[PATCH v4 2/2] x86/pti: check the return value of pti_user_pagetable_walk_pmd

[Jiang Biao]

Check the return value of pti_user_pagetable_walk_pmd() to avoid
NULL pointer dereference. And add warning for fail allocation.

Signed-off-by: Jiang Biao <jiang.bi...@zte.com.cn>
---
 arch/x86/mm/pti.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 8679c64..5cd7b82 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -205,7 +205,7 @@ static pmd_t *pti_user_pagetable_walk_pmd(unsigned long 
address)
        BUILD_BUG_ON(p4d_large(*p4d) != 0);
        if (p4d_none(*p4d)) {
                unsigned long new_pud_page = __get_free_page(gfp);
-               if (!new_pud_page)
+               if (WARN_ON_ONCE(!new_pud_page))
                        return NULL;

                set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
@@ -219,7 +219,7 @@ static pmd_t *pti_user_pagetable_walk_pmd(unsigned long 
address)
        }
        if (pud_none(*pud)) {
                unsigned long new_pmd_page = __get_free_page(gfp);
-               if (!new_pmd_page)
+               if (WARN_ON_ONCE(!new_pmd_page))
                        return NULL;

                set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
@@ -241,9 +241,13 @@ static pmd_t *pti_user_pagetable_walk_pmd(unsigned long 
address)
 static __init pte_t *pti_user_pagetable_walk_pte(unsigned long address)
 {
        gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
-       pmd_t *pmd = pti_user_pagetable_walk_pmd(address);
+       pmd_t *pmd;
        pte_t *pte;

+       pmd = pti_user_pagetable_walk_pmd(address);
+       if (!pmd)
+               return NULL;
+
        /* We can't do anything sensible if we hit a large mapping. */
        if (pmd_large(*pmd)) {
                WARN_ON(1);
--
2.7.4