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>

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index c76ef1d..278d645 100644 (file)
--- 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;
 }
 
 /*