[PATCH v5 0/7] KVM: nSVM: avoid TOC/TOU race when checking vmcb12

Emanuele Giuseppe Esposito
2021-11-03T14:06:31Z

Currently there is a TOC/TOU race between the check of vmcb12's
efer, cr0 and cr4 registers and the later save of their values in
svm_set_*, because the guest could modify the values in the meanwhile.

To solve this issue, this series introduces and uses svm->nested.save
structure in enter_svm_guest_mode to save the current value of efer,
cr0 and cr4 and later use these to set the vcpu->arch.* state.

Similarly, svm->nested.ctl contains fields that are not used, so having
a full vmcb_control_area means passing uninitialized fields.

Patches 1,3 and 8 take care of renaming and refactoring code.
Patches 2 and 6 introduce respectively vmcb_ctrl_area_cached and
vmcb_save_area_cached.
Patches 4 and 5 use vmcb_save_area_cached to avoid TOC/TOU, and patch
7 uses vmcb_ctrl_area_cached.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>

---
v5:
* rebased on kvm/queue branch

v4:
* introduce _* helpers (_nested_vmcb_check_save,
_nested_copy_vmcb_control_to_cache, _nested_copy_vmcb_save_to_cache)
that take care of additional parameters.
* svm_set_nested_state: introduce {save, ctl}_cached variables
to not pollute svm->nested.{save,ctl} state, especially if the
check fails. remove also unnecessary memset added in previous versions.
* svm_get_nested_state: change stack variable ctl introduced in this series
into a pointer that will be zeroed and freed after it has been copied to user

v3:
* merge this series with "KVM: nSVM: use vmcb_ctrl_area_cached instead
of vmcb_control_area in nested state"
* rename "nested_load_save_from_vmcb12" in
"nested_copy_vmcb_save_to_cache"
* rename "nested_load_control_from_vmcb12" in
"nested_copy_vmcb_control_to_cache"
* change check functions (nested_vmcb_valid_sregs and nested_vmcb_valid_sregs)
to accept only the vcpu parameter, since we only check
nested state now
* rename "vmcb_is_intercept_cached" in "vmcb12_is_intercept"
and duplicate the implementation instead of calling vmcb_is_intercept

v2:
* svm->nested.save is a separate struct vmcb_save_area_cached,
and not vmcb_save_area.
* update also vmcb02->cr3 with svm->nested.save.cr3

RFC:
* use svm->nested.save instead of local variables.
* not dependent anymore from "KVM: nSVM: remove useless kvm_clear_*_queue"
* simplified patches, we just use the struct and not move the check
nearer to the TOU.

Emanuele Giuseppe Esposito (7):
KVM: nSVM: move nested_vmcb_check_cr3_cr4 logic in
nested_vmcb_valid_sregs
nSVM: introduce smv->nested.save to cache save area fields
nSVM: rename nested_load_control_from_vmcb12 in
nested_copy_vmcb_control_to_cache
nSVM: use vmcb_save_area_cached in nested_vmcb_valid_sregs()
nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU
races
nSVM: introduce struct vmcb_ctrl_area_cached
nSVM: use vmcb_ctrl_area_cached instead of vmcb_control_area in struct
svm_nested_state

arch/x86/kvm/svm/nested.c | 250 ++++++++++++++++++++++++--------------
arch/x86/kvm/svm/svm.c | 7 +-
arch/x86/kvm/svm/svm.h | 59 ++++++++-
3 files changed, 213 insertions(+), 103 deletions(-)

--
2.27.0