Re: [PATCH 05/25] x86/sgx: Introduce runtime protection bits

From: Haitao Huang
Date: Wed Jan 12 2022 - 21:41:53 EST

On Wed, 12 Jan 2022 17:48:48 -0600, Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:

On Mon, Jan 10, 2022 at 09:48:15PM -0600, Haitao Huang wrote:
On Mon, 10 Jan 2022 20:15:28 -0600, Jarkko Sakkinen <jarkko@xxxxxxxxxx>
wrote:

> On Tue, Jan 11, 2022 at 04:03:32AM +0200, Jarkko Sakkinen wrote:
> > On Tue, Jan 11, 2022 at 03:55:59AM +0200, Jarkko Sakkinen wrote:
> > > On Tue, Jan 11, 2022 at 03:53:26AM +0200, Jarkko Sakkinen wrote:
> > > > On Mon, Jan 10, 2022 at 04:05:21PM -0600, Haitao Huang wrote:
> > > > > On Sat, 08 Jan 2022 10:22:30 -0600, Jarkko Sakkinen
> > <jarkko@xxxxxxxxxx>
> > > > > wrote:
> > > > >
> > > > > > On Sat, Jan 08, 2022 at 05:51:46PM +0200, Jarkko Sakkinen wrote:
> > > > > > > On Sat, Jan 08, 2022 at 05:45:44PM +0200, Jarkko Sakkinen
> > wrote:
> > > > > > > > On Fri, Jan 07, 2022 at 10:14:29AM -0600, Haitao Huang
> > wrote:
> > > > > > > > > > > > OK, so the question is: do we need both or would a
> > > > > > > mechanism just
> > > > > > > > > > > to extend
> > > > > > > > > > > > permissions be sufficient?
> > > > > > > > > > >
> > > > > > > > > > > I do believe that we need both in order to support
> > pages
> > > > > > > having only
> > > > > > > > > > > the permissions required to support their intended use
> > > > > > > during the
> > > > > > > > > > > time the
> > > > > > > > > > > particular access is required. While technically it is
> > > > > > > possible to grant
> > > > > > > > > > > pages all permissions they may need during their
> > lifetime it
> > > > > > > is safer to
> > > > > > > > > > > remove permissions when no longer required.
> > > > > > > > > >
> > > > > > > > > > So if we imagine a run-time: how EMODPR would be
> > useful, and
> > > > > > > how using it
> > > > > > > > > > would make things safer?
> > > > > > > > > >
> > > > > > > > > In scenarios of JIT compilers, once code is generated
> > into RW pages,
> > > > > > > > > modifying both PTE and EPCM permissions to RX would be
> > a good
> > > > > > > defensive
> > > > > > > > > measure. In that case, EMODPR is useful.
> > > > > > > >
> > > > > > > > What is the exact threat we are talking about?
> > > > > > >
> > > > > > > To add: it should be *significantly* critical thread,
> > given that not
> > > > > > > supporting only EAUG would leave us only one complex call
> > pattern with
> > > > > > > EACCEPT involvement.
> > > > > > >
> > > > > > > I'd even go to suggest to leave EMODPR out of the patch
> > set, and
> > > > > > > introduce
> > > > > > > it when there is PoC code for any of the existing run-time
> > that
> > > > > > > demonstrates the demand for it. Right now this way too
> > speculative.
> > > > > > >
> > > > > > > Supporting EMODPE is IMHO by factors more critical.
> > > > > >
> > > > > > At least it does not protected against enclave code because
> > an enclave
> > > > > > can
> > > > > > always choose not to EACCEPT any of the EMODPR requests. I'm
> > not only
> > > > > > confused here about the actual threat but also the potential
> > adversary
> > > > > > and
> > > > > > target.
> > > > > >
> > > > > I'm not sure I follow your thoughts here. The sequence should
> > be for enclave
> > > > > to request EMODPR in the first place through runtime to
> > kernel, then to
> > > > > verify with EACCEPT that the OS indeed has done EMODPR.
> > > > > If enclave does not verify with EACCEPT, then its own code has
> > > > > vulnerability. But this does not justify OS not providing the
> > mechanism to
> > > > > request EMODPR.
> > > >
> > > > The question is really simple: what is the threat scenario? In
> > order to use
> > > > the word "vulnerability", you would need one.
> > > >
> > > > Given the complexity of the whole dance with EMODPR it is
> > mandatory to have
> > > > one, in order to ack it to the mainline.
> > > >
> > > > > Similar to how we don't want have RWX code pages for normal Linux
> > > > > application, when an enclave loads code pages (either directly
> > or JIT
> > > > > compiled from high level code ) into EAUG'd page (which has
> > RW), we do not
> > > > > want leave pages to be RWX for code to be executable, hence
> > the need of
> > > > > EMODPR request OS to reduce the permissions to RX once the
> > code is ready to
> > > > > execute.
> > > >
> > > > You cannot compare *enforced* permissions outside the enclave,
> > and claim that
> > > > they would be equivalent to the permissions of the already
> > sandboxed code
> > > > inside the enclave, with permissions that are not enforced but
> > are based
> > > > on good will of the enclave code.
> > >
> > > To add, you can already do "EMODPR" by simply adjusting VMA
> > permissions to be
> > > more restrictive. How this would be worse than this collaboration
> > based
> > > thing?
> >
> > ... or you could even make soft version of EMODPR without using that
> > opcode
> > by writing an ioctl to update our xarray to allow lower permissions.
> > That
> > ties the hands of the process who is doing the mmap() already.
>
> E.g. why not just
>
> #define SGX_IOC_ENCLAVE_RESTRICT_PAGE_PERMISSIONS \
> _IOW(SGX_MAGIC, 0x05, struct sgx_enclave_modify_page_permissions)
> #define SGX_IOC_ENCLAVE_EXTEND_PAGE_PERMISSIONS \
> _IOW(SGX_MAGIC, 0x06, struct sgx_enclave_modify_page_permissions)
>
> struct sgx_enclave_restrict_page_permissions {
> __u64 src;
> __u64 offset;
> __u64 length;
> __u64 secinfo;
> __u64 count;
> };
> struct sgx_enclave_extend_page_permissions {
> __u64 src;
> __u64 offset;
> __u64 length;
> __u64 secinfo;
> __u64 count;
> };
>
> These would simply update the xarray and nothing else. I'd go with two
> ioctls (with the necessary checks for secinfo) in order to provide hook
> up points in the future for LSMs.
>
> This leaves only EAUG and EMODT requiring the EACCEPT handshake.
>
> /Jarkko
The trusted code base here is the enclave. It can't trust any code outside
for enforcement. There is also need for TLB shootdown.

