linux-kernel - [PATCH 5.0 229/238] KVM: nVMX: Sign extend displacements of VMX instrs mem operands

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Date:   Fri, 22 Mar 2019 12:17:28 +0100
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 5.0 229/238] KVM: nVMX: Sign extend displacements of VMX instrs mem operands

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

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

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

commit 946c522b603f281195af1df91837a1d4d1eb3bc9 upstream.

The VMCS.EXIT_QUALIFCATION field reports the displacements of memory
operands for various instructions, including VMX instructions, as a
naturally sized unsigned value, but masks the value by the addr size,
e.g. given a ModRM encoded as -0x28(%ebp), the -0x28 displacement is
reported as 0xffffffd8 for a 32-bit address size.  Despite some weird
wording regarding sign extension, the SDM explicitly states that bits
beyond the instructions address size are undefined:

    In all cases, bits of this field beyond the instructionâ€™s address
    size are undefined.

Failure to sign extend the displacement results in KVM incorrectly
treating a negative displacement as a large positive displacement when
the address size of the VMX instruction is smaller than KVM's native
size, e.g. a 32-bit address size on a 64-bit KVM.

The very original decoding, added by commit 064aea774768 ("KVM: nVMX:
Decoding memory operands of VMX instructions"), sort of modeled sign
extension by truncating the final virtual/linear address for a 32-bit
address size.  I.e. it messed up the effective address but made it work
by adjusting the final address.

When segmentation checks were added, the truncation logic was kept
as-is and no sign extension logic was introduced.  In other words, it
kept calculating the wrong effective address while mostly generating
the correct virtual/linear address.  As the effective address is what's
used in the segment limit checks, this results in KVM incorreclty
injecting #GP/#SS faults due to non-existent segment violations when
a nested VMM uses negative displacements with an address size smaller
than KVM's native address size.

Using the -0x28(%ebp) example, an EBP value of 0x1000 will result in
KVM using 0x100000fd8 as the effective address when checking for a
segment limit violation.  This causes a 100% failure rate when running
a 32-bit KVM build as L1 on top of a 64-bit KVM L0.

Fixes: f9eb4af67c9d ("KVM: nVMX: VMX instructions: add checks for #GP/#SS exceptions")
Cc: stable@...r.kernel.org
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/vmx/nested.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -4035,6 +4035,10 @@ int get_vmx_mem_address(struct kvm_vcpu
 	/* Addr = segment_base + offset */
 	/* offset = base + [index * scale] + displacement */
 	off = exit_qualification; /* holds the displacement */
+	if (addr_size == 1)
+		off = (gva_t)sign_extend64(off, 31);
+	else if (addr_size == 0)
+		off = (gva_t)sign_extend64(off, 15);
 	if (base_is_valid)
 		off += kvm_register_read(vcpu, base_reg);
 	if (index_is_valid)