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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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.

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.

I believe this is needed for LibOS runtimes (e.g.,Gramine) loading unmodified app binaries, or an enclave with JIT compiler (I think Enarx in this category?). Experts from those project can confirm or contradict. Intel SDK currently also has implementation to reduce permissions of RELRO sections in ELF binaries to ReadOnly after relocation is done. In our new EDMM user support[1] based on this patch series, we also support flows to reduce permissions using EMODPR in a generic way.

[1]https://github.com/intel/linux-sgx/pull/751



[Index of Archives]     [AMD Graphics]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite News]     [Linux Kernel]     [Linux SCSI]

  Powered by Linux