linux-kernel - [PATCH AUTOSEL 4.14 58/60] mm/gup.c: remove some BUG_ONs from get_gate

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Message-Id: <20190719041109.18262-58-sashal@kernel.org>
Date:   Fri, 19 Jul 2019 00:11:07 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Florian Weimer <fweimer@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>, linux-mm@...ck.org
Subject: [PATCH AUTOSEL 4.14 58/60] mm/gup.c: remove some BUG_ONs from get_gate_page()

From: Andy Lutomirski <luto@...nel.org>

[ Upstream commit b5d1c39f34d1c9bca0c4b9ae2e339fbbe264a9c7 ]

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.

Link: http://lkml.kernel.org/r/a1d9f4efb75b9d464e59fd6af00104b21c58f6f7.1561610798.git.luto@kernel.org
Signed-off-by: Andy Lutomirski <luto@...nel.org>
Reviewed-by: Kees Cook <keescook@...omium.org>
Reviewed-by: Andrew Morton <akpm@...ux-foundation.org>
Cc: Florian Weimer <fweimer@...hat.com>
Cc: Jann Horn <jannh@...gle.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 mm/gup.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index cee599d1692c..12b9626b1a9e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -442,11 +442,14 @@ 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;
-- 
2.20.1