Re: [PATCH] arm64: mm: mark tramp_pg_dir read-only

[Ard Biesheuvel]

On 30 May 2018 at 11:14, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Wed, May 30, 2018 at 12:48:06PM +0800, YaoJun wrote:
>> To protect against KSMA(Kernel Space Mirroring Attack), make
>> tramp_pg_dir read-only. The principle of KSMA is to insert a
>> carefully constructed PGD entry into the translation table.
>> The type of this entry is block, which maps the kernel text
>> and its access permissions bits are 01. The user process can
>> then modify kernel text directly through this mapping. In this
>> way, an arbitrary write can be converted to multiple arbitrary
>> writes.
>>
>> Signed-off-by: YaoJun <yaojun8558363@xxxxxxxxx>
>> ---
>>  arch/arm64/mm/mmu.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 2dbb2c9f1ec1..ac4b22c7e435 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -551,6 +551,10 @@ static int __init map_entry_trampoline(void)
>>       __create_pgd_mapping(tramp_pg_dir, pa_start, TRAMP_VALIAS, PAGE_SIZE,
>>                            prot, pgd_pgtable_alloc, 0);
>>
>> +     update_mapping_prot(__pa_symbol(tramp_pg_dir),
>> +                             (unsigned long)tramp_pg_dir,
>> +                             PGD_SIZE, PAGE_KERNEL_RO);
>
> Hmm, I like the idea but is there a risk that the page table has been mapped
> as part of a block entry, which we can't safely split at this point (i.e.
> we'll run into one of the BUG_ONs in the mapping code)?
>

We'd need to create a separate segment for it initially so the mapping
is already at the right granularity.