New LSM hooks

New LSM hooks

[Casey Schaufler]

Full disclosure: This is all about me and my interests.

Developers propose new LSM hooks for many reasons. Often,
it's in support of an exotic feature such as Infiniband.
Sometimes it's because someone got clever when they optimized
an otherwise mundane feature and bypassed the usual hooks,
as is being proposed for kernfs. Usually there is a particular
security module (i.e. SELinux) that is targeted for the hook.
In almost all cases a hook is provided for that LSM, but not
for any other. In many cases the developers don't even include
the LSM email list in the discussions. LSM maintainers find
out about the new hooks after the fact.

Prior to 2008, when there was only one LSM, this made perfect
sense. Since then, it's been a regular practice justified by
the notion that it's the LSM maintainer's option to use the
hook or not. That also makes sense in cases where the use is
strictly optional, as it is in binder. It does not make sense,
however, when a core system facility like kernfs is involved.

I get a double whammy on these new LSM hooks. I have to try
to figure out if Smack cares, and if it does, whether to proposed
hook will solve the problem for Smack. Because Smack uses
xattrs and process attributes differently from SELinux there are
often problems with hooks that are provided with only an SELinux
implementation. I also have to work out how the new hook will
work in the stacked security module case. There are some existing
hooks that are a special challenge there, and when a new hook is
proposed that does the same kind of things (e.g. use of secid,
secctx or list of xattrs) without considering the consequences
for other modules.

What do I want, I hear you cry? I want some sanity in the way
LSM hooks are introduced. I want some standards or conventions
on what is appropriate to pass into and out of LSM hooks. I
want push back on special purpose hooks that are required to
fix the deficiencies of a filesystem or bizarre hardware
implementation. I want to stop spending all my time trying to
deal with new, crazy LSM hooks. There are enough old ones to
keep me entertained, thank you very much.

We're about to get a bunch of new LSMs. I don't think that we
can afford to continue with the current "feature A needs a hook
for LSM S" behavior.

Re: New LSM hooks

[Paul Moore]

On Tue, Feb 5, 2019 at 12:40 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> Full disclosure: This is all about me and my interests.
>
> Developers propose new LSM hooks for many reasons. Often,
> it's in support of an exotic feature such as Infiniband.
> Sometimes it's because someone got clever when they optimized
> an otherwise mundane feature and bypassed the usual hooks,
> as is being proposed for kernfs. Usually there is a particular
> security module (i.e. SELinux) that is targeted for the hook.
> In almost all cases a hook is provided for that LSM, but not
> for any other. In many cases the developers don't even include
> the LSM email list in the discussions. LSM maintainers find
> out about the new hooks after the fact.

I think it is each LSM maintainer's job to ensure that patches which
affect the LSM hooks, either through modification or addition, are
CC'd to the LSM list for discussion.  I've tried to be good about
this, but I'm sure I've missed some over the years.

> Prior to 2008, when there was only one LSM, this made perfect
> sense. Since then, it's been a regular practice justified by
> the notion that it's the LSM maintainer's option to use the
> hook or not. That also makes sense in cases where the use is
> strictly optional, as it is in binder. It does not make sense,
> however, when a core system facility like kernfs is involved.

The term "optional" is tricky here.  In all the cases I can think of
where a LSM hook has been added to an existing bit of kernel
functionality, the hook has almost always been optional if for no
other reason than it didn't exist in the previous kernel.  I suppose
one could argue that there is a functional disparity between LSMs if
one LSM implements the hook and the others do not, but that is a
different issue.

> I get a double whammy on these new LSM hooks. I have to try
> to figure out if Smack cares, and if it does, whether to proposed
> hook will solve the problem for Smack. Because Smack uses
> xattrs and process attributes differently from SELinux there are
> often problems with hooks that are provided with only an SELinux
> implementation. I also have to work out how the new hook will
> work in the stacked security module case. There are some existing
> hooks that are a special challenge there, and when a new hook is
> proposed that does the same kind of things (e.g. use of secid,
> secctx or list of xattrs) without considering the consequences
> for other modules.

Once again, I think it is important to CC the LSM list on these sorts
of patchsets, and to get the patch author to consider other LSMs, but
I think asking the original author to preemptively add support to each
LSM for every new addition is too high a bar.  I would encourage the
individual LSMs (both maintainers and contributors) to work with patch
authors in a constructive manner to add support for new hooks when
they make sense for the given LSM.

> What do I want, I hear you cry? I want some sanity in the way
> LSM hooks are introduced. I want some standards or conventions
> on what is appropriate to pass into and out of LSM hooks. I
> want push back on special purpose hooks that are required to
> fix the deficiencies of a filesystem or bizarre hardware
> implementation. I want to stop spending all my time trying to
> deal with new, crazy LSM hooks. There are enough old ones to
> keep me entertained, thank you very much.

