linux-kernel - Re: RFC: sign the modules at install time

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Date:	Fri, 19 Oct 2012 15:04:03 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	David Miller <davem@...emloft.net>,
	David Howells <dhowells@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: RFC: sign the modules at install time

On Fri, Oct 19, 2012 at 12:58 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Tssk. I fixed it up, and now it works-for-me(tm), but some perl person
> probably really should try to make that sign-file and x509keyid merge.
> My fix made the thing even slower, doing two extra "wc -c" invocations
> since it can't do "${#..}" expansion due to the locale problem.

Hmm. I haven't seen this breakage personally yet, but looking more at
Rusty's last changes I'm pretty sure my fixes are potentially not
sufficient.

In particular, this line from Rusty's sign-file script simplification
makes me worry:

  KEYID="`perl $keyid_script $x509 keyid`"

and the problem is that the 'keyid' is a binary blob.

And I'm pretty damn sure that assigning random binary data to shell
variables is going to break eventually. In particular, I tested NUL
characters, and bash is not happy about doing things like

   a="`echo -en '\000'`"

and 'a' ends up empty, not containing a one-byte string containing a
NUL character. Not to mention that even if the shell were to handle
this correctly, if you do

  echo -n "$a"

then even *if* the shell remembered that 'a' contains one NUL
character, when it then executes the 'echo' binary, that will
certainly not be able to handle it. With a built-in 'echo' it _could_
work, with an external one it fundamentally could not. NUL is special,
and terminates strings at execve(). You cannot have NUL bytes in
arguments.

Of course, I don't know the rules for x509 key id blobs. Maybe they
aren't allowed to contain NUL anyway?

Possible solutions:

 - Linus is a worry-wart, and key id's never contain NUL.

 - somebody with the appropriate perl-fu does the perl conversion, and
keeps it all in perl.

 - somebody with (slightly less) perl-fu makes the x509 keyid_script
return a quoted string, so that we can save that *quoted* version for
the shell interatction (perhaps in a similar format that we now use
for the "prologue" variable, ie just a list of hex bytes)

otherwise we need to go back to putting the binary blob in a file
(which shell won't corrupt).

Rusty, David?

               Linus
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