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SubjectRe: [RFC PATCH v2 2/2] ELF: Add ELF program property parsing support
On Wed, Oct 09, 2019 at 01:59:13PM +0100, Dave Martin wrote:
> On Fri, Aug 30, 2019 at 09:34:17AM +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@arm.com>
> > >
> > > Reviewed-by: Kees Cook <keescook@chromium.org>
> >
> > 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.
> >
> > > Note below...
> > >
> > > > [...]
> > > > +static int parse_elf_property(const char *data, size_t *off, size_t datasz,
> > > > + struct arch_elf_state *arch,
> > > > + bool have_prev_type, u32 *prev_type)
> > > > +{
> > > > + size_t size, step;
> > > > + const struct gnu_property *pr;
> > > > + int ret;
> > > > +
> > > > + if (*off == datasz)
> > > > + return -ENOENT;
> > > > +
> > > > + if (WARN_ON(*off > datasz || *off % elf_gnu_property_align))
> > > > + return -EIO;
> > > > +
> > > > + size = datasz - *off;
> > > > + if (size < sizeof(*pr))
> > > > + return -EIO;
> > > > +
> > > > + pr = (const struct gnu_property *)(data + *off);
> > > > + if (pr->pr_datasz > size - sizeof(*pr))
> > > > + return -EIO;
> > > > +
> > > > + step = round_up(sizeof(*pr) + pr->pr_datasz, elf_gnu_property_align);
> > > > + if (step > size)
> > > > + return -EIO;
> > > > +
> > > > + /* Properties are supposed to be unique and sorted on pr_type: */
> > > > + if (have_prev_type && pr->pr_type <= *prev_type)
> > > > + return -EIO;
> > > > + *prev_type = pr->pr_type;
> > > > +
> > > > + ret = arch_parse_elf_property(pr->pr_type,
> > > > + data + *off + sizeof(*pr),
> > > > + pr->pr_datasz, ELF_COMPAT, arch);
> > >
> > > I find it slightly hard to read the "cursor" motion in this parse. It
> > > feels strange, for example, to refer twice to "data + *off" with the
> > > second including consumed *pr size. Everything is fine AFAICT in the math,
> > > though, and I haven't been able to construct a convincingly "cleaner"
> > > version. Maybe:
> > >
> > > data += *off;
> > > pr = (const struct gnu_property *)data;
> > > data += sizeof(*pr);
> > > ...
> > > ret = arch_parse_elf_property(pr->pr_type, data,
> > > pr->pr_datasz, ELF_COMPAT, arch);
> >
> > Fair point. The cursor is really *off, which I suppose I could update
> > as we go through this function, or cache in a local variable and assign
> > on the way out.
> >
> > > But that feels disjoint from the "step" calculation, so... I think what
> > > you have is fine. :)
> >
> > It's good to be as clear as possible about exactly how far we have
> > parsed, so I'll see if I can improve this when I repost.
> >
> >
> > Do you have any objection to merging patch 1 with this one? For
> > upstreaming purposes, it seems overkill for that to be a separate patch.
> >
> > I may also convert elf_gnu_property_align to upper case, since unlike
> > the other related definitions this one isn't trying to look like a
> > struct tag.
> >
> > Do you have any opinion on the WARN_ON()s? They should be un-hittable,
> > so they're documenting assumptions rather than protecting against
> > anything real. Maybe I should replace them with comments.
>
> FYI, I'm going to be inactive for a while, so I'm not going to be able
> to push this patch further.
>
> Mark Brown will be picking up the arm64 BTI series, so it will probably
> make sense if he pulls it into that series.
>
> Any thoughts?

Okay, sounds good. Mark, I think these patches are in good shape. Can
you include me on CC where you pick these up?

Thanks!

-Kees

--
Kees Cook

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