On 11/30/2012 08:50, Stephen Smalley wrote:
On 11/30/2012 08:35 AM, David Quigley wrote:
On 11/30/2012 08:28, Stephen Smalley wrote:
On 11/30/2012 08:17 AM, David Quigley wrote:
On 11/30/2012 07:57, David Quigley wrote:
On 11/30/2012 07:14, J. Bruce Fields wrote:
On Thu, Nov 29, 2012 at 09:02:49PM -0500, David Quigley wrote:
On 11/29/2012 20:50, Casey Schaufler wrote:
>On 11/29/2012 4:46 PM, David Quigley wrote:
>>On 11/29/2012 19:34, Casey Schaufler wrote:
>>>I would think that were it not for the case that access is
denied
>>>and I get an audit record for nfsd that reports a subject
>>>label of "_"
>>>(which is correct for nfsd but not the process attempting
>>>access) and
>>>an object label of "WhooHoo", which is correct. The server
side
>>>looks like it might be working right, given the information
that it
>>>has.
>>>
>>
>>Ok so this is the problem. nfsd is a kernel thread I believe.
In
>>SELinux land it has the type kernel_t which is all powerful.
We
>>don't
>>have client label transport yet (That requires RPCSECGSSv3).
Is
>>there
>>a way you can have that kernel thread running as a type that
has
>>access to everything?
>
>That would be having CAP_MAC_ADMIN and CAP_MAC_OVERRIDE in
Smackese.
>Looking at /proc/<pid-of-nfsd>/status we see CapEff of
fff...fff
>which
>is to say, all capabilities.
>
Hmm thats interesting then. You could try using rpcdebug -m
nfsd to
turn on some of the debugging to look around the internals and
figure out whats going on. If you pass -v it will give you all
of
the potential flags.
>
>>I think that is the current problem. Which makes perfect
sense. If
>>your kernel threads don't get started with max privilege then
the
>>server would be denied access on all of the file attributes
and
>>wouldn't be able to ship it over the wire properly.
>
>OK. I haven't had to do anything with kernel threads so far.
>Where is NFS setting these up? Poking around fs/nfsd looks
like
>the place, but I haven't seen anything there that makes it
look
>like they would be running without capabilities. Clearly,
that's
>what I'm seeing. It looks as if the credential of nfsd does
not
>match what /proc reports. Bother.
>
I'm not entirely sure whats up either. If you want to look for
the
NFSd threads they are in fs/nfsd/nfssvc.c. The main function
starts
on line 487.
I'm not following the discussion, but: maybe you want to look at
fs/nfsd/auth.c:nfsd_setuser() ? In particular, the
cap_{drop/raise}_nfsd_set() calls at the end.
--b.
I'm not as familiar with the capabilities code as Casey is so
I'll
leave this ball in his court. I think you are correct though and
the
problem is that NFSd is dropping and raising caps and we need to
make
sure that MAC_ADMIN and MAC_OVERRIDE is in there in the SMACK
case.
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I think I found the offending code. I can't test it for a while so
hopefully Casey can.
In include/linux/capability.h we have the following defines
# define CAP_FS_MASK_B0 (CAP_TO_MASK(CAP_CHOWN) \
| CAP_TO_MASK(CAP_MKNOD) \
| CAP_TO_MASK(CAP_DAC_OVERRIDE) \
| CAP_TO_MASK(CAP_DAC_READ_SEARCH) \
| CAP_TO_MASK(CAP_FOWNER) \
| CAP_TO_MASK(CAP_FSETID))
# define CAP_FS_MASK_B1 (CAP_TO_MASK(CAP_MAC_OVERRIDE))
#if _KERNEL_CAPABILITY_U32S != 2
# error Fix up hand-coded capability macro initializers
#else /* HAND-CODED capability initializers */
# define CAP_EMPTY_SET ((kernel_cap_t){{ 0, 0 }})
# define CAP_FULL_SET ((kernel_cap_t){{ ~0, ~0 }})
# define CAP_FS_SET ((kernel_cap_t){{ CAP_FS_MASK_B0 \
|
CAP_TO_MASK(CAP_LINUX_IMMUTABLE), \
CAP_FS_MASK_B1 } })
# define CAP_NFSD_SET ((kernel_cap_t){{ CAP_FS_MASK_B0 \
|
CAP_TO_MASK(CAP_SYS_RESOURCE), \
CAP_FS_MASK_B1 } })
So raise and drop nfsd caps uses CAP_NFSD_SET. In CAP_NFSD_SET we
have
CAP_MAC_OVERRIDE but we don't have CAP_MAC_ADMIN. I think maybe if
we
had both then Casey should be able to use the code with SMACK.
However
I'm not sure what implications this has for every other LSM.
Honestly
I'm not sure if we use either of those caps for SELinux at all (I
think
we ignore them completely).
CAP_MAC_ADMIN is used by SELinux these days, but only to control
the
ability to get or set security contexts that are not yet defined in
the policy (for package managers that lay down the security
contexts
before reloading policy and for installing a distro within a chroot
on
a build host running a different policy).
Do you think its reasonable to add that cap into the NFSd thread
then?
I'm not sure what other solution there would be. Casey needs it just
so
SMACK can work with it at all (assuming what I think is happening is
actually happening).
Looks like Smack requires CAP_MAC_ADMIN in order to set Smack
attributes on a file at all. So nfsd would require that capability
for Smack. I think this means however that setting Smack labels on
NFS files won't work in any case where root is squashed, which seems
unfortunate.
I'll leave that problem to Casey to figure out. However it seems to me
that regardless of Labeled NFS Casey should have problems with the NFS
server not being able to serve up files that are dominated by floor. I
wonder if he has every tried NFSv4 on a SMACK enabled server before. It
may have just worked because all files implicitly get labeled floor.
On the SELinux side, we don't require CAP_MAC_ADMIN to set the
SELinux attribute on a file in the normal case, only when the SELinux
attribute is not known to the security policy yet. So granting
CAP_MAC_ADMIN there means that a client will be able to set security
contexts on files that are unknown to the server. I guess that might
even be desirable in some instances where client and server policy
are
different. We do have the option of denying mac_admin permission in
policy for nfsd (kernel_t?), in which case we would block such
attempts to set unknown contexts but would still support setting of
known security contexts.
So I think it is workable, albeit a bit confusing.
Yea it is unfortunate that we have to go mucking around in capability
land but it seems that adding CAP_MAC_ADMIN should be fine and we can
deal with it in policy if we like.
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