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

[Will Deacon]

Hi Ard,

Sorry, I forgot to reply to this.

On Wed, May 30, 2018 at 11:53:20AM +0200, Ard Biesheuvel wrote:
> 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.

Why do you think that's the case? I can't see anything that guarantees this
for the page table itself.

Will