lkml.org 
[lkml]   [2018]   [Mar]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    SubjectRe: [PATCH v3 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

    ----- kernellwp@gmail.com wrote:

    > From: Wanpeng Li <wanpengli@tencent.com>
    >
    > There is no easy way to force KVM to run an instruction through the
    > emulator
    > (by design as that will expose the x86 emulator as a significant
    > attack-surface).
    > However, we do wish to expose the x86 emulator in case we are testing
    > it
    > (e.g. via kvm-unit-tests). Therefore, this patch adds a "force
    > emulation prefix"
    > that is designed to raise #UD which KVM will trap and it's #UD
    > exit-handler will
    > match "force emulation prefix" to run instruction after prefix by the
    > x86 emulator.
    > To not expose the x86 emulator by default, we add a module parameter
    > that should
    > be off by default.
    >
    > A simple testcase here:
    >
    > #include <stdio.h>
    > #include <string.h>
    >
    > #define HYPERVISOR_INFO 0x40000000
    >
    > #define CPUID(idx, eax, ebx, ecx, edx) \
    > asm volatile (\
    > "ud2a; .ascii \"kvm\"; cpuid" \
    > :"=b" (*ebx), "=a" (*eax), "=c" (*ecx), "=d" (*edx) \
    > :"0"(idx) );
    >
    > void main()
    > {
    > unsigned int eax, ebx, ecx, edx;
    > char string[13];
    >
    > CPUID(HYPERVISOR_INFO, &eax, &ebx, &ecx, &edx);
    > *(unsigned int *)(string + 0) = ebx;
    > *(unsigned int *)(string + 4) = ecx;
    > *(unsigned int *)(string + 8) = edx;
    >
    > string[12] = 0;
    > if (strncmp(string, "KVMKVMKVM\0\0\0", 12) == 0)
    > printf("kvm guest\n");
    > else
    > printf("bare hardware\n");
    > }
    >
    > Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
    > Cc: Paolo Bonzini <pbonzini@redhat.com>
    > Cc: Radim Krčmář <rkrcmar@redhat.com>
    > Cc: Andrew Cooper <andrew.cooper3@citrix.com>
    > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    > Cc: Liran Alon <liran.alon@oracle.com>
    > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
    > ---
    > arch/x86/kvm/x86.c | 18 +++++++++++++++++-
    > 1 file changed, 17 insertions(+), 1 deletion(-)
    >
    > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    > index e3a60ab..40e2f78 100644
    > --- a/arch/x86/kvm/x86.c
    > +++ b/arch/x86/kvm/x86.c
    > @@ -146,6 +146,9 @@ bool __read_mostly enable_vmware_backdoor =
    > false;
    > module_param(enable_vmware_backdoor, bool, S_IRUGO);
    > EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
    >
    > +static bool __read_mostly force_emulation_prefix = false;
    > +module_param(force_emulation_prefix, bool, S_IRUGO);
    > +
    > #define KVM_NR_SHARED_MSRS 16
    >
    > struct kvm_shared_msrs_global {
    > @@ -4843,8 +4846,21 @@
    > EXPORT_SYMBOL_GPL(kvm_write_guest_virt_system);
    > int handle_ud(struct kvm_vcpu *vcpu)
    > {
    > enum emulation_result er;
    > + int emulation_type = EMULTYPE_TRAP_UD;
    > +
    > + if (force_emulation_prefix) {
    > + char sig[5]; /* ud2; .ascii "kvm" */
    > + struct x86_exception e;
    > +
    > + kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
    > + kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e);
    > + if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
    > + emulation_type = 0;
    > + kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
    > + }
    > + }
    >
    > - er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
    > + er = emulate_instruction(vcpu, emulation_type);
    > if (er == EMULATE_USER_EXIT)
    > return 0;
    > if (er != EMULATE_DONE)
    > --
    > 2.7.4

    Reviewed-by: Liran Alon <liran.alon@oracle.com>

    \
     
     \ /
      Last update: 2018-03-28 08:56    [W:5.182 / U:0.012 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site