From: Serge E. Hallyn
Date: Thu Mar 21 2013 - 11:57:40 EST

Quoting Vivek Goyal (vgoyal@xxxxxxxxxx):
> On Thu, Mar 21, 2013 at 10:37:25AM -0500, Serge E. Hallyn wrote:
> > Quoting Vivek Goyal (vgoyal@xxxxxxxxxx):
> > ...
> > > Giving CAP_MODIFY_KERNEL to processess upon signature verification
> > > will simplify things a bit.
> > >
> > > Only thing is that signature verification alone is not sufficient. We
> > > also need to make sure after signature verification executable can
> > > not be modified in memory in any way. So that means atleast couple of
> > > things.
> >
> > Also what about context? If I construct a mounts namespace a certain
> > way, can I trick this executable into loading an old singed bzImage that
> > I had laying around?
> We were thinking that /sbin/kexec will need to verify bzImage signature
> before loading it.
> Key for verification is in kernel so idea was to take kernel's help
> in verifying signature.
> Not sure how exactly the interface should look like.
> - I was thinking may be process can mmap() the bzImage with MAP_LOCKED.
> We can create additional flag say MAP_VERIFY_SIG_POST, which tries
> to verify signature/integrity of mapped region of file after mapping and
> locking pages and mmap() fails if signature verification fails.
> There have been suggestions about creating new system call altogether.
> >
> > ISTM the only sane thing to do, if you're going down this road,
> > is to have CAP_MODIFIY_KERNEL pulled from bounding set for everyone
> > except a getty started by init on ttyS0. Then log in on serial
> > to update. Or run a damon with CAP_MODIFIY_KERNEL which listens
> > to a init_net_ns netlink socket for very basic instructions, like
> > "find and install latest signed bzImage in /boot". Then you can
> > at least trust that /boot for that daemon is not faked.
> daemon does not have the key and can't verify signature of signed
> bzImage. Even if it had the key, it can't trust the crypto code for
> signature verification as none of that is signed.

I'm not saying not to use the kernel to verify the signature.

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