linux-kernel - [PATCH 1/3] Revert "KVM: x86: mmu: Add guest physical address check in translate

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Message-Id: <20210831164224.1119728-2-seanjc@google.com>
Date:   Tue, 31 Aug 2021 09:42:22 -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+200c08e88ae818f849ce@...kaller.appspotmail.com
Subject: [PATCH 1/3] Revert "KVM: x86: mmu: Add guest physical address check
 in translate_gpa()"

Revert a misguided illegal GPA check when "translating" a non-nested GPA.
The check is woefully incomplete as it does not fill in @exception as
expected by all callers, which leads to KVM attempting to inject a bogus
exception, potentially exposing kernel stack information in the process.

 WARNING: CPU: 0 PID: 8469 at arch/x86/kvm/x86.c:525 exception_type+0x98/0xb0 arch/x86/kvm/x86.c:525
 CPU: 1 PID: 8469 Comm: syz-executor531 Not tainted 5.14.0-rc7-syzkaller #0
 RIP: 0010:exception_type+0x98/0xb0 arch/x86/kvm/x86.c:525
 Call Trace:
  x86_emulate_instruction+0xef6/0x1460 arch/x86/kvm/x86.c:7853
  kvm_mmu_page_fault+0x2f0/0x1810 arch/x86/kvm/mmu/mmu.c:5199
  handle_ept_misconfig+0xdf/0x3e0 arch/x86/kvm/vmx/vmx.c:5336
  __vmx_handle_exit arch/x86/kvm/vmx/vmx.c:6021 [inline]
  vmx_handle_exit+0x336/0x1800 arch/x86/kvm/vmx/vmx.c:6038
  vcpu_enter_guest+0x2a1c/0x4430 arch/x86/kvm/x86.c:9712
  vcpu_run arch/x86/kvm/x86.c:9779 [inline]
  kvm_arch_vcpu_ioctl_run+0x47d/0x1b20 arch/x86/kvm/x86.c:10010
  kvm_vcpu_ioctl+0x49e/0xe50 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3652

The bug has escaped notice because practically speaking the GPA check is
useless.  The GPA check in question only comes into play when KVM is
walking guest page tables (or "translating" CR3), and KVM already handles
illegal GPA checks by setting reserved bits in rsvd_bits_mask for each
PxE, or in the case of CR3 for loading PTDPTRs, manually checks for an
illegal CR3.  This particular failure doesn't hit the existing reserved
bits checks because syzbot sets guest.MAXPHYADDR=1, and IA32 architecture
simply doesn't allow for such an absurd MAXPHADDR, e.g. 32-bit paging
doesn't define any reserved PA bits checks, which KVM emulates by only
incorporating the reserved PA bits into the "high" bits, i.e. bits 63:32.

Simply remove the bogus check.  There is zero meaningful value and no
architectural justification for supporting guest.MAXPHYADDR < 32, and
properly filling the exception would introduce non-trivial complexity.

This reverts commit ec7771ab471ba6a945350353617e2e3385d0e013.

Fixes: ec7771ab471b ("KVM: x86: mmu: Add guest physical address check in translate_gpa()")
Cc: stable@...r.kernel.org
Reported-by: syzbot+200c08e88ae818f849ce@...kaller.appspotmail.com
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4853c033e6ce..4b7908187d05 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -334,12 +334,6 @@ static bool check_mmio_spte(struct kvm_vcpu *vcpu, u64 spte)
 static gpa_t translate_gpa(struct kvm_vcpu *vcpu, gpa_t gpa, u32 access,
                                   struct x86_exception *exception)
 {
-	/* Check if guest physical address doesn't exceed guest maximum */
-	if (kvm_vcpu_is_illegal_gpa(vcpu, gpa)) {
-		exception->error_code |= PFERR_RSVD_MASK;
-		return UNMAPPED_GVA;
-	}
-
         return gpa;
 }

-- 
2.33.0.259.gc128427fd7-goog