[PATCH] arm64: mm: move zero page from .bss to right before swapper_pg_dir

[Ard Biesheuvel]

On 12 September 2016 at 13:57, Mark Rutland <mark.rutland at arm.com> wrote:
> Hi,
>
> On Sun, Sep 11, 2016 at 03:38:34PM +0100, Ard Biesheuvel wrote:
>> Move the statically allocated zero page from the .bss section to right
>> before swapper_pg_dir. This allows us to refer to its physical address
>> by simply reading TTBR1_EL1 (which always points to swapper_pg_dir and
>> always has its ASID field cleared), and subtracting PAGE_SIZE.
>
> On a conflicting note, I was hoping to move the zero page into .rodata
> so as to catch any erroneous modification.
>
> Given that we can't rely on TTBR1 poiting at the swapper_pg_dir, that
> leaves us with Image size reduction vs RO-ification.
>
> Any thoughts/preference?
>

That's a good point. v2 coming up ...


>> Inspired-by: http://marc.info/?l=linux-arm-kernel&m=147282867511801
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>  arch/arm64/include/asm/mmu_context.h | 10 ++++++----
>>  arch/arm64/kernel/head.S             |  1 -
>>  arch/arm64/kernel/vmlinux.lds.S      |  2 ++
>>  arch/arm64/mm/mmu.c                  |  1 -
>>  4 files changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
>> index b1892a0dbcb0..94461ba5febd 100644
>> --- a/arch/arm64/include/asm/mmu_context.h
>> +++ b/arch/arm64/include/asm/mmu_context.h
>> @@ -49,13 +49,15 @@ static inline void contextidr_thread_switch(struct task_struct *next)
>>   */
>>  static inline void cpu_set_reserved_ttbr0(void)
>>  {
>> -     unsigned long ttbr = virt_to_phys(empty_zero_page);
>> +     unsigned long dummy;
>>
>> -     asm(
>> +     asm volatile(
>> +     "       mrs     %0, ttbr1_el1                   // get TTBR1\n"
>> +     "       sub     %0, %0, %1                      // subtract PAGE_SIZE\n"
>>       "       msr     ttbr0_el1, %0                   // set TTBR0\n"
>>       "       isb"
>> -     :
>> -     : "r" (ttbr));
>> +     : "=&r" (dummy)
>> +     : "I" (PAGE_SIZE));
>>  }
>>
>>  /*
>> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> index 8bc9458f9add..6020b884b076 100644
>> --- a/arch/arm64/kernel/head.S
>> +++ b/arch/arm64/kernel/head.S
>> @@ -449,7 +449,6 @@ __primary_switched:
>>       adr_l   x2, __bss_stop
>>       sub     x2, x2, x0
>>       bl      __pi_memset
>> -     dsb     ishst                           // Make zero page visible to PTW
>>
>>  #ifdef CONFIG_KASAN
>>       bl      kasan_early_init
>> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
>> index 659963d40bb4..a14eb8ff5144 100644
>> --- a/arch/arm64/kernel/vmlinux.lds.S
>> +++ b/arch/arm64/kernel/vmlinux.lds.S
>> @@ -193,6 +193,8 @@ SECTIONS
>>       . = ALIGN(PAGE_SIZE);
>>       idmap_pg_dir = .;
>>       . += IDMAP_DIR_SIZE;
>> +     empty_zero_page = .;
>> +     . += PAGE_SIZE;
>>       swapper_pg_dir = .;
>>       . += SWAPPER_DIR_SIZE;
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 4989948d1feb..539ce9d11325 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -53,7 +53,6 @@ EXPORT_SYMBOL(kimage_voffset);
>>   * Empty_zero_page is a special page that is used for zero-initialized data
>>   * and COW.
>>   */
>> -unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;
>>  EXPORT_SYMBOL(empty_zero_page);
>>
>>  static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
>> --
>> 2.7.4
>>
>>
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>>