Message-ID: <20250423010359.2030576-1-wangming01@loongson.cn>
Date: Wed, 23 Apr 2025 09:03:59 +0800
From: Ming Wang <wangming01@...ngson.cn>
To: Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Matthew Wilcox <willy@...radead.org>,
	Oscar Salvador <osalvador@...e.de>,
	Andrii Nakryiko <andrii@...nel.org>,
	Ryan Roberts <ryan.roberts@....com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Naoya Horiguchi <nao.horiguchi@...il.com>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Joern Engel <joern@...fs.org>,
	Hugh Dickins <hughd@...gle.com>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Cc: Huacai Chen <chenhuacai@...nel.org>,
	lixuefeng@...ngson.cn,
	Hongchen Zhang <zhanghongchen@...ngson.cn>
Subject: [PATCH] smaps: Fix crash in smaps_hugetlb_range for non-present hugetlb entries

When reading /proc/pid/smaps for a process that has mapped a hugetlbfs
file with MAP_PRIVATE, the kernel might crash inside pfn_swap_entry_to_page.
This occurs on LoongArch under specific conditions.

The root cause involves several steps:
1. When the hugetlbfs file is mapped (MAP_PRIVATE), the initial PMD
   (or relevant level) entry is often populated by the kernel during mmap()
   with a non-present entry pointing to the architecture's invalid_pte_table
   On the affected LoongArch system, this address was observed to
   be 0x90000000031e4000.
2. The smaps walker (walk_hugetlb_range -> smaps_hugetlb_range) reads
   this entry.
3. The generic is_swap_pte() macro checks `!pte_present() && !pte_none()`.
   The entry (invalid_pte_table address) is not present. Crucially,
   the generic pte_none() check (`!(pte_val(pte) & ~_PAGE_GLOBAL)`)
   returns false because the invalid_pte_table address is non-zero.
   Therefore, is_swap_pte() incorrectly returns true.
4. The code enters the `else if (is_swap_pte(...))` block.
5. Inside this block, it checks `is_pfn_swap_entry()`. Due to a bit
   pattern coincidence in the invalid_pte_table address on LoongArch,
   the embedded generic `is_migration_entry()` check happens to return
   true (misinterpreting parts of the address as a migration type).
6. This leads to a call to pfn_swap_entry_to_page() with the bogus
   swap entry derived from the invalid table address.
7. pfn_swap_entry_to_page() extracts a meaningless PFN, finds an
   unrelated struct page, checks its lock status (unlocked), and hits
   the `BUG_ON(is_migration_entry(entry) && !PageLocked(p))` assertion.

The original code's intent in the `else if` block seems aimed at handling
potential migration entries, as indicated by the inner `is_pfn_swap_entry()`
check. The issue arises because the outer `is_swap_pte()` check incorrectly
includes the invalid table pointer case on LoongArch.

This patch fixes the issue by changing the condition in
smaps_hugetlb_range() from the broad `is_swap_pte()` to the specific
`is_hugetlb_entry_migration()`.

The `is_hugetlb_entry_migration()` helper function correctly handles this
by first checking `huge_pte_none()`. Architectures like LoongArch can
provide an override for `huge_pte_none()` that specifically recognizes
the `invalid_pte_table` address as a "none" state for HugeTLB entries.
This ensures `is_hugetlb_entry_migration()` returns false for the invalid
entry, preventing the code from entering the faulty block.

This change makes the code reflect the likely original intent (handling
migration) more accurately and leverages architecture-specific helpers
(`huge_pte_none`) to correctly interpret special PTE/PMD values in the
HugeTLB context, fixing the crash on LoongArch without altering the
generic is_swap_pte() behavior.

Fixes: 25ee01a2fca0 ("mm: hugetlb: proc: add hugetlb-related fields to /proc/PID/smaps")
Co-developed-by: Hongchen Zhang <zhanghongchen@...ngson.cn>
Signed-off-by: Hongchen Zhang <zhanghongchen@...ngson.cn>
Signed-off-by: Ming Wang <wangming01@...ngson.cn>
---
 fs/proc/task_mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 994cde10e3f4..95a0093ae87c 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1027,7 +1027,7 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
 	if (pte_present(ptent)) {
 		folio = page_folio(pte_page(ptent));
 		present = true;
-	} else if (is_swap_pte(ptent)) {
+	} else if (is_hugetlb_entry_migration(ptent)) {
 		swp_entry_t swpent = pte_to_swp_entry(ptent);
 
 		if (is_pfn_swap_entry(swpent))
-- 
2.43.0

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