x86: check PMD in spurious_fault handler

Impact: fix to prevent hard lockup on bad PMD permissions

If the PMD does not have the correct permissions for a page access,
but the PTE does, the spurious fault handler will mistake the fault
as a lazy TLB transaction. This will result in an infinite loop of:

 fault -> spurious_fault check (pass) -> return to code -> fault

This patch adds a check and a warn on if the PTE passes the permissions
but the PMD does not.

[ Updated: Ingo Molnar suggested using WARN_ONCE with some text ]

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

1 files changed, 12 insertions, 1 deletions

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index c76ef1d701c..278d645d108 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -455,6 +455,7 @@ static int spurious_fault(unsigned long address,
         pud_t *pud;
         pmd_t *pmd;
         pte_t *pte;
+        int ret;
 
         /* Reserved-bit violation or user access to kernel space? */
         if (error_code & (PF_USER | PF_RSVD))
@@ -482,7 +483,17 @@ static int spurious_fault(unsigned long address,
         if (!pte_present(*pte))
                 return 0;
 
-        return spurious_fault_check(error_code, pte);
+        ret = spurious_fault_check(error_code, pte);
+        if (!ret)
+                return 0;
+
+        /*
+         * Make sure we have permissions in PMD
+         * If not, then there's a bug in the page tables.
+         */
+        ret = spurious_fault_check(error_code, (pte_t *) pmd);
+        WARN_ONCE(!ret, "PMD has incorrect permission bits\n");
+        return ret;
 }
 
 /*