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

From: Haitao Huang
Date: Mon Jan 10 2022 - 22:48:22 EST

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.