Patch "KVM: x86: inject exceptions produced by x86_decode_insn" has been added to the 3.18-stable tree

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

KVM: x86: inject exceptions produced by x86_decode_insn

to the 3.18-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-x86-inject-exceptions-produced-by-x86_decode_insn.patch
and it can be found in the queue-3.18 subdirectory.

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

From: Paolo Bonzini <[email protected]>
Date: Fri, 10 Nov 2017 10:49:38 +0100
Subject: KVM: x86: inject exceptions produced by x86_decode_insn
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From: Paolo Bonzini <[email protected]>

commit 6ea6e84309ca7e0e850b3083e6b09344ee15c290 upstream.

Sometimes, a processor might execute an instruction while another processor is updating the page tables for that instruction's code page, but before the TLB shootdown completes. The interesting case happens if the page is in the TLB.

In general, the processor will succeed in executing the instruction and nothing bad happens. However, what if the instruction is an MMIO access? If *that* happens, KVM invokes the emulator, and the emulator gets the updated page tables. If the update side had marked the code page as non present, the page table walk then will fail and so will x86_decode_insn.

Unfortunately, even though kvm_fetch_guest_virt is correctly returning X86EMUL_PROPAGATE_FAULT, x86_decode_insn's caller treats the failure as a fatal error if the instruction cannot simply be reexecuted (as is the case for MMIO). And this in fact happened sometimes when rebooting Windows 2012r2 guests. Just checking ctxt->have_exception and injecting the exception if true is enough to fix the case.

Thanks to Eduardo Habkost for helping in the debugging of this issue.

---
 arch/x86/kvm/x86.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -5372,6 +5372,8 @@ int x86_emulate_instruction(struct kvm_v
 			if (reexecute_instruction(vcpu, cr2, write_fault_to_spt,
 						emulation_type))
 				return EMULATE_DONE;
+			if (ctxt->have_exception && inject_emulated_exception(vcpu))
+				return EMULATE_DONE;
 			if (emulation_type & EMULTYPE_SKIP)
 				return EMULATE_FAIL;
 			return handle_emulation_failure(vcpu);

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

queue-3.18/kvm-x86-inject-exceptions-produced-by-x86_decode_insn.patch queue-3.18/kvm-x86-exit-to-user-mode-on-ud-intercept-when-emulator-requires.patch