linux-kernel - [PATCH 2/2] mm/kasan: avoid lazy MMU mode hazards

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Message-ID: <0d2efb7ddddbff6b288fbffeeb10166e90771718.1755528662.git.agordeev@linux.ibm.com>
Date: Mon, 18 Aug 2025 18:39:13 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Andrey Ryabinin <ryabinin.a.a@...il.com>, Daniel Axtens <dja@...ens.net>,
        Mark Rutland <mark.rutland@....com>,
        Ryan Roberts <ryan.roberts@....com>
Cc: linux-mm@...ck.org, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: [PATCH 2/2] mm/kasan: avoid lazy MMU mode hazards

Functions __kasan_populate_vmalloc() and __kasan_depopulate_vmalloc()
use apply_to_pte_range() function, which enters the lazy MMU mode. In
that mode updating PTEs may not be observed until the mode is left.

That may lead to a situation that otherwise correct reads and writes
to a PTE using ptep_get(), set_pte(), pte_clear() and other access
primitives bring wrong results when the vmalloc shadow memory is
being (de-)populated.

To avoid these hazards leave the lazy MMU mode before and re-enter it
after each PTE manipulation.

Fixes: 3c5c3cfb9ef4 ("kasan: support backing vmalloc space with real shadow memory")
Signed-off-by: Alexander Gordeev <agordeev@...ux.ibm.com>
---
 mm/kasan/shadow.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index 4d846d146d02..e2ceebf737ef 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -305,6 +305,8 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 	pte_t pte;
 	int index;
 
+	arch_leave_lazy_mmu_mode();
+
 	index = PFN_DOWN(addr - data->start);
 	page = data->pages[index];
 	__memset(page_to_virt(page), KASAN_VMALLOC_INVALID, PAGE_SIZE);
@@ -317,6 +319,8 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 	}
 	spin_unlock(&init_mm.page_table_lock);
 
+	arch_enter_lazy_mmu_mode();
+
 	return 0;
 }
 
@@ -461,6 +465,8 @@ static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 	pte_t pte;
 	int none;
 
+	arch_leave_lazy_mmu_mode();
+
 	spin_lock(&init_mm.page_table_lock);
 	pte = ptep_get(ptep);
 	none = pte_none(pte);
@@ -471,6 +477,8 @@ static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
 	if (likely(!none))
 		__free_page(pfn_to_page(pte_pfn(pte)));
 
+	arch_enter_lazy_mmu_mode();
+
 	return 0;
 }
 
-- 
2.48.1