linux-kernel - Re: [PATCH 21/21] MODSIGN: Apply signature checking to modules on module load [ver #3]

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Date:	Tue, 13 Dec 2011 12:45:40 +1030
From:	Rusty Russell <rusty@...abs.org>
To:	David Howells <dhowells@...hat.com>
Cc:	dhowells@...hat.com, keyrings@...ux-nfs.org,
	linux-crypto@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, dmitry.kasatkin@...el.com,
	zohar@...ux.vnet.ibm.com, arjan.van.de.ven@...el.com,
	alan.cox@...el.com, Jon Masters <jcm@...masters.org>
Subject: Re: [PATCH 21/21] MODSIGN: Apply signature checking to modules on module load [ver #3]

On Mon, 12 Dec 2011 16:11:27 +0000, David Howells <dhowells@...hat.com> wrote:
> Rusty Russell <rusty@...abs.org> wrote:
> 
> > OK, then you need to generate stripped modules as part of the build,
> > too.  It's a bit of a pain, sure, but hardly a showstopper.
> 
> They'd have to be maximally stripped so that mkinitrd doesn't do anything to
> them, but you'd then get the debuginfo from them into the packaging if you're
> on some distribution or other.  And you also provide an option to not strip
> them if whoever wants them unstripped.

I was thinking we generate multiple modules; people will definitely want
unstripped versions as well.

> > A signature contains a magic marker
> 
> I don't like this particularly - you can't guarantee that this won't be
> generated by the assembler quite by accident.

We don't care, in practice.  It can't generate a valid key by accident.
The only difference is that you might get "bad key" instead of "no key"
if no sigature matches.  But a validly parsable signature won't happen
in practice.

> You should find the end of the
> ELF and work from there.  It should be a simple matter of parsing the header
> and the section table only, right?  Then you can look at the file offset +
> length of the last section in the file.

Tried that first.  It's more lines (which need to be carefully
scrutinized)...

>  At that point, assuming this isn't
> coincident with the actual end of the file, you can try parsing what's
> thereafter as a signature.  If it is actual PGP, then an RFC4880 parser should
> recognise it as valid, and a signature packet should be seen.

...and you can still have padding.  At which point, I realised that we
might as well just scan the whole thing and be done.

It's a bit cheeky, but I *know* it works, and can be verified by any
reader without knowing anything about ELF or trusting the format at all.

> > A signature contains a magic marker: it signs everything up to the
> > magic marker (ie. just append them):
> > 	SUM=`md5sum drivers/block/loop.ko | cut -d\  -f1`; echo "@Module signature:$SUM" >> drivers/block/loop.ko
> 
> That's not a useful signature, but I suspect you're just showing this as an
> example.

Yeah, it was supposed to be a dumb example...

> > We can have false positives, but at worst that make us report EINVAL
> > (bad signature) instead of ENOENT (no signature).
> 
> EKEYREJECTED please; that way it's the same as RHEL does now.

OK, sure (who knew that was there?).

> > Took me longer to figure out the damn crypto API
> 
> You don't actually need to use that.  The crypto API for the moment doesn't do
> crytographic signature verification.

Sure, I was working on mainline, which is why I chose md5 (plus using
md5sum is easy).

> returns -EBADMSG if none of its parsers recognise the signature, -ENOPKG if
> the signature is recognised, but we can't handle it (for instance if it's an
> unsupported hash algorithm), -ENOKEY if we recognise it, but there's no key
> available or -EKEYREJECTED if we recognised it, found the matching key, but
> the key couldn't be used to verify the signature for some reason.
> 
> 	/* Call repeatedly to shovel data into the crypto hash */
> 	verify_sig_add_data(mod_sig, dataptr, datasize);
> 
> 	/* Call to finalise and actually perform the verification */
> 	ret = verify_sig_end(mod_sig, sig, sig_size);
> 
> or:
> 
> 	/* Call to cancel the verification */
> 	verify_sig_cancel(mod_sig);
> 
> This does all the work for you.

That looks really nice, actually!

Thanks,
Rusty.
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