[PATCH v4 00/12] KVM: x86: never write to memory from kvm_vcpu_check_block

[Sean Christopherson]

Non-x86 folks, there's nothing interesting to see here, y'all got pulled
in because removing KVM_REQ_UNHALT requires deleting kvm_clear_request()
from arch code.

Note, this based on:

	https://github.com/sean-jc/linux.git tags/kvm-x86-6.1-1

to pre-resolve conflicts with the event/exception cleanups in there.


In Paolo's words...

The following backtrace:

[ 1355.807187]  kvm_vcpu_map+0x159/0x190 [kvm]
[ 1355.807628]  nested_svm_vmexit+0x4c/0x7f0 [kvm_amd]
[ 1355.808036]  ? kvm_vcpu_block+0x54/0xa0 [kvm]
[ 1355.808450]  svm_check_nested_events+0x97/0x390 [kvm_amd]
[ 1355.808920]  kvm_check_nested_events+0x1c/0x40 [kvm] 
[ 1355.809396]  kvm_arch_vcpu_runnable+0x4e/0x190 [kvm]
[ 1355.809892]  kvm_vcpu_check_block+0x4f/0x100 [kvm]
[ 1355.811259]  kvm_vcpu_block+0x6b/0xa0 [kvm] 

can occur due to kmap being called in non-sleepable (!TASK_RUNNING) context.
The fix is to extend kvm_x86_ops->nested_ops.hv_timer_pending() to cover
all events not already checked in kvm_arch_vcpu_is_runnable(), and then
get rid of the annoying (and wrong) call to kvm_check_nested_events()
from kvm_vcpu_check_block().

Beware, this is not a complete fix, because kvm_guest_apic_has_interrupt()
might still _read_ memory from non-sleepable context.  The fix here is
probably to make kvm_arch_vcpu_is_runnable() return -EAGAIN, and in that
case do a round of kvm_vcpu_check_block() polling in sleepable context.
Nevertheless, it is a good start as it pushes the vmexit into vcpu_block().

The series also does a small cleanup pass on kvm_vcpu_check_block(),
removing KVM_REQ_UNHALT in favor of simply calling kvm_arch_vcpu_runnable()
again.  Now that kvm_check_nested_events() is not called anymore by
kvm_arch_vcpu_runnable(), it is much easier to see that KVM will never
consume the event that caused kvm_vcpu_has_events() to return true,
and therefore it is safe to evaluate it again.

The alternative of propagating the return value of
kvm_arch_vcpu_runnable() up to kvm_vcpu_{block,halt}() is inferior
because it does not quite get right the edge cases where the vCPU becomes
runnable right before schedule() or right after kvm_vcpu_check_block().
While these edge cases are unlikely to truly matter in practice, it is
also pointless to get them "wrong".

v4:
  - Make event request if INIT/SIPI is pending when GIF=>1 (SVM) and
    on nested VM-Enter (VMX).
  - Make an event request at VMXOFF iff it's necessary.
  - Keep the INIT/SIPI pending vs. blocked checks separate (for the
    above nSVM/nVMX fixes).
  - Check the result of kvm_check_nested_events() in vcpu_block().
  - Rename INIT/SIPI helpers (hopefully we'll eventually rename all of
    the related collateral, e.g. "pending_events" is so misleading).
  - Drop pending INIT/SIPI snaphsot to avoid creating weird, conflicting
    code when kvm_check_nested_events() is called by vcpu_block().

v3:
  - https://lore.kernel.org/all/20220822170659.2527086-1-pbonzini@xxxxxxxxxx
  - do not propagate the return value of  kvm_arch_vcpu_runnable() up to
    kvm_vcpu_{block,halt}()
  - move and reformat the comment in vcpu_block()

    move KVM_REQ_UNHALT removal last

Paolo Bonzini (5):
  KVM: x86: make vendor code check for all nested events
  KVM: x86: lapic does not have to process INIT if it is blocked
  KVM: x86: never write to memory from kvm_vcpu_check_block()
  KVM: mips, x86: do not rely on KVM_REQ_UNHALT
  KVM: remove KVM_REQ_UNHALT

Sean Christopherson (7):
  KVM: nVMX: Make an event request when pending an MTF nested VM-Exit
  KVM: x86: Rename and expose helper to detect if INIT/SIPI are allowed
  KVM: x86: Rename kvm_apic_has_events() to make it INIT/SIPI specific
  KVM: SVM: Make an event request if INIT or SIPI is pending when GIF is
    set
  KVM: nVMX: Make an event request if INIT or SIPI is pending on
    VM-Enter
  KVM: nVMX: Make event request on VMXOFF iff INIT/SIPI is pending
  KVM: x86: Don't snapshot pending INIT/SIPI prior to checking nested
    events

 Documentation/virt/kvm/vcpu-requests.rst | 28 +--------------
 arch/arm64/kvm/arm.c                     |  1 -
 arch/mips/kvm/emulate.c                  |  6 ++--
 arch/powerpc/kvm/book3s_pr.c             |  1 -
 arch/powerpc/kvm/book3s_pr_papr.c        |  1 -
 arch/powerpc/kvm/booke.c                 |  1 -
 arch/powerpc/kvm/powerpc.c               |  1 -
 arch/riscv/kvm/vcpu_insn.c               |  1 -
 arch/s390/kvm/kvm-s390.c                 |  2 --
 arch/x86/include/asm/kvm_host.h          |  2 +-
 arch/x86/kvm/lapic.c                     | 38 ++++++--------------
 arch/x86/kvm/lapic.h                     |  9 ++++-
 arch/x86/kvm/svm/svm.c                   |  3 +-
 arch/x86/kvm/vmx/nested.c                | 33 +++++++++--------
 arch/x86/kvm/vmx/vmx.c                   |  6 ++--
 arch/x86/kvm/x86.c                       | 46 +++++++++++++++---------
 arch/x86/kvm/x86.h                       |  5 ---
 arch/x86/kvm/xen.c                       |  1 -
 include/linux/kvm_host.h                 |  3 +-
 virt/kvm/kvm_main.c                      |  4 +--
 20 files changed, 79 insertions(+), 113 deletions(-)


base-commit: 5df50a4a9b60afba4dd2be76d0f0fb8ae8c9beab
-- 
2.37.3.968.ga6b4b080e4-goog