lguest: Fix Malicious Guest GDT Host Crash

If a Guest makes hypercall which sets a GDT entry to not present, we
currently set any segment registers using that GDT entry to 0.
Unfortunately, this is not sufficient: there are other ways of
altering GDT entries which will cause a fault.

The correct solution to do what Linux does: let them set any GDT value
they want and handle the #GP when popping causes a fault.  This has
the added benefit of making our Switcher slightly more robust in the
case of any other bugs which cause it to fault.

We kill the Guest if it causes a fault in the Switcher: it's the
Guest's responsibility to make sure it's not using segments when it
changes them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/lguest/lguest.c

@@ -323,9 +323,12 @@ static void lguest_write_gdt_entry(struct desc_struct *dt,
  * __thread variables).  So we have a hypercall specifically for this case. */
 static void lguest_load_tls(struct thread_struct *t, unsigned int cpu)
 {
+	/* There's one problem which normal hardware doesn't have: the Host
+	 * can't handle us removing entries we're currently using.  So we clear
+	 * the GS register here: if it's needed it'll be reloaded anyway. */
+	loadsegment(gs, 0);
 	lazy_hcall(LHCALL_LOAD_TLS, __pa(&t->tls_array), cpu, 0);
 }
-/*:*/
 
 /*G:038 That's enough excitement for now, back to ploughing through each of
  * the paravirt_ops (we're about 1/3 of the way through).