linux-kernel - Re: [PATCH 2/3] binfmt_elf: Verify signature of signed elf binary

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Message-ID: <871udloiku.fsf@xmission.com>
Date:	Tue, 15 Jan 2013 20:30:41 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-kernel@...r.kernel.org, zohar@...ux.vnet.ibm.com,
	pjones@...hat.com, hpa@...or.com, dhowells@...hat.com,
	jwboyer@...hat.com
Subject: Re: [PATCH 2/3] binfmt_elf: Verify signature of signed elf binary

Vivek Goyal <vgoyal@...hat.com> writes:

> If a binary is signed, verify its signature. If signature is not valid, do
> not allow execution. If binary is not signed, execution is allowed
> unconditionally.
>
> CONFIG_BINFMT_ELF_SIGNATURE controls whether elf binary signature support
> is compiled in or not.
>
> Signature are expected to be present in elf section ".section". This code
> is written along the lines of module signature verification code. Just
> that I have removed the magic string. It is not needed as signature is
> expected to be present in a specific section.
>
> I put the signature into a section, instead of appending it so that
> strip operation works fine.
>
> One signs and verifies  all the areas mapped by PT_LOAD segments of elf
> binary. Typically Elf header is mapped in first PT_LOAD segment. As adding
> .signature section can change three elf header fields (e_shoff, e_shnum
> and e_shstrndx), these fields are excluded from digest calculation

My gut feel says that a signature that we verify should reside in an ELF
segment.  Sections are for the linker not the kernel.

I don't totally know what the signature should cover but my gut feels
says the signature should come after ever non-signature segment and
cover all of the prior segments (PT_LOAD or not).  Because presumably
the loader needs to look at everything in a segment.  We can
restrict ourselves to only processing signed binaries on executables
with only PT_LOAD segments and signatures for now.

Eric
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