linux-hardening - [RFC PATCH 02/12] arm64: mm: add helpers to remap page tables read-only/read-write

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Date:   Wed, 26 Jan 2022 18:30:01 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     kvmarm@...ts.cs.columbia.edu, linux-hardening@...r.kernel.org,
        Ard Biesheuvel <ardb@...nel.org>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Fuad Tabba <tabba@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: [RFC PATCH 02/12] arm64: mm: add helpers to remap page tables read-only/read-write

Add a couple of helpers to remap a single page read-only or read-write
via its linear address. This will be used for mappings of page table
pages in the linear region.

Note that set_memory_ro/set_memory_rw operate on addresses in the
vmalloc space only, so they cannot be used here.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/arm64/include/asm/pgtable.h |  3 +++
 arch/arm64/mm/pageattr.c         | 14 ++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index c4ba047a82d2..8d3806c68687 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -34,6 +34,9 @@
 #include <linux/mm_types.h>
 #include <linux/sched.h>
 
+int set_pgtable_ro(void *addr);
+int set_pgtable_rw(void *addr);
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 #define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE
 
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index a3bacd79507a..61f4aca08b95 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -153,6 +153,20 @@ int set_memory_valid(unsigned long addr, int numpages, int enable)
 					__pgprot(PTE_VALID));
 }
 
+int set_pgtable_ro(void *addr)
+{
+	return __change_memory_common((u64)addr, PAGE_SIZE,
+				      __pgprot(PTE_RDONLY),
+				      __pgprot(PTE_WRITE));
+}
+
+int set_pgtable_rw(void *addr)
+{
+	return __change_memory_common((u64)addr, PAGE_SIZE,
+				      __pgprot(PTE_WRITE),
+				      __pgprot(PTE_RDONLY));
+}
+
 int set_direct_map_invalid_noflush(struct page *page)
 {
 	struct page_change_data data = {
-- 
2.30.2