Re: [PATCH v36 22/24] selftests/x86: Add a selftest for SGX

[Jarkko Sakkinen]

On Thu, Aug 27, 2020 at 08:20:51AM -0700, Sean Christopherson wrote:
> On Thu, Aug 27, 2020 at 12:47:04AM -0400, Nathaniel McCallum wrote:
> > > +int main(int argc, char *argv[], char *envp[])
> > > +{
> > > +       struct sgx_enclave_exception exception;
> > > +       struct vdso_symtab symtab;
> > > +       Elf64_Sym *eenter_sym;
> > > +       uint64_t result = 0;
> > > +       struct encl encl;
> > > +       unsigned int i;
> > > +       void *addr;
> > > +
> > > +       if (!encl_load("test_encl.elf", &encl))
> > > +               goto err;
> > > +
> > > +       if (!encl_measure(&encl))
> > > +               goto err;
> > > +
> > > +       if (!encl_build(&encl))
> > > +               goto err;
> > > +
> > > +       /*
> > > +        * An enclave consumer only must do this.
> > > +        */
> > > +       for (i = 0; i < encl.nr_segments; i++) {
> > > +               struct encl_segment *seg = &encl.segment_tbl[i];
> > > +
> > > +               addr = mmap((void *)encl.encl_base + seg->offset, seg->size,
> > > +                           seg->prot, MAP_SHARED | MAP_FIXED, encl.fd, 0);
> > 
> > My patch version is a bit behind (v32), but I suspect this still
> > applies. I discovered the following by accident.
> > 
> > In the Enarx code base, this invocation succeeds:
> > mmap(0x200000000000, 0x1000, PROT_READ | PROT_WRITE, MAP_SHARED |
> > MAP_FIXED, sgxfd, 0)
> > 
> > However, this one fails with -EINVAL:
> > mmap(0x200000000000, 0x1000, PROT_READ | PROT_WRITE,
> > MAP_SHARED_VALIDATE | MAP_FIXED, sgxfd, 0)
> > 
> > From man mmap:
> > 
> >        MAP_SHARED_VALIDATE (since Linux 4.15)
> >               This flag provides the same behavior as MAP_SHARED
> > except that MAP_SHARED mappings ignore unknown
> >               flags in flags.  By contrast, when creating a mapping
> > using MAP_SHARED_VALIDATE, the kernel veri‐
> >               fies  all  passed  flags  are  known  and fails the
> > mapping with the error EOPNOTSUPP for unknown
> >               flags.  This mapping type is also required to be able to
> > use some mapping flags (e.g., MAP_SYNC).
> > 
> > I can try again on a newer patch set tomorrow if need be. But the
> > documentation of mmap() doesn't match the behavior I'm seeing. A brief
> > look through the patch set didn't turn up anything obvious that could
> > be causing this.
> 
> This is a bug in sgx_get_unmapped_area().  EPC must be mapped SHARED, and
> so MAP_PRIVATE is disallowed.  The current check is:
> 
>   if (flags & MAP_PRIVATE)
>           return -EINVAL;
> 
> and the base "flags" are:
> 
>   #define MAP_SHARED      0x01            /* Share changes */
>   #define MAP_PRIVATE     0x02            /* Changes are private */
>   #define MAP_SHARED_VALIDATE 0x03        /* share + validate extension flags */
> 
> which causes the SGX check to interpret MAP_SHARED_VALIDATE as MAP_PRIVATE.
> The types are just that, types, not flag modifiers.  So the SGX code needs
> to be:
> 
>   if ((flags & MAP_TYPE) == MAP_PRIVATE)
>           return -EINVAL;

Updated, thanks.

/Jarkko