Re: [RFCv2] arm64: support HAVE_ARCH_RARE_WRITE and HAVE_ARCH_RARE_WRITE_MEMCPY

[Ho-Eun Ryu]

> On 31 Mar 2017, at 6:25 PM, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> 
> On 30 March 2017 at 15:39, Hoeun Ryu <hoeun.ryu@xxxxxxxxx> wrote:
>> 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@xxxxxxxxx>
>> ---
>> 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);
>> +
> 
> If these hook into a generic framework, shouldn't these already be
> declared in a generic header file?

the generic header file doesn't declare arch-specific version of api.

> 
>> #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 = {
> 
> static please

OK, add static. 

> 
>> +       .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)
> 
> These functions are not called from the same compilation unit, so what
> do you think __always_inline buys you here?

The first patch of Kee's series ([PATCH 01/11] Introduce rare_write() infrastructure) [1] says some requirements and one of these is the arch-specific helpers should be inline to avoid making them ROP targets.

So I'd make sure the helpers are inline.

[1] : http://www.openwall.com/lists/kernel-hardening/2017/03/29/16

> 
>> +{
>> +       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)
> 
> static please

OK, add static. 

> 
>> +{
>> +       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();
>> +
> 
> This places the rodata alias at a fixed offset from the original,
> right, even under KASLR? Given that they are completely independent, I
> think you can use a random offset here rather than the same offset we
> use for vmlinux.

What I did here is just to randomize the base of rw alias mapping when kaslr is enabled.
You're right it's completely independent from kaslr though, I would like to add randomization for rw alias base address by using simple solution in early boot stage.

I' ll think again to achieve randomization or just remove it.

> 
> 
>> +       BUG_ON(rodata_rw_alias_start + size > TASK_SIZE_64);
>> +
>> +       rare_write_mm.pgd = pgd;
> 
> For correctness, you should add a call to mm_init_cpumask() here,
> although it doesn't matter in practice on arm64.

OK, I'll fix this. 

> 
>> +       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());
> 
> This needs to be updated after the changes that are now queued in the
> arm64 tree for v4.12
> 
> BTW you can allow page/cont mappings in all cases, no need for the
> debug_pagealloc_enabled() check

I will rebase this onto the version you mentioned later.

> 
>> +}
>> +
>> /*
>>  * 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
>> 

Thank you very much for the review