[PATCH v2] binder: use cred instead of task for selinux checks

[Stephen Smalley]

On Mon, Oct 4, 2021 at 8:27 PM Jann Horn <jannh at google.com> wrote:
>
> On Tue, Oct 5, 2021 at 1:38 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
> > On 10/4/2021 3:28 PM, Jann Horn wrote:
> > > On Mon, Oct 4, 2021 at 6:19 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> > >> On 10/1/2021 3:58 PM, Jann Horn wrote:
> > >>> On Fri, Oct 1, 2021 at 10:10 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> > >>>> On 10/1/2021 12:50 PM, Jann Horn wrote:
> > >>>>> On Fri, Oct 1, 2021 at 9:36 PM Jann Horn <jannh at google.com> wrote:
> > >>>>>> On Fri, Oct 1, 2021 at 8:46 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> > >>>>>>> On 10/1/2021 10:55 AM, Todd Kjos wrote:
> > >>>>>>>> Save the struct cred associated with a binder process
> > >>>>>>>> at initial open to avoid potential race conditions
> > >>>>>>>> when converting to a security ID.
> > >>>>>>>>
> > >>>>>>>> Since binder was integrated with selinux, it has passed
> > >>>>>>>> 'struct task_struct' associated with the binder_proc
> > >>>>>>>> to represent the source and target of transactions.
> > >>>>>>>> The conversion of task to SID was then done in the hook
> > >>>>>>>> implementations. It turns out that there are race conditions
> > >>>>>>>> which can result in an incorrect security context being used.
> > >>>>>>> In the LSM stacking patch set I've been posting for a while
> > >>>>>>> (on version 29 now) I use information from the task structure
> > >>>>>>> to ensure that the security information passed via the binder
> > >>>>>>> interface is agreeable to both sides. Passing the cred will
> > >>>>>>> make it impossible to do this check. The task information
> > >>>>>>> required is not appropriate to have in the cred.
> > >>>>>> Why not? Why can't you put the security identity of the task into the creds?
> > >>>>> Ah, I get it now, you're concerned about different processes wanting
> > >>>>> to see security contexts formatted differently (e.g. printing the
> > >>>>> SELinux label vs printing the AppArmor label), right?
> > >>>> That is correct.
> > >>>>
> > >>>>> But still, I don't think you can pull that information from the
> > >>>>> receiving task. Maybe the easiest solution would be to also store that
> > >>>>> in the creds? Or you'd have to manually grab that information when
> > >>>>> /dev/binder is opened.
> > >>>> I'm storing the information in the task security blob because that's
> > >>>> the appropriate scope. Today the LSM hook is given both task_struct's.
> > >>> Which is wrong, because you have no idea who the semantic "recipient
> > >>> task" is - any task that has a mapping of the binder fd can
> > >>> effectively receive transactions from it.
> > >>>
> > >>> (And the current "sender task" is also wrong, because binder looks at
> > >>> the task that opened the binder device, not the task currently
> > >>> performing the action.)
> > >> I'm confused. Are you saying that the existing binder code is
> > >> completely broken? Are you saying that neither "task" is correct?
> > > Yeah, basically
> >
> > Well, hot biscuits and gravy!
> >
> > >  - but luckily the actual impact this has is limited by
> > > the transitions that SELinux permits. If domain1 has no way to
> > > transition to domain2, then it can't abuse this bug to pretend to be
> > > domain2. I do have a reproducer that lets Android's "shell" domain
> > > send a binder transaction that appears to come from "runas", but
> > > luckily "runas" has no interesting privileges with regards to binder,
> > > so that's not exploitable.
> >
> > You're counting on the peculiarities of the SELinux policy you're
> > assuming is used to mask the fact that the hook isn't really doing
> > what it is supposed to?  Ouch.
>
> I'm not saying I like the current situation - I do think that this
> needs to change. I'm just saying it probably isn't *exploitable*, and
> exploitability often hinges on these little circumstantial details.
>
> > >> How does passing the creds from the wrong tasks "fix" the problem?
> > > This patch is not passing the creds from the "wrong" tasks at all. It
> > > relies on the basic idea that when a security context opens a
> > > resource, and then hands that resource to another context for
> > > read/write operations, then you can effectively treat this as a
> > > delegation of privileges from the original opener, and perform access
> > > checks against the credentials using which the resource was opened.
> >
> > OK. I can understand that without endorsing it.
> >
> > > In particular, we already have those semantics in the core kernel for
> > > ->read() and ->write() VFS operations - they are *not allowed* to look
> > > at the credentials of the caller, and if they want to make security
> > > checks, they have to instead check against file->f_cred, which are the
> > > credentials using which the file was originally opened. (Yes, some
> > > places still get that wrong.) Passing a file descriptor to another
> > > task is a delegation of access, and the other task can then call
> > > syscalls like read() / write() / mmap() on the file descriptor without
> > > needing to have any access to the underlying file.
> >
> > A mechanism sufficiently entrenched.
>
> It's not just "entrenched", it is a fundamental requirement for being
> able to use file descriptor passing with syscalls like write(). If
> task A gives a file descriptor to task B, then task B must be able to
> write() to that FD without having to worry that the FD actually refers
> to some sort of special file that interprets the written data as some
> type of command, or something like that, and that this leads to task B
> unknowingly passing through access checks.
>
> > > You can't really attribute binder transactions to specific tasks that
> > > are actually involved in the specific transaction, neither on the
> > > sending side nor on the receiving side, because binder is built around
> > > passing data through memory mappings. Memory mappings can be accessed
> > > by multiple tasks, and even a task that does not currently have it
> > > mapped could e.g. map it at a later time. And on top of that you have
> > > the problem that the receiving task might also go through privileged
> > > execve() transitions.
> >
> > OK. I'm curious now as to why the task_struct was being passed to the
> > hook in the first place.
>
> Probably because that's what most other LSM hooks looked like and the
> authors/reviewers of the patch didn't realize that this model doesn't
> really work for binder? FWIW, these hooks were added in commit
> 79af73079d75 ("Add security hooks to binder and implement the hooks
> for SELinux."). The commit message also just talks about "processes".

Note that in the same code path (binder_transaction), sender_euid is
set from proc->tsk and security_ctx is based on proc->tsk. If we are
changing the hooks to operate on the opener cred, then presumably we
should be doing that for sender_euid and replace the
security_task_getsecid_obj() call with security_cred_getsecid()?

NB Mandatory Access Control doesn't allow uncontrolled delegation,
hence typically checks against the subject credential either at
delegation/transfer or use or both. That's true in other places too,
e.g. file_permission, socket_sendmsg.