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authorRick Edgecombe <rick.p.edgecombe@intel.com>2019-04-26 17:23:58 -0700
committerPaolo Bonzini <pbonzini@redhat.com>2019-04-27 09:48:52 +0200
commitf2fde6a5bcfcfcfca8eef59666b20454da055ad7 (patch)
tree1d9a76d43afd1866d8ae03912c7c0d66da687a45 /arch/ia64
parentb6aa57c69cb26ea0160c51f7cf45f1af23542686 (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>
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