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Subject[RFCv2] arm64: support HAVE_ARCH_RARE_WRITE and HAVE_ARCH_RARE_WRITE_MEMCPY
Date
 This patch might be a part of Kees Cook's rare_write infrastructure series
for [1] for arm64 architecture.

This implementation is based on Mark Rutland's suggestions [2], which is
that a special userspace mm that maps only __start/end_rodata as RW permi-
ssion is prepared during early boot time (paging_init) and __arch_rare_-
write_begin() switches to the mm [2].

rare_write_mm address space is added for the special purpose and a page
global directory is also prepared for it. The mm remaps __start_rodata ~
__end_rodata to rodata_rw_alias_start which starts from TASK_SIZE_64 / 4
+ kaslr_offset().

It passes LKDTM's rare write test.

[1] : http://www.openwall.com/lists/kernel-hardening/2017/02/27/5
[2] : https://lkml.org/lkml/2017/3/29/704

Signed-off-by: Hoeun Ryu <hoeun.ryu@gmail.com>
---
arch/arm64/Kconfig | 2 +
arch/arm64/include/asm/pgtable.h | 4 ++
arch/arm64/mm/mmu.c | 101 +++++++++++++++++++++++++++++++++++++++
3 files changed, 107 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index f2b0b52..6e2c592 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -102,6 +102,8 @@ config ARM64
select HAVE_SYSCALL_TRACEPOINTS
select HAVE_KPROBES
select HAVE_KRETPROBES
+ select HAVE_ARCH_RARE_WRITE
+ select HAVE_ARCH_RARE_WRITE_MEMCPY
select IOMMU_DMA if IOMMU_SUPPORT
select IRQ_DOMAIN
select IRQ_FORCED_THREADING
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index c213fdbd0..1514933 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -741,6 +741,10 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
#define kc_vaddr_to_offset(v) ((v) & ~VA_START)
#define kc_offset_to_vaddr(o) ((o) | VA_START)

+unsigned long __arch_rare_write_begin(void);
+unsigned long __arch_rare_write_end(void);
+void __arch_rare_write_memcpy(void *dst, const void *src, __kernel_size_t len);
+
#endif /* !__ASSEMBLY__ */

#endif /* __ASM_PGTABLE_H */
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 91502e3..86b25c9 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -570,6 +570,105 @@ static void __init map_kernel(pgd_t *pgd)
kasan_copy_shadow(pgd);
}

+struct mm_struct rare_write_mm = {
+ .mm_rb = RB_ROOT,
+ .mm_users = ATOMIC_INIT(2),
+ .mm_count = ATOMIC_INIT(1),
+ .mmap_sem = __RWSEM_INITIALIZER(rare_write_mm.mmap_sem),
+ .page_table_lock= __SPIN_LOCK_UNLOCKED(rare_write_mm.page_table_lock),
+ .mmlist = LIST_HEAD_INIT(rare_write_mm.mmlist),
+};
+
+#ifdef CONFIG_ARM64_PTDUMP_DEBUGFS
+#include <asm/ptdump.h>
+
+static struct ptdump_info rare_write_ptdump_info = {
+ .mm = &rare_write_mm,
+ .markers = (struct addr_marker[]){
+ { 0, "rare-write start" },
+ { TASK_SIZE_64, "rare-write end" }
+ },
+ .base_addr = 0,
+};
+
+static int __init ptdump_init(void)
+{
+ return ptdump_debugfs_register(&rare_write_ptdump_info,
+ "rare_write_page_tables");
+}
+device_initcall(ptdump_init);
+
+#endif
+
+__always_inline unsigned long __arch_rare_write_begin(void)
+{
+ struct mm_struct *mm = &rare_write_mm;
+
+ preempt_disable();
+
+ __switch_mm(mm);
+
+ if (system_uses_ttbr0_pan()) {
+ update_saved_ttbr0(current, mm);
+ cpu_switch_mm(mm->pgd, mm);
+ }
+
+ return 0;
+}
+
+__always_inline unsigned long __arch_rare_write_end(void)
+{
+ struct mm_struct *mm = current->active_mm;
+
+ __switch_mm(mm);
+
+ if (system_uses_ttbr0_pan()) {
+ cpu_set_reserved_ttbr0();
+ if (mm != &init_mm)
+ update_saved_ttbr0(current, mm);
+ }
+
+ preempt_enable_no_resched();
+
+ return 0;
+}
+
+static unsigned long rodata_rw_alias_start __ro_after_init = TASK_SIZE_64 / 4;
+
+__always_inline void __arch_rare_write_memcpy(void *dst, const void *src,
+ __kernel_size_t len)
+{
+ unsigned long __dst = (unsigned long)dst;
+
+ __dst -= (unsigned long)__start_rodata;
+ __dst += rodata_rw_alias_start;
+
+ memcpy((void *)__dst, src, len);
+}
+
+void __init rare_write_init(void)
+{
+ phys_addr_t pgd_phys = early_pgtable_alloc();
+ pgd_t *pgd = (pgd_t *)__phys_to_virt(pgd_phys);
+ phys_addr_t pa_start = __pa_symbol(__start_rodata);
+ unsigned long size = __end_rodata - __start_rodata;
+
+ BUG_ON(!PAGE_ALIGNED(pa_start));
+ BUG_ON(!PAGE_ALIGNED(size));
+
+ rodata_rw_alias_start += kaslr_offset();
+
+ BUG_ON(rodata_rw_alias_start + size > TASK_SIZE_64);
+
+ rare_write_mm.pgd = pgd;
+ init_new_context(NULL, &rare_write_mm);
+
+ __create_pgd_mapping(pgd,
+ pa_start, rodata_rw_alias_start, size,
+ __pgprot(pgprot_val(PAGE_KERNEL) | PTE_NG),
+ early_pgtable_alloc, debug_pagealloc_enabled());
+}
+
/*
* paging_init() sets up the page tables, initialises the zone memory
* maps and sets up the zero page.
@@ -603,6 +702,8 @@ void __init paging_init(void)
*/
memblock_free(__pa_symbol(swapper_pg_dir) + PAGE_SIZE,
SWAPPER_DIR_SIZE - PAGE_SIZE);
+
+ rare_write_init();
}

/*
--
2.7.4
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