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SubjectRe: [RFCv2] arm64: support HAVE_ARCH_RARE_WRITE and HAVE_ARCH_RARE_WRITE_MEMCPY
On 30 March 2017 at 15:39, Hoeun Ryu <hoeun.ryu@gmail.com> 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@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);
> +

If these hook into a generic framework, shouldn't these already be
declared in a generic header file?

> #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

> + .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?

> +{
> + 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

> +{
> + 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.


> + 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.

> + 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

> +}
> +
> /*
> * 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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