Re: [RFCv2] arm64: support HAVE_ARCH_RARE_WRITE and HAVE_ARCH_RARE_WRITE_MEMCPY

From: Ard Biesheuvel
Date: Mon Apr 03 2017 - 03:11:22 EST

On 3 April 2017 at 05:17, Ho-Eun Ryu <hoeun.ryu@xxxxxxxxx> wrote:
>
>> 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
>

OK, that explains a number of issues with this patch.

Please understand that marking a function inline has *no* effect
whatsoever if the C definition is not visible to the caller. This is
why the C declarations in the header file are not made generic: these
functions are supposed to be *defined* as static inline in header
files.

So please move these implementations into the header files, and mark
them static inline. This means the rare_write_mm structure should
*not* be made static either.


>>
>>> +{
>>> + 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
>