Yeah, good luck with that.  I think trying to legislate this too much
is a sure path to failure.  Our saving grace is that the LSM hook
boundary is not a kernel/userspace boundary so we are free to change
it as needed, we just need to make sure if we change an existing hook
we don't break any of the existing in-tree LSMs.  We've done this in
the past without too much problem, even other subsystems have done
this for us (without much notification to the LSMs) and it has
generally worked out okay.

I'm all for increased collaboration between LSMs, e.g. requiring hooks
changes to CC the LSM list, but I think mandating anything beyond that
is a fool's errand.

-- 
paul moore
www.paul-moore.com

Re: New LSM hooks

[Casey Schaufler]

On 2/5/2019 10:15 AM, Paul Moore wrote:
> On Tue, Feb 5, 2019 at 12:40 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
>> Full disclosure: This is all about me and my interests.
>>
>> Developers propose new LSM hooks for many reasons. Often,
>> it's in support of an exotic feature such as Infiniband.
>> Sometimes it's because someone got clever when they optimized
>> an otherwise mundane feature and bypassed the usual hooks,
>> as is being proposed for kernfs. Usually there is a particular
>> security module (i.e. SELinux) that is targeted for the hook.
>> In almost all cases a hook is provided for that LSM, but not
>> for any other. In many cases the developers don't even include
>> the LSM email list in the discussions. LSM maintainers find
>> out about the new hooks after the fact.
> I think it is each LSM maintainer's job to ensure that patches which
> affect the LSM hooks, either through modification or addition, are
> CC'd to the LSM list for discussion.  I've tried to be good about
> this, but I'm sure I've missed some over the years.

It's often not the LSM maintainer, but the developer of
the feature that introduces the hook. That's the case that
has been a problem.


>> Prior to 2008, when there was only one LSM, this made perfect
>> sense. Since then, it's been a regular practice justified by
>> the notion that it's the LSM maintainer's option to use the
>> hook or not. That also makes sense in cases where the use is
>> strictly optional, as it is in binder. It does not make sense,
>> however, when a core system facility like kernfs is involved.
> The term "optional" is tricky here.  In all the cases I can think of
> where a LSM hook has been added to an existing bit of kernel
> functionality, the hook has almost always been optional if for no
> other reason than it didn't exist in the previous kernel.  I suppose
> one could argue that there is a functional disparity between LSMs if
> one LSM implements the hook and the others do not, but that is a
> different issue.

Agreed that there's ambiguity about what should be done for
an "optional" feature. If RedHat adds support for a feature
they have every reason to make sure it works correctly with
SELinux and no incentive to implement AppArmor hooks. But is
overlayfs an "optional" feature? If the container developers
who wanted overlayfs only cared about an SELinux environment
does that excuse them from introducing a feature that didn't
work with Smack?

>> I get a double whammy on these new LSM hooks. I have to try
>> to figure out if Smack cares, and if it does, whether to proposed
>> hook will solve the problem for Smack. Because Smack uses
>> xattrs and process attributes differently from SELinux there are
>> often problems with hooks that are provided with only an SELinux
>> implementation. I also have to work out how the new hook will
>> work in the stacked security module case. There are some existing
>> hooks that are a special challenge there, and when a new hook is
>> proposed that does the same kind of things (e.g. use of secid,
>> secctx or list of xattrs) without considering the consequences
>> for other modules.
> Once again, I think it is important to CC the LSM list on these sorts
> of patchsets, and to get the patch author to consider other LSMs, but
> I think asking the original author to preemptively add support to each
> LSM for every new addition is too high a bar.

Two edged sword. If someone introduced an LSM that worked
with btrfs but broke ext4 and xfs it wouldn't be likely to
make it upstream. I wouldn't expect the developer who wants
to introduce a new hook to provide all the LSM versions, but
I would like that developer to work with the LSM maintainers
before the patch is submitted.

>   I would encourage the
> individual LSMs (both maintainers and contributors) to work with patch
> authors in a constructive manner to add support for new hooks when
> they make sense for the given LSM.

Yes.

>> What do I want, I hear you cry? I want some sanity in the way
>> LSM hooks are introduced. I want some standards or conventions
>> on what is appropriate to pass into and out of LSM hooks. I
>> want push back on special purpose hooks that are required to
>> fix the deficiencies of a filesystem or bizarre hardware
>> implementation. I want to stop spending all my time trying to
>> deal with new, crazy LSM hooks. There are enough old ones to
>> keep me entertained, thank you very much.
> Yeah, good luck with that.

Hey, if you don't play, you can't win. ;)

> I think trying to legislate this too much
> is a sure path to failure.

There's not doubt that "too much" legislation would be a problem.
Right now, we have *no* guidance or guidelines.


