[PATCH AUTOSEL 4.14 58/60] mm/gup.c: remove some BUG_ONs from get_gate_page()

From: Sasha Levin
Date: Fri Jul 19 2019 - 00:13:05 EST

From: Andy Lutomirski <luto@xxxxxxxxxx>

[ 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@xxxxxxxxxx
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Florian Weimer <fweimer@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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);
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;