KVM: SVM: Skip WRMSR fastpath on VM-Exit if next RIP isn't valid

Skip the WRMSR fastpath in SVM's VM-Exit handler if the next RIP isn't
valid, e.g. because KVM is running with nrips=false.  SVM must decode and
emulate to skip the WRMSR if the CPU doesn't provide the next RIP.
Getting the instruction bytes to decode the WRMSR requires reading guest
memory, which in turn means dereferencing memslots, and that isn't safe
because KVM doesn't hold SRCU when the fastpath runs.

Don't bother trying to enable the fastpath for this case, e.g. by doing
only the WRMSR and leaving the "skip" until later.  NRIPS is supported on
all modern CPUs (KVM has considered making it mandatory), and the next
RIP will be valid the vast, vast majority of the time.

  =============================
  WARNING: suspicious RCU usage
  6.0.0-smp--4e557fcd3d80-skip #13 Tainted: G           O
  -----------------------------
  include/linux/kvm_host.h:954 suspicious rcu_dereference_check() usage!

  other info that might help us debug this:

  rcu_scheduler_active = 2, debug_locks = 1
  1 lock held by stable/206475:
   #0: ffff9d9dfebcc0f0 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x8b/0x620 [kvm]

  stack backtrace:
  CPU: 152 PID: 206475 Comm: stable Tainted: G           O       6.0.0-smp--4e557fcd3d80-skip #13
  Hardware name: Google, Inc. Arcadia_IT_80/Arcadia_IT_80, BIOS 10.48.0 01/27/2022
  Call Trace:
   <TASK>
   dump_stack_lvl+0x69/0xaa
   dump_stack+0x10/0x12
   lockdep_rcu_suspicious+0x11e/0x130
   kvm_vcpu_gfn_to_memslot+0x155/0x190 [kvm]
   kvm_vcpu_gfn_to_hva_prot+0x18/0x80 [kvm]
   paging64_walk_addr_generic+0x183/0x450 [kvm]
   paging64_gva_to_gpa+0x63/0xd0 [kvm]
   kvm_fetch_guest_virt+0x53/0xc0 [kvm]
   __do_insn_fetch_bytes+0x18b/0x1c0 [kvm]
   x86_decode_insn+0xf0/0xef0 [kvm]
   x86_emulate_instruction+0xba/0x790 [kvm]
   kvm_emulate_instruction+0x17/0x20 [kvm]
   __svm_skip_emulated_instruction+0x85/0x100 [kvm_amd]
   svm_skip_emulated_instruction+0x13/0x20 [kvm_amd]
   handle_fastpath_set_msr_irqoff+0xae/0x180 [kvm]
   svm_vcpu_run+0x4b8/0x5a0 [kvm_amd]
   vcpu_enter_guest+0x16ca/0x22f0 [kvm]
   kvm_arch_vcpu_ioctl_run+0x39d/0x900 [kvm]
   kvm_vcpu_ioctl+0x538/0x620 [kvm]
   __se_sys_ioctl+0x77/0xc0
   __x64_sys_ioctl+0x1d/0x20
   do_syscall_64+0x3d/0x80
   entry_SYSCALL_64_after_hwframe+0x63/0xcd

Fixes: 404d5d7bff0d ("KVM: X86: Introduce more exit_fastpath_completion enum values")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220930234031.1732249-1-seanjc@google.com

1 files changed, 8 insertions, 2 deletions

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 91352d692845..6ffadbd57744 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3895,8 +3895,14 @@ static int svm_vcpu_pre_run(struct kvm_vcpu *vcpu)
 
 static fastpath_t svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
 {
-	if (to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
-	    to_svm(vcpu)->vmcb->control.exit_info_1)
+	struct vmcb_control_area *control = &to_svm(vcpu)->vmcb->control;
+
+	/*
+	 * Note, the next RIP must be provided as SRCU isn't held, i.e. KVM
+	 * can't read guest memory (dereference memslots) to decode the WRMSR.
+	 */
+	if (control->exit_code == SVM_EXIT_MSR && control->exit_info_1 &&
+	    nrips && control->next_rip)
 		return handle_fastpath_set_msr_irqoff(vcpu);
 
 	return EXIT_FASTPATH_NONE;