linux-kernel - [PATCH 1/5] x86/pti: check the return value of pti_user_pagetable_walk

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Date:   Mon, 16 Jul 2018 12:03:36 +0800
From:   Jiang Biao <jiang.biao2@....com.cn>
To:     tglx@...utronix.de, mingo@...hat.com
Cc:     dave.hansen@...ux.intel.com, luto@...nel.org, hpa@...or.com,
        x86@...nel.org, albcamus@...il.com, linux-kernel@...r.kernel.org,
        zhong.weidong@....com.cn, jiang.biao2@....com.cn
Subject: [PATCH 1/5] x86/pti: check the return value of pti_user_pagetable_walk_p4d

pti_user_pagetable_walk_p4d() may return NULL, we should check the
return value to avoid NULL pointer dereference.

Signed-off-by: Jiang Biao <jiang.biao2@....com.cn>
---
 arch/x86/mm/pti.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 4d418e7..be9e5bc 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -195,8 +195,10 @@ 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);
 	pud_t *pud;
+	p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
+	if (WARN_ON(!p4d))
+		return NULL;
 
 	BUILD_BUG_ON(p4d_large(*p4d) != 0);
 	if (p4d_none(*p4d)) {
@@ -354,6 +356,9 @@ static void __init pti_clone_p4d(unsigned long addr)
 	pgd_t *kernel_pgd;
 
 	user_p4d = pti_user_pagetable_walk_p4d(addr);
+	if (WARN_ON(!user_p4d))
+		return;
+	
 	kernel_pgd = pgd_offset_k(addr);
 	kernel_p4d = p4d_offset(kernel_pgd, addr);
 	*user_p4d = *kernel_p4d;
-- 
2.7.4