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Subject[PATCH 1/5] 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>
---
drivers/lguest/page_tables.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -373,8 +373,10 @@ unsigned long guest_pa(struct lg_cpu *cp
/* 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))


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