linux-kernel - [PATCH 2/9] KVM: x86: Emulate triple fault shutdown if RSM emulation fails

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Date:   Wed,  9 Jun 2021 11:56:12 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+fb0b6a7e8713aeb0319c@...kaller.appspotmail.com
Subject: [PATCH 2/9] KVM: x86: Emulate triple fault shutdown if RSM emulation fails

Use the recently introduced KVM_REQ_TRIPLE_FAULT to properly emulate
shutdown if RSM from SMM fails.

Note, entering shutdown after clearing the SMM flag and restoring NMI
blocking is architecturally correct with respect to AMD's APM, which KVM
also uses for SMRAM layout and RSM NMI blocking behavior.  The APM says:

  An RSM causes a processor shutdown if an invalid-state condition is
  found in the SMRAM state-save area. Only an external reset, external
  processor-initialization, or non-maskable external interrupt (NMI) can
  cause the processor to leave the shutdown state.

Of note is processor-initialization (INIT) as a valid shutdown wake
event, as INIT is blocked by SMM, implying that entering shutdown also
forces the CPU out of SMM.

For recent Intel CPUs, restoring NMI blocking is technically wrong, but
so is restoring NMI blocking in the first place, and Intel's RSM
"architecture" is such a mess that just about anything is allowed and can
be justified as micro-architectural behavior.

Per the SDM:

  On Pentium 4 and later processors, shutdown will inhibit INTR and A20M
  but will not change any of the other inhibits. On these processors,
  NMIs will be inhibited if no action is taken in the SMI handler to
  uninhibit them (see Section 34.8).

where Section 34.8 says:

  When the processor enters SMM while executing an NMI handler, the
  processor saves the SMRAM state save map but does not save the
  attribute to keep NMI interrupts disabled. Potentially, an NMI could be
  latched (while in SMM or upon exit) and serviced upon exit of SMM even
  though the previous NMI handler has still not completed.

I.e. RSM unconditionally unblocks NMI, but shutdown on RSM does not,
which is in direct contradiction of KVM's behavior.  But, as mentioned
above, KVM follows AMD architecture and restores NMI blocking on RSM, so
that micro-architectural detail is already lost.

And for Pentium era CPUs, SMI# can break shutdown, meaning that at least
some Intel CPUs fully leave SMM when entering shutdown:

  In the shutdown state, Intel processors stop executing instructions
  until a RESET#, INIT# or NMI# is asserted.  While Pentium family
  processors recognize the SMI# signal in shutdown state, P6 family and
  Intel486 processors do not.

In other words, the fact that Intel CPUs have implemented the two
extremes gives KVM carte blanche when it comes to honoring Intel's
architecture for handling shutdown during RSM.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/emulate.c     | 12 +++++++-----
 arch/x86/kvm/kvm_emulate.h |  1 +
 arch/x86/kvm/x86.c         |  6 ++++++
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 5e5de05a8fbf..0603a2c79093 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2683,7 +2683,7 @@ static int em_rsm(struct x86_emulate_ctxt *ctxt)
 	 * state-save area.
 	 */
 	if (ctxt->ops->pre_leave_smm(ctxt, buf))
-		return X86EMUL_UNHANDLEABLE;
+		goto emulate_shutdown;
 
 #ifdef CONFIG_X86_64
 	if (emulator_has_longmode(ctxt))
@@ -2692,14 +2692,16 @@ static int em_rsm(struct x86_emulate_ctxt *ctxt)
 #endif
 		ret = rsm_load_state_32(ctxt, buf);
 
-	if (ret != X86EMUL_CONTINUE) {
-		/* FIXME: should triple fault */
-		return X86EMUL_UNHANDLEABLE;
-	}
+	if (ret != X86EMUL_CONTINUE)
+		goto emulate_shutdown;
 
 	ctxt->ops->post_leave_smm(ctxt);
 
 	return X86EMUL_CONTINUE;
+
+emulate_shutdown:
+	ctxt->ops->triple_fault(ctxt);
+	return X86EMUL_UNHANDLEABLE;
 }
 
 static void
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index 3e870bf9ca4d..9c34aa60e45f 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -233,6 +233,7 @@ struct x86_emulate_ops {
 	int (*pre_leave_smm)(struct x86_emulate_ctxt *ctxt,
 			     const char *smstate);
 	void (*post_leave_smm)(struct x86_emulate_ctxt *ctxt);
+	void (*triple_fault)(struct x86_emulate_ctxt *ctxt);
 	int (*set_xcr)(struct x86_emulate_ctxt *ctxt, u32 index, u64 xcr);
 };
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 54d212fe9b15..cda148cf06fa 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7123,6 +7123,11 @@ static void emulator_post_leave_smm(struct x86_emulate_ctxt *ctxt)
 	kvm_smm_changed(emul_to_vcpu(ctxt));
 }
 
+static void emulator_triple_fault(struct x86_emulate_ctxt *ctxt)
+{
+	kvm_make_request(KVM_REQ_TRIPLE_FAULT, emul_to_vcpu(ctxt));
+}
+
 static int emulator_set_xcr(struct x86_emulate_ctxt *ctxt, u32 index, u64 xcr)
 {
 	return __kvm_set_xcr(emul_to_vcpu(ctxt), index, xcr);
@@ -7172,6 +7177,7 @@ static const struct x86_emulate_ops emulate_ops = {
 	.set_hflags          = emulator_set_hflags,
 	.pre_leave_smm       = emulator_pre_leave_smm,
 	.post_leave_smm      = emulator_post_leave_smm,
+	.triple_fault        = emulator_triple_fault,
 	.set_xcr             = emulator_set_xcr,
 };
 
-- 
2.32.0.rc1.229.g3e70b5a671-goog