[PATCH 6.12.y] mm/page_table_check: skip zero pages

[Sasha Levin <sashal@kernel.org>]

From: Zhiling Zou <roxy520tt@gmail.com>

[ Upstream commit 33afd07412a1efe5b750a3fd5eb0272ebdd12761 ]

page_table_check_set() accounts pages by whether they are PageAnon().  The
shared zero page is a special page, not an ordinary file-backed page.
Read faults on private anonymous mappings can install many read-only PTEs
that point at the zero page, but page_table_check currently accounts them
in file_map_count.

That lets an unprivileged process populate enough zero-page mappings to
overflow file_map_count and trip the BUG_ON() in page_table_check_set().

Skip zero pages in page_table_check accounting.  They do not need the
anonymous/file mapping conflict checks that page_table_check performs for
ordinary pages, and this keeps the existing counter size and page_ext
layout unchanged.

Link: https://lore.kernel.org/1f8848512d2e3ded944f8d595c29faee8fdaeab0.1784645969.git.roxy520tt@gmail.com
Fixes: df4e817b7108 ("mm: page table check")
Signed-off-by: Zhiling Zou <roxy520tt@gmail.com>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: Codex:gpt-5.4
Cc: Ye Liu <liuye@kylinos.cn>
Cc: <stable@vger.kernel.org>
[ sashal: Replaced unavailable is_huge_zero_pfn(pfn) with is_huge_zero_folio(pfn_folio(pfn)). ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/page_table_check.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 2efa910eef23e..1076f8e055f31 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -67,7 +67,8 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
 	unsigned long i;
 	bool anon;
 
-	if (!pfn_valid(pfn))
+	if (!pfn_valid(pfn) || is_zero_pfn(pfn) ||
+	    is_huge_zero_folio(pfn_folio(pfn)))
 		return;
 
 	page = pfn_to_page(pfn);
@@ -107,7 +108,8 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
 	unsigned long i;
 	bool anon;
 
-	if (!pfn_valid(pfn))
+	if (!pfn_valid(pfn) || is_zero_pfn(pfn) ||
+	    is_huge_zero_folio(pfn_folio(pfn)))
 		return;
 
 	page = pfn_to_page(pfn);
-- 
2.53.0