> Our saving grace is that the LSM hook
> boundary is not a kernel/userspace boundary so we are free to change
> it as needed, we just need to make sure if we change an existing hook
> we don't break any of the existing in-tree LSMs.  We've done this in
> the past without too much problem, even other subsystems have done
> this for us (without much notification to the LSMs) and it has
> generally worked out okay.

Sure, but it's a royal pain to deal with the wild inconsistency
of interfaces we have today. I would like to discourage new
hooks that do things like expose filesystem or network protocol
implementation details to the LSM interface.

> I'm all for increased collaboration between LSMs, e.g. requiring hooks
> changes to CC the LSM list, but I think mandating anything beyond that
> is a fool's errand.

VFS manages this, and I wouldn't call Al a fool.

Re: New LSM hooks

[Paul Moore]

On Tue, Feb 5, 2019 at 3:04 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> On 2/5/2019 10:15 AM, Paul Moore wrote:
> > On Tue, Feb 5, 2019 at 12:40 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> >> Full disclosure: This is all about me and my interests.
> >>
> >> Developers propose new LSM hooks for many reasons. Often,
> >> it's in support of an exotic feature such as Infiniband.
> >> Sometimes it's because someone got clever when they optimized
> >> an otherwise mundane feature and bypassed the usual hooks,
> >> as is being proposed for kernfs. Usually there is a particular
> >> security module (i.e. SELinux) that is targeted for the hook.
> >> In almost all cases a hook is provided for that LSM, but not
> >> for any other. In many cases the developers don't even include
> >> the LSM email list in the discussions. LSM maintainers find
> >> out about the new hooks after the fact.
> >
> > I think it is each LSM maintainer's job to ensure that patches which
> > affect the LSM hooks, either through modification or addition, are
> > CC'd to the LSM list for discussion.  I've tried to be good about
> > this, but I'm sure I've missed some over the years.
>
> It's often not the LSM maintainer, but the developer of
> the feature that introduces the hook. That's the case that
> has been a problem.

I believe that will always be a problem, no matter what we do.  The
point I was trying to make was that everyone, especially the
maintainers, need to watch for this when patches are posted and make
sure the patch author posts to the LSM list in addition to any of the
relevant LSM specific lists.

> >> Prior to 2008, when there was only one LSM, this made perfect
> >> sense. Since then, it's been a regular practice justified by
> >> the notion that it's the LSM maintainer's option to use the
> >> hook or not. That also makes sense in cases where the use is
> >> strictly optional, as it is in binder. It does not make sense,
> >> however, when a core system facility like kernfs is involved.
> >
> > The term "optional" is tricky here.  In all the cases I can think of
> > where a LSM hook has been added to an existing bit of kernel
> > functionality, the hook has almost always been optional if for no
> > other reason than it didn't exist in the previous kernel.  I suppose
> > one could argue that there is a functional disparity between LSMs if
> > one LSM implements the hook and the others do not, but that is a
> > different issue.
>
> Agreed that there's ambiguity about what should be done for
> an "optional" feature. If RedHat adds support for a feature
> they have every reason to make sure it works correctly with
> SELinux and no incentive to implement AppArmor hooks.

I want to be very clear on this point, I don't care who submits the
patches, or who may pay them to do it.  From an upstream perspective,
patches are patches.  The individual patch submitter may not see
things quite this same way, but it is everyone's responsibility to
make sure that we do the Right Thing, regardless of who may be paying
us for our work.

Earlier in the thread I advocated for each of the LSM maintainers to
work with patchset authors who introduce new LSM hooks but don't
provide an implementation for their LSM.  I think it is that
cooperation and discussion which is important.

> ... But is overlayfs an "optional" feature?

I'm not using overlayfs on this Linux system right now, so I would
have to say "yes".  I know it will shock many who read this, but you
can still run a Linux system these days without containers.

> If the container developers
> who wanted overlayfs only cared about an SELinux environment
> does that excuse them from introducing a feature that didn't
> work with Smack?

There is a big difference between "doesn't work" and "not yet
implemented".  I'm not sure where things stand with respect to
overlayfs support in the various LSMs, but have you or any of the
other LSM maintainers tried working with Vivek to add support wherever
it may be missing?  He's a nice guy, you should send him mail.

> >> I get a double whammy on these new LSM hooks. I have to try
> >> to figure out if Smack cares, and if it does, whether to proposed
> >> hook will solve the problem for Smack. Because Smack uses
> >> xattrs and process attributes differently from SELinux there are
> >> often problems with hooks that are provided with only an SELinux
> >> implementation. I also have to work out how the new hook will
> >> work in the stacked security module case. There are some existing
> >> hooks that are a special challenge there, and when a new hook is
> >> proposed that does the same kind of things (e.g. use of secid,
> >> secctx or list of xattrs) without considering the consequences
> >> for other modules.
> >
> > Once again, I think it is important to CC the LSM list on these sorts
> > of patchsets, and to get the patch author to consider other LSMs, but
> > I think asking the original author to preemptively add support to each
> > LSM for every new addition is too high a bar.
>
> Two edged sword. If someone introduced an LSM that worked
> with btrfs but broke ext4 and xfs it wouldn't be likely to
> make it upstream. I wouldn't expect the developer who wants
> to introduce a new hook to provide all the LSM versions, but
> I would like that developer to work with the LSM maintainers
> before the patch is submitted.

