x86/pti: Check the return value of pti_user_pagetable_walk_p4d()

CVE-2017-5754

pti_user_pagetable_walk_p4d() can return NULL, so the return value should
be checked to prevent a NULL pointer dereference.

Add the check and a warning when the P4D allocation fails.

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: dave.hansen@linux.intel.com
Cc: luto@kernel.org
Cc: hpa@zytor.com
Cc: albcamus@gmail.com
Cc: zhong.weidong@zte.com.cn
Link: https://lkml.kernel.org/r/1532045192-49622-1-git-send-email-jiang.biao2@zte.com.cn
(cherry picked from commit b2b7d986a89b6c94b1331a909de1217214fb08c1)
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index f84d0f6b1649fa388097f9384080bbaf82f8b38d..3aa6bab21d0d57d54cd4eb01e8c00424866dbc0f 100644 (file)
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -177,7 +177,7 @@ static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
 
        if (pgd_none(*pgd)) {
                unsigned long new_p4d_page = __get_free_page(gfp);
-               if (!new_p4d_page)
+               if (WARN_ON_ONCE(!new_p4d_page))
                        return NULL;
 
                set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
@@ -196,9 +196,13 @@ static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
 static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
 {
        gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
-       p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
+       p4d_t *p4d;
        pud_t *pud;
 
+       p4d = pti_user_pagetable_walk_p4d(address);
+       if (!p4d)
+               return NULL;
+
        BUILD_BUG_ON(p4d_large(*p4d) != 0);
        if (p4d_none(*p4d)) {
                unsigned long new_pud_page = __get_free_page(gfp);
@@ -360,6 +364,9 @@ static void __init pti_clone_p4d(unsigned long addr)
        pgd_t *kernel_pgd;
 
        user_p4d = pti_user_pagetable_walk_p4d(addr);
+       if (!user_p4d)
+               return;
+
        kernel_pgd = pgd_offset_k(addr);
        kernel_p4d = p4d_offset(kernel_pgd, addr);
        *user_p4d = *kernel_p4d;