arm64: kasan: ensure that the KASAN zero page is mapped read-only

When switching from the early KASAN shadow region, which maps the
entire shadow space read-write, to the permanent KASAN shadow region,
which uses a zero page to shadow regions that are not subject to
instrumentation, the lowest level table kasan_zero_pte[] may be
reused unmodified, which means that the mappings of the zero page
that it contains will still be read-write.

So update it explicitly to map the zero page read only when we
activate the permanent mapping.

Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index cf038c7..cab7a5b 100644 (file)
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -120,6 +120,7 @@ static void __init cpu_set_ttbr1(unsigned long ttbr1)
 void __init kasan_init(void)
 {
        struct memblock_region *reg;
+       int i;
 
        /*
         * We are going to perform proper setup of shadow memory.
@@ -155,6 +156,14 @@ void __init kasan_init(void)
                                pfn_to_nid(virt_to_pfn(start)));
        }
 
+       /*
+        * KAsan may reuse the contents of kasan_zero_pte directly, so we
+        * should make sure that it maps the zero page read-only.
+        */
+       for (i = 0; i < PTRS_PER_PTE; i++)
+               set_pte(&kasan_zero_pte[i],
+                       pfn_pte(virt_to_pfn(kasan_zero_page), PAGE_KERNEL_RO));
+
        memset(kasan_zero_page, 0, PAGE_SIZE);
        cpu_set_ttbr1(__pa(swapper_pg_dir));
        flush_tlb_all();