With that last sentence above I think we're pretty much saying the
same thing, the only real difference is that I don't particularly care
when the collaboration takes place so long as it takes place.  In my
opinion it would be foolish to hold off committing working support in
one LSM because support for another remains a work in progress.  The
LSM interface is not a protected interface that needs to be preserved,
if it is broken or doesn't meet our needs, we change it.  We've been
doing this for years, and regardless of what may come from this I
expect existing and future hooks will continue to change over time.
Predicting the future is impossible, agreeing to work together to deal
with the future as it comes is much easier.

> >   I would encourage the
> > individual LSMs (both maintainers and contributors) to work with patch
> > authors in a constructive manner to add support for new hooks when
> > they make sense for the given LSM.
>
> Yes.
>
> >> What do I want, I hear you cry? I want some sanity in the way
> >> LSM hooks are introduced. I want some standards or conventions
> >> on what is appropriate to pass into and out of LSM hooks. I
> >> want push back on special purpose hooks that are required to
> >> fix the deficiencies of a filesystem or bizarre hardware
> >> implementation. I want to stop spending all my time trying to
> >> deal with new, crazy LSM hooks. There are enough old ones to
> >> keep me entertained, thank you very much.
> > Yeah, good luck with that.
>
> Hey, if you don't play, you can't win. ;)
>
> > I think trying to legislate this too much
> > is a sure path to failure.
>
> There's not doubt that "too much" legislation would be a problem.
> Right now, we have *no* guidance or guidelines.

Sigh.  Be very careful what you wish for, we've got some pretty good
convention and established practices here, I would hate for your quest
for regulation to break what we've got going.  Especially when people
will use that regulation as a substitute for discussion.

> > Our saving grace is that the LSM hook
> > boundary is not a kernel/userspace boundary so we are free to change
> > it as needed, we just need to make sure if we change an existing hook
> > we don't break any of the existing in-tree LSMs.  We've done this in
> > the past without too much problem, even other subsystems have done
> > this for us (without much notification to the LSMs) and it has
> > generally worked out okay.
>
> Sure, but it's a royal pain to deal with the wild inconsistency
> of interfaces we have today.

We are writing hooks that deal with the entire system, there is
*always* going to be inconsistency; just look at the subsystems.
Further, just like the rest of the kernel, the hooks evolve over time
and often that evolution happens at different paces because the
different pieces of the kernel move at different paces.

> I would like to discourage new
> hooks that do things like expose filesystem or network protocol
> implementation details to the LSM interface.

Until something comes along and we need to break that abstraction.
Once again, if we're going to mandate anything, let's mandate that we
need to discuss changes to the LSM hooks and leave it at that.

> > I'm all for increased collaboration between LSMs, e.g. requiring hooks
> > changes to CC the LSM list, but I think mandating anything beyond that
> > is a fool's errand.
>
> VFS manages this, and I wouldn't call Al a fool.

You sir, are no Al Viro. :)  (and for the record, none of us are IMHO)

While the VFS subsystem may be the closest analogy to the LSM
subsystem, I think the two subsystems are quite different.  If nothing
else, the other kernel developers see the need for a working
filesystem and are willing to make compromises to keep their
filesystem working.  I think we can all name at least one subsystem
that would very much like to see the LSMs go away, and in order to
keep things functioning we sometimes have to get "creative", and that
can mean breaking abstractions.

-- 
paul moore
www.paul-moore.com

Re: New LSM hooks

[James Morris]

On Tue, 5 Feb 2019, Paul Moore wrote:

> I believe that will always be a problem, no matter what we do.  The
> point I was trying to make was that everyone, especially the
> maintainers, need to watch for this when patches are posted and make
> sure the patch author posts to the LSM list in addition to any of the
> relevant LSM specific lists.

Right, and there is no way a new LSM hook should ever be added to the 
kernel without review and ack/signoffs from folks on the LSM list 
(especially those who are maintainers of in-tree LSMs).

Casey, do you have any examples of this happening?


-- 
James Morris
<jmorris@namei.org>

Re: New LSM hooks

[Stephen Smalley]

On 2/5/19 8:11 PM, James Morris wrote:
> On Tue, 5 Feb 2019, Paul Moore wrote:
> 
>> I believe that will always be a problem, no matter what we do.  The
>> point I was trying to make was that everyone, especially the
>> maintainers, need to watch for this when patches are posted and make
>> sure the patch author posts to the LSM list in addition to any of the
>> relevant LSM specific lists.
> 
> Right, and there is no way a new LSM hook should ever be added to the
> kernel without review and ack/signoffs from folks on the LSM list
> (especially those who are maintainers of in-tree LSMs).
> 
> Casey, do you have any examples of this happening?

