KVM: x86: Introduce segmented_write_std

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>
Cc: stable@vger.kernel.org
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>

1 files changed, 18 insertions, 4 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 56628a44668b..f36d0fa6b885 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -818,6 +818,20 @@ 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);
+}
+
 /*
  * Prefetch the remaining bytes of the instruction without crossing page
  * boundary if they are not in fetch_cache yet.
@@ -3685,8 +3699,8 @@ 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)
@@ -3932,7 +3946,7 @@ static int em_fxsave(struct x86_emulate_ctxt *ctxt)
         else
                 size = offsetof(struct fxregs_state, xmm_space[0]);
 
-        return segmented_write(ctxt, ctxt->memop.addr.mem, &fx_state, size);
+        return segmented_write_std(ctxt, ctxt->memop.addr.mem, &fx_state, size);
 }
 
 static int fxrstor_fixup(struct x86_emulate_ctxt *ctxt,
@@ -3974,7 +3988,7 @@ static int em_fxrstor(struct x86_emulate_ctxt *ctxt)
         if (rc != X86EMUL_CONTINUE)
                 return rc;
 
-        rc = segmented_read(ctxt, ctxt->memop.addr.mem, &fx_state, 512);
+        rc = segmented_read_std(ctxt, ctxt->memop.addr.mem, &fx_state, 512);
         if (rc != X86EMUL_CONTINUE)
                 return rc;