linux-kernel - [PATCH v1] mm/madvise: Always set ptes via arch helpers

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Message-ID: <20250307123307.262298-1-ryan.roberts@arm.com>
Date: Fri,  7 Mar 2025 12:33:06 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Ryan Roberts <ryan.roberts@....com>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: [PATCH v1] mm/madvise: Always set ptes via arch helpers

Instead of writing a pte directly into the table, use the set_pte_at()
helper, which gives the arch visibility of the change.

In this instance we are guaranteed that the pte was originally none and
is being modified to a not-present pte, so there was unlikely to be a
bug in practice (at least not on arm64). But it's bad practice to write
the page table memory directly without arch involvement.

Cc: <stable@...r.kernel.org>
Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism")
Signed-off-by: Ryan Roberts <ryan.roberts@....com>
---
 mm/madvise.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 388dc289b5d1..6170f4acc14f 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1101,7 +1101,7 @@ static int guard_install_set_pte(unsigned long addr, unsigned long next,
 	unsigned long *nr_pages = (unsigned long *)walk->private;

 	/* Simply install a PTE marker, this causes segfault on access. */
-	*ptep = make_pte_marker(PTE_MARKER_GUARD);
+	set_pte_at(walk->mm, addr, ptep, make_pte_marker(PTE_MARKER_GUARD));
 	(*nr_pages)++;

 	return 0;
--
2.43.0