Most of the times I've seen that it has come from vfs folks or other 
subsystems as part of some major reworking of that subsystem rather than 
from security module developers, e.g. the mount hooks overhaul.

Re: New LSM hooks

[Casey Schaufler]

On 2/6/2019 5:20 AM, Stephen Smalley wrote:
> On 2/5/19 8:11 PM, James Morris wrote:
>> On Tue, 5 Feb 2019, Paul Moore wrote:
>>
>>> I believe that will always be a problem, no matter what we do.  The
>>> point I was trying to make was that everyone, especially the
>>> maintainers, need to watch for this when patches are posted and make
>>> sure the patch author posts to the LSM list in addition to any of the
>>> relevant LSM specific lists.
>>
>> Right, and there is no way a new LSM hook should ever be added to the
>> kernel without review and ack/signoffs from folks on the LSM list
>> (especially those who are maintainers of in-tree LSMs).
>>
>> Casey, do you have any examples of this happening?
>
> Most of the times I've seen that it has come from vfs folks or other subsystems as part of some major reworking of that subsystem rather than from security module developers, e.g. the mount hooks overhaul.

David Howells did contact me directly on the mount hook
changes well in advance. I'm more concerned with special
purpose hooks like we have for binder, kernfs, nfs, tun,
Infiniband and bpf. I'm not saying that we never need to
provide hooks with a single user, but you do have to
wonder about security_ismaclabel().

Re: New LSM hooks

[Stephen Smalley]

On 2/6/19 12:24 PM, Casey Schaufler wrote:
> On 2/6/2019 5:20 AM, Stephen Smalley wrote:
>> On 2/5/19 8:11 PM, James Morris wrote:
>>> On Tue, 5 Feb 2019, Paul Moore wrote:
>>>
>>>> I believe that will always be a problem, no matter what we do.  The
>>>> point I was trying to make was that everyone, especially the
>>>> maintainers, need to watch for this when patches are posted and make
>>>> sure the patch author posts to the LSM list in addition to any of the
>>>> relevant LSM specific lists.
>>>
>>> Right, and there is no way a new LSM hook should ever be added to the
>>> kernel without review and ack/signoffs from folks on the LSM list
>>> (especially those who are maintainers of in-tree LSMs).
>>>
>>> Casey, do you have any examples of this happening?
>>
>> Most of the times I've seen that it has come from vfs folks or other subsystems as part of some major reworking of that subsystem rather than from security module developers, e.g. the mount hooks overhaul.
> 
> David Howells did contact me directly on the mount hook
> changes well in advance. I'm more concerned with special
> purpose hooks like we have for binder, kernfs, nfs, tun,
> Infiniband and bpf. I'm not saying that we never need to
> provide hooks with a single user, but you do have to
> wonder about security_ismaclabel().

binder is an IPC mechanism, so just like all the other IPC mechanisms, 
we needed hooks to mediate it.  No different than having hooks for 
System V IPC or Unix sockets. If LSM were a proper abstraction layer 
like the Flask architecture or even the BSD MAC framework, then we'd 
have some general purpose access control interface with a small number 
of entrypoints to invoke, but instead we just push all of that behind 
the LSM interface.  I wasn't an advocate for the LSM approach at the 
time but it is what was adopted and we have to live with it.

kernfs doesn't follow the usual rules for inode setup, so we have to 
play by its rules. Not our choice to make, and there is a reason why 
kernfs is the way it is.

NFSv4.2 chose to support MAC labels as a first class citizen rather than 
just xattrs (and there was a reason for that too), so we need a 
different path for getting and setting those labels.

And so on for tun, infiniband, and bpf.  Remember that LSM by design 
does nothing more than expose the kernel objects and operations to the 
security modules and leaves it up to the module writer to do everything 
else.

