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SubjectRe: [PATCH v4 1/2] fs/binfmt_elf: Fix AT_PHDR for unusual ELF files
On Thu, Jan 27, 2022 at 08:23:51AM -0800, Kees Cook wrote:
> On Thu, Jan 27, 2022 at 09:56:43PM +0900, Akira Kawata wrote:
> > On Wed, Jan 26, 2022 at 09:01:30PM -0800, Kees Cook wrote:
> > > [...]
> > > 1) The ELF spec says e_phoff is 0 if there's no program header table.
> > >
> > > The old code would just pass the load_addr as a result. This patch will
> > > now retain the same result (phdr_addr defaults to 0). I wonder if there
> > > is a bug in this behavior, though? (To be addressed in a different patch
> > > if needed...)
> > >
> >
> > It is better to return NULL from load_elf_phdrs when e_phoff == 0, I
> > think.
>
> Yeah, right now it just returns a pointer to file offset 0.
>
> I also wonder if we should sanity-check e_phoff vs PT_PHDR? Right now
> Linux ignores PT_PHDR. Should we reject loading when e_phoff != PT_PHDR
> file offset? (And I wonder if there are "broken" binaries right now that
> have bad PT_PHDR segments that have gone unnoticed...)

I agree that unnoticed broken binaries exist. I checked glibc rtld and
there is no check of e_phoff != PT_PHDR file offset.

>
> And now I'm thinking about the excellent ELF loading analysis at:
> https://nathanotterness.com/2021/10/tiny_elf_modernized.html
>
> ;)

I think you have interested in https://shinh.skr.jp/obf/bingolf.html
also.

>
> --
> Kees Cook

Akira Kawata

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