[PATCH v3] cred: Propagate security_prepare_creds() error code

[Paul Moore]

On Wed, Jun 15, 2022 at 6:30 AM Christian Brauner <brauner at kernel.org> wrote:
>
> On Tue, Jun 14, 2022 at 01:59:08PM -0500, Frederick Lawler wrote:
> > On 6/14/22 11:30 AM, Eric W. Biederman wrote:
> > > Frederick Lawler <fred at cloudflare.com> writes:
> > >
> > > > On 6/13/22 11:44 PM, Eric W. Biederman wrote:
> > > > > Frederick Lawler <fred at cloudflare.com> writes:
> > > > >
> > > > > > Hi Eric,
> > > > > >
> > > > > > On 6/13/22 12:04 PM, Eric W. Biederman wrote:
> > > > > > > Frederick Lawler <fred at cloudflare.com> writes:
> > > > > > >
> > > > > > > > While experimenting with the security_prepare_creds() LSM hook, we
> > > > > > > > noticed that our EPERM error code was not propagated up the callstack.
> > > > > > > > Instead ENOMEM is always returned.  As a result, some tools may send a
> > > > > > > > confusing error message to the user:
> > > > > > > >
> > > > > > > > $ unshare -rU
> > > > > > > > unshare: unshare failed: Cannot allocate memory
> > > > > > > >
> > > > > > > > A user would think that the system didn't have enough memory, when
> > > > > > > > instead the action was denied.
> > > > > > > >
> > > > > > > > This problem occurs because prepare_creds() and prepare_kernel_cred()
> > > > > > > > return NULL when security_prepare_creds() returns an error code. Later,
> > > > > > > > functions calling prepare_creds() and prepare_kernel_cred() return
> > > > > > > > ENOMEM because they assume that a NULL meant there was no memory
> > > > > > > > allocated.
> > > > > > > >
> > > > > > > > Fix this by propagating an error code from security_prepare_creds() up
> > > > > > > > the callstack.
> > > > > > > Why would it make sense for security_prepare_creds to return an error
> > > > > > > code other than ENOMEM?
> > > > > > >    > That seems a bit of a violation of what that function is supposed to do
> > > > > > >
> > > > > >
> > > > > > The API allows LSM authors to decide what error code is returned from the
> > > > > > cred_prepare hook. security_task_alloc() is a similar hook, and has its return
> > > > > > code propagated.
> > > > > It is not an api.  It is an implementation detail of the linux kernel.
> > > > > It is a set of convenient functions that do a job.
> > > > > The general rule is we don't support cases without an in-tree user.  I
> > > > > don't see an in-tree user.
> > > > >
> > > > > > I'm proposing we follow security_task_allocs() pattern, and add visibility for
> > > > > > failure cases in prepare_creds().
> > > > > I am asking why we would want to.  Especially as it is not an API, and I
> > > > > don't see any good reason for anything but an -ENOMEM failure to be
> > > > > supported.
> > > > >
> > > > We're writing a LSM BPF policy, and not a new LSM. Our policy aims to solve
> > > > unprivileged unshare, similar to Debian's patch [1]. We're in a position such
> > > > that we can't use that patch because we can't block _all_ of our applications
> > > > from performing an unshare. We prefer a granular approach. LSM BPF seems like a
> > > > good choice.
> > >
> > > I am quite puzzled why doesn't /proc/sys/user/max_user_namespaces work
> > > for you?
> > >
> >
> > We have the following requirements:
> >
> > 1. Allow list criteria
> > 2. root user must be able to create namespaces whenever
> > 3. Everything else not in 1 & 2 must be denied
> >
> > We use per task attributes to determine whether or not we allow/deny the
> > current call to unshare().
> >
> > /proc/sys/user/max_user_namespaces limits are a bit broad for this level of
> > detail.
> >
> > > > Because LSM BPF exposes these hooks, we should probably treat them as an
> > > > API. From that perspective, userspace expects unshare to return a EPERM
> > > > when the call is denied permissions.
> > >
> > > The BPF code gets to be treated as a out of tree kernel module.
> > >
> > > > > Without an in-tree user that cares it is probably better to go the
> > > > > opposite direction and remove the possibility of return anything but
> > > > > memory allocation failure.  That will make it clearer to implementors
> > > > > that a general error code is not supported and this is not a location
> > > > > to implement policy, this is only a hook to allocate state for the LSM.
> > > > >
> > > >
> > > > That's a good point, and it's possible we're using the wrong hook for the
> > > > policy. Do you know of other hooks we can look into?
>
> Fwiw, from this commit it wasn't very clear what you wanted to achieve
> with this. It might be worth considering adding a new security hook for
> this. Within msft it recently came up SELinux might have an interest in
> something like this as well.

Just to clarify things a bit, I believe SELinux would have an interest
in a LSM hook capable of implementing an access control point for user
namespaces regardless of Microsoft's current needs.  I suspect due to
the security relevant nature of user namespaces most other LSMs would
be interested as well; it seems like a well crafted hook would be
welcome by most folks I think.

-- 
paul-moore.com