Re: New LSM hooks

[Casey Schaufler]

On 2/6/2019 9:44 AM, Stephen Smalley wrote:
> On 2/6/19 12:24 PM, Casey Schaufler wrote:
>> On 2/6/2019 5:20 AM, Stephen Smalley wrote:
>>> On 2/5/19 8:11 PM, James Morris wrote:
>>>> On Tue, 5 Feb 2019, Paul Moore wrote:
>>>>
>>>>> I believe that will always be a problem, no matter what we do.  The
>>>>> point I was trying to make was that everyone, especially the
>>>>> maintainers, need to watch for this when patches are posted and make
>>>>> sure the patch author posts to the LSM list in addition to any of the
>>>>> relevant LSM specific lists.
>>>>
>>>> Right, and there is no way a new LSM hook should ever be added to the
>>>> kernel without review and ack/signoffs from folks on the LSM list
>>>> (especially those who are maintainers of in-tree LSMs).
>>>>
>>>> Casey, do you have any examples of this happening?
>>>
>>> Most of the times I've seen that it has come from vfs folks or other subsystems as part of some major reworking of that subsystem rather than from security module developers, e.g. the mount hooks overhaul.
>>
>> David Howells did contact me directly on the mount hook
>> changes well in advance. I'm more concerned with special
>> purpose hooks like we have for binder, kernfs, nfs, tun,
>> Infiniband and bpf. I'm not saying that we never need to
>> provide hooks with a single user, but you do have to
>> wonder about security_ismaclabel().
>
> binder is an IPC mechanism, so just like all the other IPC mechanisms, we needed hooks to mediate it.  No different than having hooks for System V IPC or Unix sockets. If LSM were a proper abstraction layer like the Flask architecture or even the BSD MAC framework, then we'd have some general purpose access control interface with a small number of entrypoints to invoke, but instead we just push all of that behind the LSM interface.  I wasn't an advocate for the LSM approach at the time but it is what was adopted and we have to live with it.

I didn't get what I wanted, either.

>
> kernfs doesn't follow the usual rules for inode setup, so we have to play by its rules. Not our choice to make, and there is a reason why kernfs is the way it is.

I don't believe that kernfs has to be as unusual as it is,
and should never have been included without working LSM
integration.

>
> NFSv4.2 chose to support MAC labels as a first class citizen rather than just xattrs (and there was a reason for that too), so we need a different path for getting and setting those labels.

And I *still* disagree with those reasons.

>
> And so on for tun, infiniband, and bpf.  Remember that LSM by design does nothing more than expose the kernel objects and operations to the security modules and leaves it up to the module writer to do everything else.

Re: New LSM hooks

[Casey Schaufler]

On 2/5/2019 5:11 PM, James Morris wrote:
> On Tue, 5 Feb 2019, Paul Moore wrote:
>
>> I believe that will always be a problem, no matter what we do.  The
>> point I was trying to make was that everyone, especially the
>> maintainers, need to watch for this when patches are posted and make
>> sure the patch author posts to the LSM list in addition to any of the
>> relevant LSM specific lists.
> Right, and there is no way a new LSM hook should ever be added to the 
> kernel without review and ack/signoffs from folks on the LSM list 
> (especially those who are maintainers of in-tree LSMs).
>
> Casey, do you have any examples of this happening?

overlayfs (according to my records - which may be flawed)
is a prime example. Inifiniband hooks were reviewed/acked
for SELinux, but there was never an attempt made to work
with other security module maintainers. Yes, they were posted
to LSM, but under the title "SELinux support for Infiniband".

Re: New LSM hooks

[Stephen Smalley]

On 2/6/19 11:30 AM, Casey Schaufler wrote:
> On 2/5/2019 5:11 PM, James Morris wrote:
>> On Tue, 5 Feb 2019, Paul Moore wrote:
>>
>>> I believe that will always be a problem, no matter what we do.  The
>>> point I was trying to make was that everyone, especially the
>>> maintainers, need to watch for this when patches are posted and make
>>> sure the patch author posts to the LSM list in addition to any of the
>>> relevant LSM specific lists.
>> Right, and there is no way a new LSM hook should ever be added to the
>> kernel without review and ack/signoffs from folks on the LSM list
>> (especially those who are maintainers of in-tree LSMs).
>>
>> Casey, do you have any examples of this happening?
> 
> overlayfs (according to my records - which may be flawed)
> is a prime example. Inifiniband hooks were reviewed/acked
> for SELinux, but there was never an attempt made to work
> with other security module maintainers. Yes, they were posted
> to LSM, but under the title "SELinux support for Infiniband".

overlayfs hooks were also posted to and discussed on lsm list, including 
comments from you.  Admittedly the cover patch said Overlayfs SELinux 
Support but the individual patches for the hooks were "security, 
overlayfs: provide copy up security hook for unioned files" and 
"security,overlayfs: Provide security hook for copy up of xattrs for 
overlay file".

In either case, did you request a change that was ignored?

