Re: [RFC PATCH v2 2/2] ELF: Add ELF program property parsing support

[Yu-cheng Yu]

On Fri, 2019-08-30 at 09:34 +0100, Dave Martin wrote:
> On Fri, Aug 30, 2019 at 06:37:45AM +0100, Kees Cook wrote:
> > On Fri, Aug 23, 2019 at 06:23:40PM +0100, Dave Martin wrote:
> > > ELF program properties will needed for detecting whether to enable
> > > optional architecture or ABI features for a new ELF process.
> > > 
> > > For now, there are no generic properties that we care about, so do
> > > nothing unless CONFIG_ARCH_USE_GNU_PROPERTY=y.
> > > 
> > > Otherwise, the presence of properties using the PT_PROGRAM_PROPERTY
> > > phdrs entry (if any), and notify each property to the arch code.
> > > 
> > > For now, the added code is not used.
> > > 
> > > Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
> > 
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> 
> Thanks for the review.
> 
> Do you have any thoughts on Yu-Cheng Yu's comments?  It would be nice to
> early-terminate the scan if we can, but my feeling so far was that the
> scan is cheap, the number of properties is unlikely to be more than a
> smallish integer, and the code separation benefits of just calling the
> arch code for every property probably likely outweigh the costs of
> having to iterate over every property.  We could always optimise it
> later if necessary.
> 
> I need to double-check that there's no way we can get stuck in an
> infinite loop with the current code, though I've not seen it in my
> testing.  I should throw some malformed notes at it though.

Here is my arch_parse_elf_property() and objdump of the property.
The parser works fine.

Thanks,
Yu-cheng




int arch_parse_elf_property(u32 type, const void *data, size_t datasz,
          
                   bool compat, struct arch_elf_state *state)
{
        if (type
!= GNU_PROPERTY_X86_FEATURE_1_AND)
                return -ENOENT;

        if (datasz < sizeof(unsigned int))
                return -ENOEXEC;

        state->gnu_property = *(unsigned int *)data;
        return 0;
}

Contents of section .note.gnu.property:
 400338 04000000 30000000 05000000 474e5500  ....0.......GNU.
 400348 020000c0 04000000 03000000 00000000  ................
 400358 000001c0 04000000 00000000 00000000  ................
 400368 010001c0 04000000 01000000 00000000  ................