On 09/08/19 18:00, Adalbert Lazăr wrote:
> From: Mihai Donțu <mdontu@xxxxxxxxxxxxxxx>
>
> It can happened for us to end up emulating the VMCALL instruction as a
> result of the handling of an EPT write fault. In this situation, the
> emulator will try to unconditionally patch the correct hypercall opcode
> bytes using emulator_write_emulated(). However, this last call uses the
> fault GPA (if available) or walks the guest page tables at RIP,
> otherwise. The trouble begins when using KVMI, when we forbid the use of
> the fault GPA and fallback to the guest pt walk: in Windows (8.1 and
> newer) the page that we try to write into is marked read-execute and as
> such emulator_write_emulated() fails and we inject a write #PF, leading
> to a guest crash.
>
> The fix is rather simple: check the existing instruction bytes before
> doing the patching. This does not change the normal KVM behaviour, but
> does help when using KVMI as we no longer inject a write #PF.
Fixing the hypercall is just an optimization. Can we just hush and
return to the guest if emulator_write_emulated returns
X86EMUL_PROPAGATE_FAULT?
Paolo
> CC: Joerg Roedel <joro@xxxxxxxxxx>
> Signed-off-by: Mihai Donțu <mdontu@xxxxxxxxxxxxxxx>
> Signed-off-by: Adalbert Lazăr <alazar@xxxxxxxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 23 ++++++++++++++++++++---
> 1 file changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 04b1d2916a0a..965c4f0108eb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7363,16 +7363,33 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> }
> EXPORT_SYMBOL_GPL(kvm_emulate_hypercall);
>
> +#define KVM_HYPERCALL_INSN_LEN 3
> +
> static int emulator_fix_hypercall(struct x86_emulate_ctxt *ctxt)
> {
> + int err;
> struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> - char instruction[3];
> + char buf[KVM_HYPERCALL_INSN_LEN];
> + char instruction[KVM_HYPERCALL_INSN_LEN];
> unsigned long rip = kvm_rip_read(vcpu);
>
> + err = emulator_read_emulated(ctxt, rip, buf, sizeof(buf),
> + &ctxt->exception);
> + if (err != X86EMUL_CONTINUE)
> + return err;
> +
> kvm_x86_ops->patch_hypercall(vcpu, instruction);
> + if (!memcmp(instruction, buf, sizeof(instruction)))
> + /*
> + * The hypercall instruction is the correct one. Retry
> + * its execution maybe we got here as a result of an
> + * event other than #UD which has been resolved in the
> + * mean time.
> + */
> + return X86EMUL_CONTINUE;
>
> - return emulator_write_emulated(ctxt, rip, instruction, 3,
> - &ctxt->exception);
> + return emulator_write_emulated(ctxt, rip, instruction,
> + sizeof(instruction), &ctxt->exception);
> }
>
> static int dm_request_for_irq_injection(struct kvm_vcpu *vcpu)
>
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