linux-kernel - [PATCH] mm/gup: Remove some BUG_ONs from get_gate

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Date:   Wed, 26 Jun 2019 21:47:30 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Florian Weimer <fweimer@...hat.com>,
        Jann Horn <jannh@...gle.com>, Andy Lutomirski <luto@...nel.org>
Subject: [PATCH] mm/gup: Remove some BUG_ONs from get_gate_page()

If we end up without a PGD or PUD entry backing the gate area, don't
BUG -- just fail gracefully.

It's not entirely implausible that this could happen some day on
x86.  It doesn't right now even with an execute-only emulated
vsyscall page because the fixmap shares the PUD, but the core mm
code shouldn't rely on that particular detail to avoid OOPSing.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 mm/gup.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ddde097cf9e4..9883b598fd6f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -585,11 +585,14 @@ static int get_gate_page(struct mm_struct *mm, unsigned long address,
 		pgd = pgd_offset_k(address);
 	else
 		pgd = pgd_offset_gate(mm, address);
-	BUG_ON(pgd_none(*pgd));
+	if (pgd_none(*pgd))
+		return -EFAULT;
 	p4d = p4d_offset(pgd, address);
-	BUG_ON(p4d_none(*p4d));
+	if (p4d_none(*p4d))
+		return -EFAULT;
 	pud = pud_offset(p4d, address);
-	BUG_ON(pud_none(*pud));
+	if (pud_none(*pud))
+		return -EFAULT;
 	pmd = pmd_offset(pud, address);
 	if (!pmd_present(*pmd))
 		return -EFAULT;
-- 
2.21.0