From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 19 Jun 2018 16:50:01 +0100 From: Will Deacon Subject: Re: [PATCH] arm64: mm: mark tramp_pg_dir read-only Message-ID: <20180619155001.GP13984@arm.com> References: <20180530044806.18449-1-yaojun8558363@gmail.com> <20180530091457.GB2452@arm.com> <20180619152035.GM13984@arm.com> <20180619152805.GN13984@arm.com> <20180619153754.GO13984@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: To: Ard Biesheuvel Cc: YaoJun , Kernel Hardening , Catalin Marinas , linux-arm-kernel , Linux Kernel Mailing List List-ID: On Tue, Jun 19, 2018 at 05:40:26PM +0200, Ard Biesheuvel wrote: > On 19 June 2018 at 17:37, Will Deacon wrote: > > On Tue, Jun 19, 2018 at 05:29:03PM +0200, Ard Biesheuvel wrote: > >> On 19 June 2018 at 17:28, Will Deacon wrote: > >> > On Tue, Jun 19, 2018 at 05:23:41PM +0200, Ard Biesheuvel wrote: > >> >> On 19 June 2018 at 17:20, Will Deacon 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 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 > >> >> >> >> --- > >> >> >> >> 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. > >> > > >> > Ah right, with NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS, I agree that we're good. > >> > Ideally, we'd move {idmap,swapper,tramp}_pg_dir into .rodata... > >> > > >> > >> idmap and tramp yes, but swapper needs to be modifiable at runtime, no? > > > > Right, but couldn't we swizzle the permissions in e.g. set_pmd? We could > > even predicate that on a sanity check of the prot. > > > > Swizzle the permissions of the entire .rodata segment? That sounds > doable, but there is a whole class of data that belongs in this > category, and I think PaX/grsecurity had an API for that (but I don't > think anyone is upstreaming that yet). So let's not reinvent that > wheel for swapper_pg_dir only. I wasn't thinking of the whole .rodata segment -- just the page containing the entry being modified, but ok. I was hoping to avoid getting involved with the PaX/grsec stuff as it seems rather toxic from what I've seen. Will From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Tue, 19 Jun 2018 16:50:01 +0100 Subject: [PATCH] arm64: mm: mark tramp_pg_dir read-only In-Reply-To: References: <20180530044806.18449-1-yaojun8558363@gmail.com> <20180530091457.GB2452@arm.com> <20180619152035.GM13984@arm.com> <20180619152805.GN13984@arm.com> <20180619153754.GO13984@arm.com> Message-ID: <20180619155001.GP13984@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Tue, Jun 19, 2018 at 05:40:26PM +0200, Ard Biesheuvel wrote: > On 19 June 2018 at 17:37, Will Deacon wrote: > > On Tue, Jun 19, 2018 at 05:29:03PM +0200, Ard Biesheuvel wrote: > >> On 19 June 2018 at 17:28, Will Deacon wrote: > >> > On Tue, Jun 19, 2018 at 05:23:41PM +0200, Ard Biesheuvel wrote: > >> >> On 19 June 2018 at 17:20, Will Deacon 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 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 > >> >> >> >> --- > >> >> >> >> 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. > >> > > >> > Ah right, with NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS, I agree that we're good. > >> > Ideally, we'd move {idmap,swapper,tramp}_pg_dir into .rodata... > >> > > >> > >> idmap and tramp yes, but swapper needs to be modifiable at runtime, no? > > > > Right, but couldn't we swizzle the permissions in e.g. set_pmd? We could > > even predicate that on a sanity check of the prot. > > > > Swizzle the permissions of the entire .rodata segment? That sounds > doable, but there is a whole class of data that belongs in this > category, and I think PaX/grsecurity had an API for that (but I don't > think anyone is upstreaming that yet). So let's not reinvent that > wheel for swapper_pg_dir only. I wasn't thinking of the whole .rodata segment -- just the page containing the entry being modified, but ok. I was hoping to avoid getting involved with the PaX/grsec stuff as it seems rather toxic from what I've seen. Will