lguest: fix BUG_ON() in invalid guest page table.

If we discover the entry is invalid, we kill the guest, but we must
avoid calling gpte_addr() on the invalid pmd, otherwise:

	kernel BUG at drivers/lguest/page_tables.c:157!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index a35d8d100165..bfb39bb56ef1 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -669,8 +669,10 @@ unsigned long guest_pa(struct lg_cpu *cpu, unsigned long vaddr)
 
 #ifdef CONFIG_X86_PAE
         gpmd = lgread(cpu, gpmd_addr(gpgd, vaddr), pmd_t);
-        if (!(pmd_flags(gpmd) & _PAGE_PRESENT))
+        if (!(pmd_flags(gpmd) & _PAGE_PRESENT)) {
                 kill_guest(cpu, "Bad address %#lx", vaddr);
+                return -1UL;
+        }
         gpte = lgread(cpu, gpte_addr(cpu, gpmd, vaddr), pte_t);
 #else
         gpte = lgread(cpu, gpte_addr(cpu, gpgd, vaddr), pte_t);