Patch "KVM: PPC: Book3S HV: Save/restore TM state in H_CEDE" has been added to the 4.7-stable tree

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

KVM: PPC: Book3S HV: Save/restore TM state in H_CEDE

to the 4.7-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-save-restore-tm-state-in-h_cede.patch
and it can be found in the queue-4.7 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, 22 Jun 2016 15:52:55 +1000
Subject: KVM: PPC: Book3S HV: Save/restore TM state in H_CEDE

From: Paul Mackerras <[email protected]>

commit 93d17397e4e2182fdaad503e2f9da46202c0f1c3 upstream.

It turns out that if the guest does a H_CEDE while the CPU is in a transactional state, and the H_CEDE does a nap, and the nap loses the architected state of the CPU (which is is allowed to do), then we lose the checkpointed state of the virtual CPU. In addition, the transactional-memory state recorded in the MSR gets reset back to non-transactional, and when we try to return to the guest, we take a TM bad thing type of program interrupt because we are trying to transition from non-transactional to transactional with a hrfid instruction, which is not permitted.

The result of the program interrupt occurring at that point is that the host CPU will hang in an infinite loop with interrupts disabled. Thus this is a denial of service vulnerability in the host which can be triggered by any guest (and depending on the guest kernel, it can potentially triggered by unprivileged userspace in the guest).

This vulnerability has been assigned the ID CVE-2016-5412.

To fix this, we save the TM state before napping and restore it on exit from the nap, when handling a H_CEDE in real mode. The case where H_CEDE exits to host virtual mode is already OK (as are other hcalls which exit to host virtual mode) because the exit path saves the TM state.

---
 arch/powerpc/kvm/book3s_hv_rmhandlers.S |   13 +++++++++++++
 1 file changed, 13 insertions(+)

@@ -2037,6 +2037,13 @@ _GLOBAL(kvmppc_h_cede)		/* r3 = vcpu poi
 	/* save FP state */
 	bl	kvmppc_save_fp
 
+#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
+BEGIN_FTR_SECTION
+	ld	r9, HSTATE_KVM_VCPU(r13)
+	bl	kvmppc_save_tm
+END_FTR_SECTION_IFSET(CPU_FTR_TM)
+#endif
+
 	/*
 	 * Set DEC to the smaller of DEC and HDEC, so that we wake
 	 * no later than the end of our timeslice (HDEC interrupts

@@ -2113,6 +2120,12 @@ kvm_end_cede:
 	bl	kvmhv_accumulate_time
 #endif
 
+#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
+BEGIN_FTR_SECTION
+	bl	kvmppc_restore_tm
+END_FTR_SECTION_IFSET(CPU_FTR_TM)
+#endif
+
 	/* load up FP state */
 	bl	kvmppc_load_fp
 

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

queue-4.7/kvm-ppc-book3s-hv-pull-out-tm-state-save-restore-into-separate-procedures.patch queue-4.7/kvm-ppc-book3s-hv-save-restore-tm-state-in-h_cede.patch