Patch "KVM: PPC: Book3S HV: Don't lose hardware R/C bit updates in H_PROTECT" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

KVM: PPC: Book3S HV: Don't lose hardware R/C bit updates in H_PROTECT

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-ppc-book3s-hv-don-t-lose-hardware-r-c-bit-updates-in-h_protect.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Paul Mackerras <[email protected]>
Date: Wed, 16 Nov 2016 16:43:28 +1100
Subject: KVM: PPC: Book3S HV: Don't lose hardware R/C bit updates in H_PROTECT

From: Paul Mackerras <[email protected]>

commit f064a0de1579fabded8990bed93971e30deb9ecb upstream.

The hashed page table MMU in POWER processors can update the R (reference) and C (change) bits in a HPTE at any time until the HPTE has been invalidated and the TLB invalidation sequence has completed. In kvmppc_h_protect, which implements the H_PROTECT hypercall, we read the HPTE, modify the second doubleword, invalidate the HPTE in memory, do the TLB invalidation sequence, and then write the modified value of the second doubleword back to memory. In doing so we could overwrite an R/C bit update done by hardware between when we read the HPTE and when the TLB invalidation completed. To fix this we re-read the second doubleword after the TLB invalidation and OR in the (possibly) new values of R and C. We can use an OR since hardware only ever sets R and C, never clears them.

This race was found by code inspection. In principle this bug could cause occasional guest memory corruption under host memory pressure.

---
 arch/powerpc/kvm/book3s_hv_rm_mmu.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -653,6 +653,8 @@ long kvmppc_h_protect(struct kvm_vcpu *v
 					      HPTE_V_ABSENT);
 			do_tlbies(kvm, &rb, 1, global_invalidates(kvm, flags),
 				  true);
+			/* Don't lose R/C bit updates done by hardware */
+			r |= be64_to_cpu(hpte[1]) & (HPTE_R_R | HPTE_R_C);
 			hpte[1] = cpu_to_be64(r);
 		}
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/kvm-ppc-book3s-hv-don-t-lose-hardware-r-c-bit-updates-in-h_protect.patch queue-4.8/kvm-ppc-book3s-hv-save-restore-xer-in-checkpointed-register-state.patch