[PATCH 4.18 094/235] x86/pti: Check the return value of pti_user_pagetable_walk_p4d()

[Greg Kroah-Hartman]

4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jiang Biao <jiang.biao2@xxxxxxxxxx>

[ Upstream commit b2b7d986a89b6c94b1331a909de1217214fb08c1 ]

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@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: dave.hansen@xxxxxxxxxxxxxxx
Cc: luto@xxxxxxxxxx
Cc: hpa@xxxxxxxxx
Cc: albcamus@xxxxxxxxx
Cc: zhong.weidong@xxxxxxxxxx
Link: https://lkml.kernel.org/r/1532045192-49622-1-git-send-email-jiang.biao2@xxxxxxxxxx
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/mm/pti.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -177,7 +177,7 @@ static p4d_t *pti_user_pagetable_walk_p4
 
 	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_p4
 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);
@@ -355,6 +359,9 @@ static void __init pti_clone_p4d(unsigne
 	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;