn1 << PAGE_SHIFT) |
 		((_page_cachable_default >> _CACHE_SHIFT) << ENTRYLO_C_SHIFT) |
-		(tlb->tlb_lo[1] & ENTRYLO_D) |
-		(tlb->tlb_lo[1] & ENTRYLO_V);
+		(tlb_lo[1] & ENTRYLO_D) |
+		(tlb_lo[1] & ENTRYLO_V);
 
 	kvm_debug("@ %#lx tlb_lo0: 0x%08lx tlb_lo1: 0x%08lx\n", vcpu->arch.pc,
 		  tlb->tlb_lo[0], tlb->tlb_lo[1]);