Re: New LSM hooks

[Casey Schaufler]

On 2/6/2019 9:06 AM, Stephen Smalley wrote:
> On 2/6/19 11:30 AM, Casey Schaufler wrote:
>> On 2/5/2019 5:11 PM, James Morris wrote:
>>> On Tue, 5 Feb 2019, Paul Moore wrote:
>>>
>>>> I believe that will always be a problem, no matter what we do.  The
>>>> point I was trying to make was that everyone, especially the
>>>> maintainers, need to watch for this when patches are posted and make
>>>> sure the patch author posts to the LSM list in addition to any of the
>>>> relevant LSM specific lists.
>>> Right, and there is no way a new LSM hook should ever be added to the
>>> kernel without review and ack/signoffs from folks on the LSM list
>>> (especially those who are maintainers of in-tree LSMs).
>>>
>>> Casey, do you have any examples of this happening?
>>
>> overlayfs (according to my records - which may be flawed)
>> is a prime example. Inifiniband hooks were reviewed/acked
>> for SELinux, but there was never an attempt made to work
>> with other security module maintainers. Yes, they were posted
>> to LSM, but under the title "SELinux support for Infiniband".
>
> overlayfs hooks were also posted to and discussed on lsm list, including comments from you.  Admittedly the cover patch said Overlayfs SELinux Support but the individual patches for the hooks were "security, overlayfs: provide copy up security hook for unioned files" and "security,overlayfs: Provide security hook for copy up of xattrs for overlay file".

OK, apologies all around. My records are still better
than my memory, but neither is perfect.

>
> In either case, did you request a change that was ignored?

Smack support was definitely not included. I started
getting complaints almost immediately when overlayfs
hit upstream. 

Re: New LSM hooks

[Stephen Smalley]

On 2/6/19 12:44 PM, Casey Schaufler wrote:
> On 2/6/2019 9:06 AM, Stephen Smalley wrote:
>> On 2/6/19 11:30 AM, Casey Schaufler wrote:
>>> On 2/5/2019 5:11 PM, James Morris wrote:
>>>> On Tue, 5 Feb 2019, Paul Moore wrote:
>>>>
>>>>> I believe that will always be a problem, no matter what we do.  The
>>>>> point I was trying to make was that everyone, especially the
>>>>> maintainers, need to watch for this when patches are posted and make
>>>>> sure the patch author posts to the LSM list in addition to any of the
>>>>> relevant LSM specific lists.
>>>> Right, and there is no way a new LSM hook should ever be added to the
>>>> kernel without review and ack/signoffs from folks on the LSM list
>>>> (especially those who are maintainers of in-tree LSMs).
>>>>
>>>> Casey, do you have any examples of this happening?
>>>
>>> overlayfs (according to my records - which may be flawed)
>>> is a prime example. Inifiniband hooks were reviewed/acked
>>> for SELinux, but there was never an attempt made to work
>>> with other security module maintainers. Yes, they were posted
>>> to LSM, but under the title "SELinux support for Infiniband".
>>
>> overlayfs hooks were also posted to and discussed on lsm list, including comments from you.  Admittedly the cover patch said Overlayfs SELinux Support but the individual patches for the hooks were "security, overlayfs: provide copy up security hook for unioned files" and "security,overlayfs: Provide security hook for copy up of xattrs for overlay file".
> 
> OK, apologies all around. My records are still better
> than my memory, but neither is perfect.
> 
>>
>> In either case, did you request a change that was ignored?
> 
> Smack support was definitely not included. I started
> getting complaints almost immediately when overlayfs
> hit upstream.

You can't expect the developer to supply a Smack implementation of the 
hook if they aren't using Smack themselves.

On the other hand, absence of a hook implementation shouldn't break 
existing users.  So if that was the case, then there was a bug in the 
hook's fallback behavior.

RE: New LSM hooks

[Edwin Zimmerman]

On Tuesday, February 05, 2019 12:40 PM Casey Schaufler wrote:
> Full disclosure: This is all about me and my interests.
> 
> Developers propose new LSM hooks for many reasons. Often,
> it's in support of an exotic feature such as Infiniband.
> Sometimes it's because someone got clever when they optimized
> an otherwise mundane feature and bypassed the usual hooks,
> as is being proposed for kernfs. Usually there is a particular
> security module (i.e. SELinux) that is targeted for the hook.
> In almost all cases a hook is provided for that LSM, but not
> for any other. In many cases the developers don't even include
> the LSM email list in the discussions. LSM maintainers find
> out about the new hooks after the fact.
> 
> Prior to 2008, when there was only one LSM, this made perfect
> sense. Since then, it's been a regular practice justified by
> the notion that it's the LSM maintainer's option to use the
> hook or not. That also makes sense in cases where the use is
> strictly optional, as it is in binder. It does not make sense,
> however, when a core system facility like kernfs is involved.
> 
> I get a double whammy on these new LSM hooks. I have to try
> to figure out if Smack cares, and if it does, whether to proposed
> hook will solve the problem for Smack. Because Smack uses
> xattrs and process attributes differently from SELinux there are
> often problems with hooks that are provided with only an SELinux
> implementation. I also have to work out how the new hook will
> work in the stacked security module case. There are some existing
> hooks that are a special challenge there, and when a new hook is
> proposed that does the same kind of things (e.g. use of secid,
> secctx or list of xattrs) without considering the consequences
> for other modules.
> 
> What do I want, I hear you cry? I want some sanity in the way
> LSM hooks are introduced. I want some standards or conventions
> on what is appropriate to pass into and out of LSM hooks. I
> want push back on special purpose hooks that are required to
> fix the deficiencies of a filesystem or bizarre hardware
> implementation. I want to stop spending all my time trying to
> deal with new, crazy LSM hooks. There are enough old ones to
> keep me entertained, thank you very much.

