On 19 June 2018 at 17:20, Will Deacon <[email protected]> wrote: > 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 <[email protected]> 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 <[email protected]> >> >> --- >> >> 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. >
We'd need to pass NO_BLOCK_MAPPINGS to map_kernel_segment(), obviously, but that shouldn't hurt since that segment is relatively tiny anyway.
