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%3Ba=su...

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 stable@vger.kernel.org know about it.

From 6ea6e84309ca7e0e850b3083e6b09344ee15c290 Mon Sep 17 00:00:00 2001

From: Paolo Bonzini pbonzini@redhat.com Date: Fri, 10 Nov 2017 10:49:38 +0100 Subject: KVM: x86: inject exceptions produced by x86_decode_insn MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit

From: Paolo Bonzini pbonzini@redhat.com

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.

Reported-by: Yanan Fu yfu@redhat.com Cc: Eduardo Habkost ehabkost@redhat.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Radim Krčmář rkrcmar@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

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

--- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -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 pbonzini@redhat.com 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