On Fri, Aug 23, 2019 at 03:57:36AM +0300, Jarkko Sakkinen wrote:
> On Fri, 2019-08-23 at 03:39 +0300, Jarkko Sakkinen wrote:
> > On Thu, 2019-08-22 at 09:34 -0700, Sean Christopherson wrote:
> > > On Thu, Aug 22, 2019 at 07:31:39PM +0300, Jarkko Sakkinen wrote:
> > > > On Wed, 2019-08-21 at 20:55 -0700, Sean Christopherson wrote:
> > > > > Why are we validating the TCS protection bits? Hardware ignores them, so
> > > > > why do we care? sgx_ioc_enclave_add_page() sets the internal protection
> > > > > bits so there's no danger of putting the wrong thing in the page tables.
> > > >
> > > > I think that in this commit I got it wrong but I think this is awkward:
> > > >
> > > > /*
> > > > * TCS pages must always RW set for CPU access while the SECINFO
> > > > * permissions are *always* zero - the CPU ignores the user provided
> > > > * values and silently overwrites with zero permissions.
> > > > */
> > > > if ((secinfo.flags & SGX_SECINFO_PAGE_TYPE_MASK) == SGX_SECINFO_TCS)
> > > > prot |= PROT_READ | PROT_WRITE;
> > > >
> > > > In my opinion the right thing to do would be check that SECINFO has *at
> > > > minimum* RW and return -EINVAL if not.
> > >
> > > Based on Serge's comment, hardware updates MRENCLAVE with SECINFO *after*
> > > it overwrites the flags for TCS pages. I.e. requiring RW for the TCS
> > > would result in every enclave failing EINIT due to an invalid measurement.
> > > It'd be fairly easy to verify this if we want to triple check that that is
> > > indeed hardware behavior.
> >
> > This is from the signing tool that I wrote back in 2016 used in the
> > selftest:
> >
> > struct mreadd {
> > uint64_t tag;
> > uint64_t offset;
> > uint64_t flags; /* SECINFO flags */
> > uint8_t reserved[40];
> > } __attribute__((__packed__));
> >
> > static bool mrenclave_eadd(EVP_MD_CTX *ctx, uint64_t offset, uint64_t flags)
> > {
> > struct mreadd mreadd;
> >
> > memset(&mreadd, 0, sizeof(mreadd));
> > mreadd.tag = MREADD;
> > mreadd.offset = offset;
> > mreadd.flags = flags;
> >
> > return mrenclave_update(ctx, &mreadd);
> > }
> >
> > If MRENCLAVE was updated after the overwrite, this would not work.
> >
> > The least confusing semantics would be to require RW, no more or less.
>
> OK, it is how Serge said.
>
> This can we verified from the SDM easily (SCRATCH_SECINFO gets zeros
> is extended after that).
>
> And also from my signing tool :-)
>
> for (offset = 0; offset < sb.st_size; offset += 0x1000) {
> if (!offset)
> flags = SGX_SECINFO_TCS;
> else
> flags = SGX_SECINFO_REG | SGX_SECINFO_R |
> SGX_SECINFO_W | SGX_SECINFO_X;
>
> OK, so this looks like that my patch does exactly the right thing,
> right?
That's my understanding as well. Definitely worthy of a comment
explaining all of the above.
