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>
#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)
* 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)
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 |=