I agree.  We need a roadmap of where we want the LSM infrastructure to go.
Without such a guide, LSM code is going to end up as a rats nest of
confusing, partially implemented, partially supported code.

Here's my suggestion for starters. According to kernel documentation, new 
LSMs must be documented before being accepted.  Perhaps we need a 
similar requirement for LSM hooks.  As I see it, LSMs are security additions,
not functionality patches for the rest of the kernel or for special hardware or
whatever.  Therefore, I also suggest that all new hooks be implemented in 
at least two LSMs before being accepted.

-Edwin Zimmerman

> We're about to get a bunch of new LSMs. I don't think that we
> can afford to continue with the current "feature A needs a hook
> for LSM S" behavior.

Re: New LSM hooks

[Casey Schaufler]

On 2/5/2019 10:28 AM, Edwin Zimmerman wrote:
> On Tuesday, February 05, 2019 12:40 PM Casey Schaufler wrote:
>> Full disclosure: This is all about me and my interests.
>> ...
>>
>> What do I want, I hear you cry? I want some sanity in the way
>> LSM hooks are introduced. I want some standards or conventions
>> on what is appropriate to pass into and out of LSM hooks. I
>> want push back on special purpose hooks that are required to
>> fix the deficiencies of a filesystem or bizarre hardware
>> implementation. I want to stop spending all my time trying to
>> deal with new, crazy LSM hooks. There are enough old ones to
>> keep me entertained, thank you very much.
> I agree.  We need a roadmap of where we want the LSM infrastructure to go.
> Without such a guide, LSM code is going to end up as a rats nest of
> confusing, partially implemented, partially supported code.

We've achieved rat's nest already. I'm working to untangle it as
part of the stacking work.

> Here's my suggestion for starters. According to kernel documentation, new 
> LSMs must be documented before being accepted.  Perhaps we need a 
> similar requirement for LSM hooks.

That would be handy. The documentation would need to cover
the purpose for the hook and how a security module would be
expected to use it.

> As I see it, LSMs are security additions,
> not functionality patches for the rest of the kernel or for special hardware or
> whatever.  Therefore, I also suggest that all new hooks be implemented in 
> at least two LSMs before being accepted.

I can't say this makes sense. The binder hooks are only
useful for Android, and requiring Smack or AppArmor hooks
be implemented isn't reasonable. It would be reasonable for
the kernfs hook, as the kernfs hook is a workaround for the
fact that kernfs doesn't use inodes.

Re: New LSM hooks

[Paul Moore]

On Tue, Feb 5, 2019 at 2:26 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> On 2/5/2019 10:28 AM, Edwin Zimmerman wrote:
> > Here's my suggestion for starters. According to kernel documentation, new
> > LSMs must be documented before being accepted.  Perhaps we need a
> > similar requirement for LSM hooks.
>
> That would be handy. The documentation would need to cover
> the purpose for the hook and how a security module would be
> expected to use it.

We have a weak version of this now with the comments in
include/linux/lsm_hooks.h, and as far as I recall we've pushed back on
people who have changed the hooks without some documentation in the
comments.  Whatever we end up doing, let's try to keep this going as a
practice.

-- 
paul moore
www.paul-moore.com

RE: New LSM hooks

[Edwin Zimmerman]

On Tuesday, February 05, 2019 12:40 PM Casey Schaufler wrote:
> On 2/5/2019 10:28 AM, Edwin Zimmerman wrote:
> > On Tuesday, February 05, 2019 12:40 PM Casey Schaufler wrote:
> >...
> > Here's my suggestion for starters. According to kernel documentation, new
> > LSMs must be documented before being accepted.  Perhaps we need a
> > similar requirement for LSM hooks.
> 
> That would be handy. The documentation would need to cover
> the purpose for the hook and how a security module would be
> expected to use it.
> 
> > As I see it, LSMs are security additions,
> > not functionality patches for the rest of the kernel or for special hardware or
> > whatever.  Therefore, I also suggest that all new hooks be implemented in
> > at least two LSMs before being accepted.
> 
> I can't say this makes sense. The binder hooks are only
> useful for Android, and requiring Smack or AppArmor hooks
> be implemented isn't reasonable. It would be reasonable for
> the kernfs hook, as the kernfs hook is a workaround for the
> fact that kernfs doesn't use inodes.

You have a good point there.  I withdraw my "two LSMs" suggestions.