Re: [Ksummit-discuss] Last minute nominations: mcgrof and toshi

[Jason Cooper <jason@lakedaemon.net>]

On Mon, Aug 01, 2016 at 04:13:50PM -0700, Andy Lutomirski wrote:
> On Aug 1, 2016 4:04 PM, "Jason Cooper" <jason@lakedaemon.net> wrote:
> >
> > On Mon, Aug 01, 2016 at 03:36:51PM -0700, Andy Lutomirski wrote:
> > > On Mon, Aug 1, 2016 at 3:21 PM, Mimi Zohar <zohar@linux.vnet.ibm.com>
> wrote:
> > > > On Mo, 2016-08-01 at 10:59 -0700, Andy Lutomirski wrote:
> > > >
> > > >> Mimi, I'm curious: I don't fully understand what is covered by IMA
> > > >> policy.  How does the IMA kernel_read_file stuff deal with symlinks?
> > > >> For example, if I symlink /lib/firmware/iwlwifi-8265-21.ucode to
> > > >> /home/badguy/iwlwifi-8265-21.ucode, what happens?  What if I symlink
> > > >> /lib/firmware/iwlwifi-8265-21.ucode to /home/badguy/something_else?
> > > >> Or even /lib/modules/kernel/foo/bar.ko to /home/badguy/evil.ko?  The
> > > >> interesting case is where the "badguy" user is duly authorized to
> > > >> write to /home/badguy and holds whatever keys may be needed.
> > > >
> > > > Lets step back a second.  In order for a key to be added to the IMA
> > > > keyring, the key must be signed by a key on the builtin keyring.  The
> > > > key on the builtin keyring can be compiled into the kernel image or
> > > > added post build using Mehmet Kayaalp's patches.
> > > >
> > > > True, any key on the IMA keyring could be used to verify file
> signatures
> > > > (in IMA terminology appraise the file's integrity).  The enumeration
> is
> > > > a first step to making sure that only properly signed code is read by
> > > > the kernel.  The next step requires finer grain key management.  In
> > > > general, pathname based policies are not a good idea.  Whatever method
> > > > is defined, it should not be limited to just firmware or files read by
> > > > the kernel, but to all files.
> > > >
> > >
> > > Unless I'm mistaken (which is quite possible), IMA is primarily
> > > intended to appraise the content of POSIX filesystems.  So, if IMA is
> > > in use, then doing:
> > >
> > > $ cat /foo/bar
> > >
> > > should only succeed if /foo/bar is signed according to loaded policy.
> > > It's the system administrator's decision what filesystem is actually
> > > mounted at /foo, and root can presumably mess around with application
> > > expectations by, say, bind-mounting something over /foo.
> > >
> > > Modules and firmware are special: even root should not be able to
> > > avoid the full signature policy.  This means that, for example:
> > >
> > > # mount --bind /evil /lib/firmware
> > >
> > > should not result in violating policy.  So the pathname should not be
> > > used as such.  However, firmware is a bit special in that the driver
> > > chooses the pathname to request, and it really does uniquely identify
> > > the intended firmware.  So, when a driver asks for:
> > >
> > > "iwlwifi-whatever.ucode"
> > >
> > > and the driver core tries to read "/lib/firmware/iwlwifi-whatever.ucode"
> > >
> > > it's entirely possible that we'll follow a symlink and end up
> > > elsewhere (Fedora, for example, does exactly this), but the file
> > > that's loaded should be appraised (or verified using a non-IMA means,
> > > etc.) to verify that whatever blob gets found is actually signed by
> > > the holder of an authorized key for the purpose of being used as
> > > "iwlwifi-whatever.ucode".
> >
> > Assuming Andy's lightweight signature scheme, it would probably be best
> > to do a lookup based on the sha256 hash of the file.  Then pathnames
> > don't matter, and bad files don't even get to the signature checking
> > code.
> >
> 
> I'm not sure I understand what you mean.  What table would we look the hash
> up in?  What are we finding in that table?

>From the other subthread:

> Then, to verify a signature, the kernel hashes the blob, generates its
> own linux_blob_signed_data, memcmps it to the one that Kyle signed
> (and rejects if they differ *at all*), and then verifies the
> signature.  (Do not try to be clever and parse the supplied
> linux_blob_signed_data -- there is a long and storied history of
> equivalent ideas being implemented incorrectly, and I can dig out
> literature references if you really want.  Just generate your own and
> memcmp it, which leaves no room for ambiguity.)
> 

So, I'm suggesting that when "the kernel hashes the blob", it use that
hash to locate *which* "Kyle-signed" linux_blob_signed_data it needs to
compare against.  That's all, just removing the filename from the
equation. :-)

thx,

Jason.