Re: [PATCH v6 3/6] fs/procfs: add build ID fetching to PROCMAP_QUERY API

[Andi Kleen]

> 1) non-executable file-backed VMA still has build ID associated with
> it. Note, build ID is extracted from the backing file's content, not
> from VMA itself. The part of ELF file that contains build ID isn't
> necessarily mmap()'ed at all

That's true, but there should be at least one executable mapping
for any useful ELF file.

Basically such a check guarantee that you cannot tell anything
about a non x mapping not related to ELF.

> 
> 2) What sort of exploitation are we talking about here? it's not
> enough for backing file to have correct 4 starting bytes (0x7f"ELF"),
> we still have to find correct PT_NOTE segment, and .note.gnu.build-id
> section within it, that has correct type (3) and key name "GNU".

There's a timing side channel, you can tell where the checks
stop. I don't think it's a big problem, but it's still better to avoid
such leaks in the first place as much as possible.

> 
> I'm trying to understand what we are protecting against here.
> Especially that opening /proc/<pid>/maps already requires
> PTRACE_MODE_READ permissions anyways (or pid should be self).

While that's true for the standard security permission model there might
be non standard ones where the relationship is more complicated.

-Andi