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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Hugh Dickens <hughd@google.com>,
        Greg Ungerer <gerg@linux-m68k.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.15 120/247] m68knommu: set ZERO_PAGE() to the allocated zeroed page
Date:   Mon, 13 Jun 2022 12:10:22 +0200
Message-Id: <20220613094926.603648110@linuxfoundation.org>
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From: Greg Ungerer <gerg@linux-m68k.org>

[ Upstream commit dc068f46217970d9516f16cd37972a01d50dc055 ]

The non-MMU m68k pagetable ZERO_PAGE() macro is being set to the
somewhat non-sensical value of "virt_to_page(0)". The zeroth page
is not in any way guaranteed to be a page full of "0". So the result
is that ZERO_PAGE() will almost certainly contain random values.

We already allocate a real "empty_zero_page" in the mm setup code shared
between MMU m68k and non-MMU m68k. It is just not hooked up to the
ZERO_PAGE() macro for the non-MMU m68k case.

Fix ZERO_PAGE() to use the allocated "empty_zero_page" pointer.

I am not aware of any specific issues caused by the old code.

Link: https://lore.kernel.org/linux-m68k/2a462b23-5b8e-bbf4-ec7d-778434a3b9d7@google.com/T/#t
Reported-by: Hugh Dickens <hughd@google.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/m68k/include/asm/pgtable_no.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/m68k/include/asm/pgtable_no.h b/arch/m68k/include/asm/pgtable_no.h
index 87151d67d91e..bce5ca56c388 100644
--- a/arch/m68k/include/asm/pgtable_no.h
+++ b/arch/m68k/include/asm/pgtable_no.h
@@ -42,7 +42,8 @@ extern void paging_init(void);
  * ZERO_PAGE is a global shared page that is always zero: used
  * for zero-mapped memory areas etc..
  */
-#define ZERO_PAGE(vaddr)	(virt_to_page(0))
+extern void *empty_zero_page;
+#define ZERO_PAGE(vaddr)	(virt_to_page(empty_zero_page))
 
 /*
  * All 32bit addresses are effectively valid for vmalloc...
-- 
2.35.1