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

[Casey Schaufler]

On 10/5/2021 8:21 AM, Stephen Smalley wrote:
> 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.

Terrific. Now I'm even less convinced that either the proposed change
or the existing code make sense. It's also disturbing that the change
log claims that the reason for the change is fix a race condition when
in fact it changes the data being sent to the hook completely. I, for one,
had assumed that the cred being passed was the cred from the task. There
is certainly nothing in the description to make me think otherwise.