Re: [PATCH] x86/sgx: Remove checks for platform limits from sgx_validate_secs()

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Tue, Oct 06, 2020 at 06:16:17PM +0300, Jarkko Sakkinen wrote:
> On Mon, Oct 05, 2020 at 03:46:26PM -0700, Sean Christopherson wrote:
> > On Mon, Oct 05, 2020 at 02:38:40PM +0300, Jarkko Sakkinen wrote:
> > > On Sun, Oct 04, 2020 at 11:00:28PM -0500, Haitao Huang wrote:
> > > > On Sun, 04 Oct 2020 21:08:19 -0500, Jarkko Sakkinen
> > > > <jarkko.sakkinen@linux.intel.com> wrote:
> > > > 
> > > > > Remove from sgx_validate_secs():
> > > > > 
> > > > > 	if (secs->miscselect & sgx_misc_reserved_mask ||
> > > > > 	    secs->attributes & sgx_attributes_reserved_mask ||
> > > > > 	    secs->xfrm & sgx_xfrm_reserved_mask)
> > > > > 		return -EINVAL;
> > > > > 
> > > > > SECS can surpass the platform limits because it's the SIGSTRUCT that
> > > > > defines the limits that are used at run-time.
> > > > > 
> > > > > What SECS does is that it defines the overall limits that must apply for
> > > > > any platform, i.e.  SECS limits and platform limits are orthogonal. They
> > > > > are not dependent.
> > > > > 
> > > > > Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> > > > > Cc: Jethro Beekman <jethro@fortanix.com>
> > > > > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > > > > Suggested-by: Haitao Huang <haitao.huang@linux.intel.com>
> > > > > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > > > > ---
> > > > >  arch/x86/kernel/cpu/sgx/ioctl.c | 5 -----
> > > > >  1 file changed, 5 deletions(-)
> > > > > 
> > > > > diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c
> > > > > b/arch/x86/kernel/cpu/sgx/ioctl.c
> > > > > index 6b3cc8483008..008752cb54f0 100644
> > > > > --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> > > > > +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> > > > > @@ -86,11 +86,6 @@ static int sgx_validate_secs(const struct sgx_secs
> > > > > *secs)
> > > > >  	if (secs->base & (secs->size - 1))
> > > > >  		return -EINVAL;
> > > > > -	if (secs->miscselect & sgx_misc_reserved_mask ||
> > > > > -	    secs->attributes & sgx_attributes_reserved_mask ||
> > > > > -	    secs->xfrm & sgx_xfrm_reserved_mask)
> > > > > -		return -EINVAL;
> > > > > -
> > > > >  	if (secs->size > max_size)
> > > > >  		return -EINVAL;
> > > > > 
> > > > 
> > > > Looks good to me.
> > > > Thanks
> > > > Haitao
> > > 
> > > Thanks, I'll merge it then.
> > 
> > You can't remove this wholesale, the kernel should still disallow access to
> > features that are unknown to the kernel and/or are explicitly disallowed by
> > the kernel.  E.g. see SGX_ATTR_RESERVED_MASK and SGX_MISC_RESERVED_MASK.
> > 
> > Dropping sgx_xfrm_reserved_mask is ok because the CPU explicitly checks that
> > XFRM is a strict subset of the current XCR0, though that makes me wonder what
> > it does with XSS...
> 
> If possible send a patch so I can get a changelog reference.
> 
> Only thing you need to put as long description is permlink to my
> response in this thread.

Sean, I can do the fix, don't worry about it but before I make a backlog
entry I need to challenge this just a bit.

SIGSTRUCT will tell what features we want to run the enclave with, not
SECS, so why you see it a problem that the features are not validated
in SGX_IOC_ENCLAVE_CREATE? I.e. the enclave binary could potentially be
used in different platforms with different kernel versions.

/Jarkko