diff options
| author | Rick Edgecombe <rick.p.edgecombe@intel.com> | 2019-04-26 17:23:58 -0700 |
|---|---|---|
| committer | Paolo Bonzini <pbonzini@redhat.com> | 2019-04-27 09:48:52 +0200 |
| commit | f2fde6a5bcfcfcfca8eef59666b20454da055ad7 (patch) | |
| tree | 1d9a76d43afd1866d8ae03912c7c0d66da687a45 | |
| parent | b6aa57c69cb26ea0160c51f7cf45f1af23542686 (diff) | |
KVM: VMX: Move RSB stuffing to before the first RET after VM-Exit
The not-so-recent change to move VMX's VM-Exit handing to a dedicated
"function" unintentionally exposed KVM to a speculative attack from the
guest by executing a RET prior to stuffing the RSB. Make RSB stuffing
happen immediately after VM-Exit, before any unpaired returns.
Alternatively, the VM-Exit path could postpone full RSB stuffing until
its current location by stuffing the RSB only as needed, or by avoiding
returns in the VM-Exit path entirely, but both alternatives are beyond
ugly since vmx_vmexit() has multiple indirect callers (by way of
vmx_vmenter()). And putting the RSB stuffing immediately after VM-Exit
makes it much less likely to be re-broken in the future.
Note, the cost of PUSH/POP could be avoided in the normal flow by
pairing the PUSH RAX with the POP RAX in __vmx_vcpu_run() and adding an
a POP to nested_vmx_check_vmentry_hw(), but such a weird/subtle
dependency is likely to cause problems in the long run, and PUSH/POP
will take all of a few cycles, which is peanuts compared to the number
of cycles required to fill the RSB.
Fixes: 453eafbe65f7 ("KVM: VMX: Move VM-Enter + VM-Exit handling to non-inline sub-routines")
Reported-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Co-developed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
| -rw-r--r-- | arch/x86/kvm/vmx/vmenter.S | 12 | ||||
| -rw-r--r-- | arch/x86/kvm/vmx/vmx.c | 3 |
2 files changed, 12 insertions, 3 deletions
diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S index 7b272738c576..d4cb1945b2e3 100644 --- a/arch/x86/kvm/vmx/vmenter.S +++ b/arch/x86/kvm/vmx/vmenter.S @@ -3,6 +3,7 @@ #include <asm/asm.h> #include <asm/bitsperlong.h> #include <asm/kvm_vcpu_regs.h> +#include <asm/nospec-branch.h> #define WORD_SIZE (BITS_PER_LONG / 8) @@ -77,6 +78,17 @@ ENDPROC(vmx_vmenter) * referred to by VMCS.HOST_RIP. */ ENTRY(vmx_vmexit) +#ifdef CONFIG_RETPOLINE + ALTERNATIVE "jmp .Lvmexit_skip_rsb", "", X86_FEATURE_RETPOLINE + /* Preserve guest's RAX, it's used to stuff the RSB. */ + push %_ASM_AX + + /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */ + FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE + + pop %_ASM_AX +.Lvmexit_skip_rsb: +#endif ret ENDPROC(vmx_vmexit) diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index bcdd69d90a8c..0c955bb286ff 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -6462,9 +6462,6 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) x86_spec_ctrl_restore_host(vmx->spec_ctrl, 0); - /* Eliminate branch target predictions from guest mode */ - vmexit_fill_RSB(); - /* All fields are clean at this point */ if (static_branch_unlikely(&enable_evmcs)) current_evmcs->hv_clean_fields |= |