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From:   Pasha Tatashin <pasha.tatashin@soleen.com>
To:     pasha.tatashin@soleen.com, akpm@linux-foundation.org,
        corbet@lwn.net, linux-mm@kvack.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, rick.p.edgecombe@intel.com
Subject: [PATCH v2 3/3] mm/page_table_check: Check writable zero page in page table check
Date:   Sat, 22 Jul 2023 23:15:08 +0000
Message-ID: <20230722231508.1030269-4-pasha.tatashin@soleen.com>
In-Reply-To: <20230722231508.1030269-1-pasha.tatashin@soleen.com>
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From: Rick Edgecombe <rick.p.edgecombe@intel.com>

The zero page should remain all zero, so that it can be mapped as
read-only for read faults of memory that should be zeroed. If it is ever
mapped writable to userspace, it could become non-zero and so other apps
would unexpectedly get non-zero data. So the zero page should never be
mapped writable to userspace. Check for this condition in
page_table_check_set().

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 mm/page_table_check.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index ad4447e999f8..db1ed36f7203 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -114,6 +114,8 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
 	if (!pfn_valid(pfn))
 		return;
 
+	PAGE_TABLE_CHECK_WARN(is_zero_pfn(pfn) && rw);
+
 	page = pfn_to_page(pfn);
 	page_ext = page_ext_get(page);
 
-- 
2.41.0.487.g6d72f3e995-goog