To answer your earlier question about threat, the threat is
malicious/compromised code inside enclave. Yes, you can say the whole thing
is sand-boxed, but the runtime inside enclave could load complex upper layer
code. Therefore the runtime needs to have a trusted mechanism to ensure
code pages not writable so that there is less/no chance for compromised
malicious enclave to modify existing code pages. I still consider it to be
similar to normal Linux elf-loader/dynamic linker relying on mmap/mprotect
and trusting OS to enforce permissions, but here the enclave runtime only
trust the HW provided mechanism: EMODPR to change EPCM records and EACCEPT
to verify.

So what if:

1. User space does EMODPR ioctl.
2. Enclave does EACCEPT.
3. Enclave does EMODPE.

Could you elaborate on your exact concern here? EMODPE won't be able to restrict permissions, only add, so no way to cancel what's done by EMODPR if that's your concern.

And EMODPE would only affect EPCM not PTE. So if OS set PTE no matching EPCM, the enclave won't be able to use the page for added access.

The problem here is the asymmetry of these operations. If EMODPE also
required EACCEPT from the run-time, EMODPR would also make sense.


The asymmetry is on the user space side as Reinette stated in her reply. I could not see why this a relevant concern for kernel.

Please give a code example on how EMODPR improves trust.

It's not that EMODPR itself improves trust. What I try to say is that the enclave runtime can use EACCET to verify EPCM permissions which is trusted, and not relying on PTE permissions which is controlled by OS. It must do EACCEPT for EMODPR and other ENCLS ops like EMODT,EAUG, etc. as enclave security model considers OS untrusted.

EMODPR is the only way to restrict permissions in EPCM for enclave pages. So if it is not supported by kernel then there is no way for enclave runtimes to support the use cases I stated previously. That means RWX required in EPCM for dynamic loaded/JIT compiled code pages.

Thanks
Haitao