linux-kernel - Re: [PATCH v2] fs: binfmt_elf: Add ELF header consistency checks

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Message-ID: <CAGG-pUTec1mzz-nXSGVyMHPZ8PQ=hMrbTLnnRoLZ8ju=EKo98g@mail.gmail.com>
Date:	Thu, 21 Nov 2013 08:52:13 -0200
From:	Geyslan Gregório Bem <geyslan@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	"open list:FILESYSTEMS (VFS...), linux-kernel@...r.kernel.org (open
	list)" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	kernel-br <kernel-br@...glegroups.com>
Subject: Re: [PATCH v2] fs: binfmt_elf: Add ELF header consistency checks

2013/11/20 Al Viro <viro@...iv.linux.org.uk>:
> On Wed, Nov 20, 2013 at 09:34:31PM -0300, Geyslan G. Bem wrote:
>> The member 'e_ehsize' that holds the ELF header size is compared
>> with the elfhdr struct size. If not equal, goes out.
>> If 'e_phoff' holds 0 the object has no program header table, so
>> goes out.
>> Ensures the file being loaded has the correct data encoding, checking
>> 'e_ident[EI_DATA]' against 'ELF_DATA'.
>>
>> Besides the checks being in accordance with the ELF Specifications,
>> they increase the binary consistency reducing the use of malformed ones.
>
> This is completely misguided.  We are allowed to reject such binaries,
> but what's the point of doing that?

Viro, First of all, thanks for reply.

The security (or anti-security) guys are used to mess up with the not checked
header fields for their "benefits": anti-debugging, injection and so on.

Concerning to 'e_phoff': when it is 0 the check avoids that 'elf_phdr' been read
from a erroneous offset (ELF header). I know that without this check the binary
will goes out anyway. But it reduces wasting cpu cycles.

Regarding 'e_ident[EI_DATA]': that check also prevents a farther code reading
when the binary, although been the correct arch, is compiled with a different
data encoding (MSB vs LSB).

So checking besides increase the binary consistency, guarantee some mislead
and fewer cpu cycles.

-- 
Regards,

Geyslan G. Bem
hackingbits.com
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