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mm/page_table_check: skip zero pages

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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: df4e817b71 ("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>
This commit is contained in:
Zhiling Zou 2026-07-21 23:56:38 +08:00 committed by Andrew Morton
parent 0224133dba
commit 33afd07412
1 changed files with 2 additions and 2 deletions
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mm/page_table_check.c
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@ -67,7 +67,7 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
struct page *page;
bool anon;
if (!pfn_valid(pfn))
if (!pfn_valid(pfn) || is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
return;
page = pfn_to_page(pfn);
@ -102,7 +102,7 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
struct page *page;
bool anon;
if (!pfn_valid(pfn))
if (!pfn_valid(pfn) || is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
return;
page = pfn_to_page(pfn);