[PATCH] mm/gup: Remove some BUG_ONs from get_gate_page()

[Andy Lutomirski]

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@xxxxxxxxxx>
---
 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