Date:   Mon, 22 Aug 2022 13:06:52 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     mlevitsk@...hat.com, seanjc@...gle.com
Subject: [PATCH v3 0/7] KVM: x86: never write to memory from kvm_vcpu_check_block

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".

Paolo

v2->v3: 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 (6):
  KVM: x86: check validity of argument to KVM_SET_MP_STATE
  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 (1):
  KVM: nVMX: Make an event request when pending an MTF nested VM-Exit

 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          |  3 +-
 arch/x86/kvm/i8259.c                     |  4 +-
 arch/x86/kvm/lapic.h                     |  2 +-
 arch/x86/kvm/vmx/nested.c                |  9 +++-
 arch/x86/kvm/vmx/vmx.c                   |  6 ++-
 arch/x86/kvm/x86.c                       | 53 ++++++++++++++++++------
 arch/x86/kvm/x86.h                       |  5 ---
 arch/x86/kvm/xen.c                       |  1 -
 include/linux/kvm_host.h                 |  3 +-
 virt/kvm/kvm_main.c                      |  4 +-
 19 files changed, 63 insertions(+), 69 deletions(-)

-- 
2.31.1

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