KVM: x86: Introduce segmented_write_std

commit 129a72a0d3c8e139a04512325384fe5ac119e74d upstream.

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*!

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: 96051572c819194c37a8367624b285be10297eca
Fixes: 283c95d0e3891b64087706b344a4b545d04a6e62
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Steve Rutherford <srutherford@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 364f020a5d68d28a0ac0224a4d2a69dca4421891..c96485054f6ba3a623a3c0a88d7de5020fbcb7aa 100644 (file)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -906,6 +906,20 @@ static int segmented_read_std(struct x86_emulate_ctxt *ctxt,
        return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception);
 }
 
+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);
+}
+
 /*
  * Fetch the next byte of the instruction being emulated which is pointed to
  * by ctxt->_eip, then increment ctxt->_eip.
@@ -3361,8 +3375,8 @@ 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)