linux-kernel - Re: [PATCH v4 1/2] fs/binfmt_elf: Fix AT

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Message-ID: <202201270816.5030A2A4B5@keescook>
Date:   Thu, 27 Jan 2022 08:23:51 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Akira Kawata <akirakawata1@...il.com>
Cc:     akpm@...ux-foundation.org, adobriyan@...il.com,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        lukas.bulwahn@...il.com, kernel test robot <lkp@...el.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] fs/binfmt_elf: Fix AT_PHDR for unusual ELF files

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...)

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

;)

-- 
Kees Cook