lguest: fix spurious BUG_ON() on invalid guest stack.

Impact: fix crash on misbehaving guest

gpte_addr() contains a BUG_ON(), insisting that the present flag is
set.  We need to return before we call it if that isn't the case.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index 576a8318221c..82ff484bd8c8 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -373,8 +373,10 @@ unsigned long guest_pa(struct lg_cpu *cpu, unsigned long vaddr)
         /* First step: get the top-level Guest page table entry. */
         gpgd = lgread(cpu, gpgd_addr(cpu, vaddr), pgd_t);
         /* Toplevel not present?  We can't map it in. */
-        if (!(pgd_flags(gpgd) & _PAGE_PRESENT))
+        if (!(pgd_flags(gpgd) & _PAGE_PRESENT)) {
                 kill_guest(cpu, "Bad address %#lx", vaddr);
+                return -1UL;
+        }
 
         gpte = lgread(cpu, gpte_addr(gpgd, vaddr), pte_t);
         if (!(pte_flags(gpte) & _PAGE_PRESENT))