[PATCH] KVM: x86: Introduce segmented_write_std

[Colin King]

CVE-2017-2584

Introduces segemented_write_std.

Switches from emulated reads/writes to standard read/writes in fxsave,
fxrstor, sgdt, and sidt.  This fixes CVE-2017-2584, a longstanding
kernel memory leak.

Since commit 283c95d0e389 ("KVM: x86: emulate FXSAVE and FXRSTOR",
2016-11-09), which is luckily not yet in any final release, this would
also be an exploitable kernel memory *write*!

Note: backport removes changes to functions em_sgdt and fxrstor_fixup
as these don't exist in Trusty and were introduced in 2016 with commit
283c95d0e389 ("KVM: x86: emulate FXSAVE and FXRSTOR")

Reported-by: Dmitry Vyukov <dvyukov at google.com>
Cc: stable at vger.kernel.org
Fixes: 96051572c819194c37a8367624b285be10297eca
Fixes: 283c95d0e3891b64087706b344a4b545d04a6e62
Suggested-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Steve Rutherford <srutherford at google.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
(backported from upstream fix 129a72a0d3c8e139a04512325384fe5ac119e74d)
Signed-off-by: Colin Ian King <colin.king at canonical.com>
---
 arch/x86/kvm/emulate.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 2952452..d924843 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -809,6 +809,20 @@ static int do_insn_fetch(struct x86_emulate_ctxt *ctxt,
 		goto done;						\
 })
 
+static int segmented_write_std(struct x86_emulate_ctxt *ctxt,
+			       struct segmented_address addr,
+			       void *data,
+			       unsigned int size)
+{
+	int rc;
+	ulong linear;
+
+	rc = linearize(ctxt, addr, size, true, &linear);
+	if (rc != X86EMUL_CONTINUE)
+		return rc;
+	return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception);
+}
+
 /*
  * Given the 'reg' portion of a ModRM byte, and a register block, return a
  * pointer into the block that addresses the relevant register.
@@ -3244,8 +3258,8 @@ static int emulate_store_desc_ptr(struct x86_emulate_ctxt *ctxt,
 	}
 	/* Disable writeback. */
 	ctxt->dst.type = OP_NONE;
-	return segmented_write(ctxt, ctxt->dst.addr.mem,
-			       &desc_ptr, 2 + ctxt->op_bytes);
+	return segmented_write_std(ctxt, ctxt->dst.addr.mem,
+				   &desc_ptr, 2 + ctxt->op_bytes);
 }
 
 static int em_sgdt(struct x86_emulate_ctxt *ctxt)
-- 
2.7.4