Re: Make gelf_getphdr more robust?

[Florian Weimer <fweimer at redhat dot com>]

On 02/06/2014 02:20 PM, Mark Wielaard wrote:

>> The check against maxsize is insufficient, it's also required to check
>> that phdr->p_filesz <= maxsize - phdr->p_offset.  Would it make sense to
>> do both checks inside gelf_getphdr?
>
> I think this case is actually caught now by default by the integrated
> robustify patches from last month, specifically:
> https://git.fedorahosted.org/cgit/elfutils.git/commit/?id=720383c53b435de6647edd78060dd7d38ade25a5
>
> + /* First see whether the information in the ELF header is
> + valid and it does not ask for too much. */
> + if (unlikely (ehdr->e_phoff >= elf->maximum_size)
> +     || unlikely (elf->maximum_size - ehdr->e_phoff < size))
> + {
> +   /* Something is wrong. */
> +   __libelf_seterrno (ELF_E_INVALID_PHDR);
> +   goto out;
> + }
>
> So that would make the maxsize check in readelf.c redundant.

Looks like this.  I was testing the version in Fedora 20, 
elfutils-0.158-1.fc20.x86_64.

>> And the printf call can leak data or crash if filedata + phdr->p_offset
>> is not NUL terminated (which obviously needs a crafted ELF file).  This
>> can't be fixed in gelf_getphdr, alas.
>
> Yes, it would probably be good to catch this in eu-readelf.
>
> Did you catch this by code inspection or did you stumble upon a bad ELF
> file that made it crash?

I noticed this issue in the source code when I ivestigated why my own 
code complained about about a malformed interp header in a debuginfo 
package.  I have built a crafted ELF image derived from /usr/bin/gs that 
crashes eu-readelf (running under valgrind), but it's nothing that I 
found in the wild.

-- 
Florian Weimer / Red Hat Product Security Team