Re: [PATCH v14 09/19] x86/mm: x86/sgx: Signal SEGV_SGXERR for #PFs w/ PF_SGX

[Jarkko Sakkinen]

On Wed, Sep 26, 2018 at 01:16:59PM -0700, Dave Hansen wrote:
> On 09/26/2018 11:12 AM, Andy Lutomirski wrote:
> >> e omniscient.
> >>
> >> How about this?  With formatting changes since it's long-winded...
> >>
> >>        /*
> >>         * Access is blocked by the Enclave Page Cache Map (EPCM), i.e. the
> >>         * access is allowed by the PTE but not the EPCM.  This usually happens
> >>         * when the EPCM is yanked out from under us, e.g. by hardware after a
> >>         * suspend/resume cycle.  In any case, software, i.e. the kernel, can't
> >>         * fix the source of the fault as the EPCM can't be directly modified
> >>         * by software.  Handle the fault as an access error in order to signal
> >>         * userspace, e.g. so that userspace can rebuild their enclave(s), even
> >>         * though userspace may not have actually violated access permissions.
> >>         */
> >>
> > Looks good to me.
> 
> Including the actual architectural definition of the bit might add some
> clarity.  The SDM explicitly says (Vol 3a section 4.7):
> 
> 	The fault resulted from violation of SGX-specific access-control
> 	requirements.
> 
> Which totally squares with returning true from access_error().
> 
> There's also a tidbit that says:
> 
> 	This flag is 1 if the exception is unrelated to paging and
> 	resulted from violation of SGX-specific access-control
> 	requirements. ... such a violation can occur only if there
> 	is no ordinary page fault...
> 
> This is pretty important.  It means that *none* of the other
> paging-related stuff that we're doing applies.
> 
> We also need to clarify how this can happen.  Is it through something
> than an app does, or is it solely when the hardware does something under
> the covers, like suspend/resume.

When you change page permissions lets say with mprotect after the and
try to do an invalid access according to the EPCM permissions this can
happen.

/Jarkko