linux-kernel - [PATCH 4.16 26/26] Revert "KVM: X86: Fix SMRAM accessing even if VM is shutdown"

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Message-Id: <20180425103315.890929918@linuxfoundation.org>
Date:   Wed, 25 Apr 2018 12:33:35 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 4.16 26/26] Revert "KVM: X86: Fix SMRAM accessing even if VM is shutdown"

4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Sean Christopherson <sean.j.christopherson@...el.com>

commit 2c151b25441ae5c2da66472abd165af785c9ecd2 upstream.

The bug that led to commit 95e057e25892eaa48cad1e2d637b80d0f1a4fac5
was a benign warning (no adverse affects other than the warning
itself) that was detected by syzkaller.  Further inspection shows
that the WARN_ON in question, in handle_ept_misconfig(), is
unnecessary and flawed (this was also briefly discussed in the
original patch: https://patchwork.kernel.org/patch/10204649).

  * The WARN_ON is unnecessary as kvm_mmu_page_fault() will WARN
    if reserved bits are set in the SPTEs, i.e. it covers the case
    where an EPT misconfig occurred because of a KVM bug.

  * The WARN_ON is flawed because it will fire on any system error
    code that is hit while handling the fault, e.g. -ENOMEM can be
    returned by mmu_topup_memory_caches() while handling a legitmate
    MMIO EPT misconfig.

The original behavior of returning -EFAULT when userspace munmaps
an HVA without first removing the memslot is correct and desirable,
i.e. KVM is letting userspace know it has generated a bad address.
Returning RET_PF_EMULATE masks the WARN_ON in the EPT misconfig path,
but does not fix the underlying bug, i.e. the WARN_ON is bogus.

Furthermore, returning RET_PF_EMULATE has the unwanted side effect of
causing KVM to attempt to emulate an instruction on any page fault
with an invalid HVA translation, e.g. a not-present EPT violation
on a VM_PFNMAP VMA whose fault handler failed to insert a PFN.

  * There is no guarantee that the fault is directly related to the
    instruction, i.e. the fault could have been triggered by a side
    effect memory access in the guest, e.g. while vectoring a #DB or
    writing a tracing record.  This could cause KVM to effectively
    mask the fault if KVM doesn't model the behavior leading to the
    fault, i.e. emulation could succeed and resume the guest.

  * If emulation does fail, KVM will return EMULATION_FAILED instead
    of -EFAULT, which is a red herring as the user will either debug
    a bogus emulation attempt or scratch their head wondering why we
    were attempting emulation in the first place.

TL;DR: revert to returning -EFAULT and remove the bogus WARN_ON in
handle_ept_misconfig in a future patch.

This reverts commit 95e057e25892eaa48cad1e2d637b80d0f1a4fac5.

Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/kvm/mmu.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3031,7 +3031,7 @@ static int kvm_handle_bad_page(struct kv
 		return RET_PF_RETRY;
 	}

-	return RET_PF_EMULATE;
+	return -EFAULT;
 }

 static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,