KVM: x86: Forcibly leave nested if RSM to L2 hits shutdown

Leave nested mode before synthesizing shutdown (a.k.a. TRIPLE_FAULT) if
RSM fails when resuming L2 (a.k.a. guest mode).  Architecturally, shutdown
on RSM occurs _before_ the transition back to guest mode on both Intel and
AMD.

On Intel, per the SDM pseudocode, SMRAM state is loaded before critical
VMX state:

  restore state normally from SMRAM;
  ...
  CR4.VMXE := value stored internally;
  IF internal storage indicates that the logical processor had been in
     VMX operation (root or non-root)
  THEN
     enter VMX operation (root or non-root);
     restore VMX-critical state as defined in Section 32.14.1;
     ...
     restore current VMCS pointer;
  FI;

AMD's APM is both less clearcut and more explicit.  Because AMD CPUs save
VMCB and guest state in SMRAM itself, given the lack of anything in the
APM to indicate a shutdown in guest mode is possible, a straightforward
reading of the clause on invalid state is that _what_ state is invalid is
irrelevant, i.e. all roads lead to shutdown.

  An RSM causes a processor shutdown if an invalid-state condition is
  found in the SMRAM state-save area.

This fixes a bug found by syzkaller where synthesizing shutdown for L2
led to a nested VM-Exit (if L1 is intercepting shutdown), which in turn
caused KVM to complain about trying to cancel a nested VM-Enter (see
commit 759cbd5 ("KVM: x86: nSVM/nVMX: set nested_run_pending on VM
entry which is a result of RSM").

Note, Paolo pointed out that KVM shouldn't set nested_run_pending until
after loading SMRAM state.  But as above, that's only half the story, KVM
shouldn't transition to guest mode either.  Unfortunately, fixing that
mess requires rewriting the nVMX and nSVM RSM flows to not piggyback
their nested VM-Enter flows, as executing the nested VM-Enter flows after
loading state from SMRAM would clobber much of said state.

For now, add a FIXME to call out that transitioning to guest mode before
loading state from SMRAM is wrong.

Link: https://lore.kernel.org/all/CABgObfYaUHXyRmsmg8UjRomnpQ0Jnaog9-L2gMjsjkqChjDYUQ@mail.gmail.com
Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]
Reported-by: Zheyu Ma <[email protected]>
Closes: https://lore.kernel.org/all/CAMhUBjmXMYsEoVYw_M8hSZjBMHh24i88QYm-RY6HDta5YZ7Wgw@mail.gmail.com
Analyzed-by: Michal Wilczynski <[email protected]>
Cc: Kishen Maloor <[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: sean-jc

arch/x86/kvm/smm.c

@@ -624,17 +624,31 @@ int emulator_leave_smm(struct x86_emulate_ctxt *ctxt)
 #endif
 
 	/*
-	 * Give leave_smm() a chance to make ISA-specific changes to the vCPU
-	 * state (e.g. enter guest mode) before loading state from the SMM
-	 * state-save area.
+	 * FIXME: When resuming L2 (a.k.a. guest mode), the transition to guest
+	 * mode should happen _after_ loading state from SMRAM.  However, KVM
+	 * piggybacks the nested VM-Enter flows (which is wrong for many other
+	 * reasons), and so nSVM/nVMX would clobber state that is loaded from
+	 * SMRAM and from the VMCS/VMCB.
 	 */
 	if (kvm_x86_call(leave_smm)(vcpu, &smram))
 		return X86EMUL_UNHANDLEABLE;
 
 #ifdef CONFIG_X86_64
 	if (guest_cpuid_has(vcpu, X86_FEATURE_LM))
-		return rsm_load_state_64(ctxt, &smram.smram64);
+		ret = rsm_load_state_64(ctxt, &smram.smram64);
 	else
 #endif
-		return rsm_load_state_32(ctxt, &smram.smram32);
+		ret = rsm_load_state_32(ctxt, &smram.smram32);
+
+	/*
+	 * If RSM fails and triggers shutdown, architecturally the shutdown
+	 * occurs *before* the transition to guest mode.  But due to KVM's
+	 * flawed handling of RSM to L2 (see above), the vCPU may already be
+	 * in_guest_mode().  Force the vCPU out of guest mode before delivering
+	 * the shutdown, so that L1 enters shutdown instead of seeing a VM-Exit
+	 * that architecturally shouldn't be possible.
+	 */
+	if (ret != X86EMUL_CONTINUE && is_guest_mode(vcpu))
+		kvm_leave_nested(vcpu);
+	return ret;
 }

arch/x86/kvm/x86.c

@@ -833,12 +833,6 @@ static void kvm_queue_exception_vmexit(struct kvm_vcpu *vcpu, unsigned int vecto
 	ex->payload = payload;
 }
 
-/* Forcibly leave the nested mode in cases like a vCPU reset */
-static void kvm_leave_nested(struct kvm_vcpu *vcpu)
-{
-	kvm_x86_ops.nested_ops->leave_nested(vcpu);
-}
-
 static void kvm_multiple_exception(struct kvm_vcpu *vcpu,
 		unsigned nr, bool has_error, u32 error_code,
 	        bool has_payload, unsigned long payload, bool reinject)

arch/x86/kvm/x86.h

@@ -108,6 +108,12 @@ static inline unsigned int __shrink_ple_window(unsigned int val,
 void kvm_service_local_tlb_flush_requests(struct kvm_vcpu *vcpu);
 int kvm_check_nested_events(struct kvm_vcpu *vcpu);
 
+/* Forcibly leave the nested mode in cases like a vCPU reset */
+static inline void kvm_leave_nested(struct kvm_vcpu *vcpu)
+{
+	kvm_x86_ops.nested_ops->leave_nested(vcpu);
+}
+
 static inline bool kvm_vcpu_has_run(struct kvm_vcpu *vcpu)
 {
 	return vcpu->arch.last_vmentry_cpu != -1;