Re: [PATCH 4/4] security: binder: Add transfer_charge SElinux hook

[Casey Schaufler <casey@schaufler-ca.com>]

On 1/9/2023 4:30 PM, T.J. Mercier wrote:
> On Mon, Jan 9, 2023 at 2:28 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
>> On 1/9/2023 1:38 PM, T.J. Mercier wrote:
>>> Any process can cause a memory charge transfer to occur to any other
>>> process when transmitting a file descriptor through binder. This should
>>> only be possible for central allocator processes,
>> How is a "central allocator process" identified?
> Any process with the transfer_charge permission. On Android this is
> the graphics allocator HAL which would have this added to its policy.

OK. You're putting SELinux policy directly into the name of the LSM hook.

>
>> If I have a LSM that
>> is not SELinux (e.g. AppArmor, Smack) or no LSM at all, how can/should this
>> be enforced?
> Sorry, why would you be expecting enforcement with no LSM?

Because the LSM is supposed to be a set of *additional* restrictions.
If there are no restrictions when there's no LSM, you can't add to
existing restrictions. If binder works correctly without any restrictions
that's fine. It seems odd that you'd add SELinux restrictions if there
are no basic restrictions. If, on the other hand, binder doesn't have
native restrictions because it always assumes SELinux, it ought to have
a CONFIG dependency on SELinux.

None of which is really important.

>  Are you
> suggesting that this check should be different than the ones that
> already exist for Binder here?
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/lsm_hook_defs.h#n29

This one seems just a little bit more like an implementation of
SELinux policy in the hook than some of the others. If there is no way
to identify the special process without SELinux policy it's going to
be really difficult for a different LSM to utilize the hook.

>
>> Why isn't binder enforcing this restriction itself?
> Binder has no direct knowledge of which process has been designated as
> an allocator / charge transferrer. That is defined externally by
> whoever configures the system.

So the attribute isn't a binder attribute, it's an SELinux attribute?
That isn't appropriate in the LSM interface, at least not explicitly.

>
>>>  so a new SELinux
>>> permission is added to restrict which processes are allowed to initiate
>>> these charge transfers.
>> Which is all perfectly reasonable if you have SELinux.
>>
>>> Signed-off-by: T.J. Mercier <tjmercier@google.com>
>>> ---
>>>  drivers/android/binder.c            | 5 +++++
>>>  include/linux/lsm_hook_defs.h       | 2 ++
>>>  include/linux/lsm_hooks.h           | 6 ++++++
>>>  include/linux/security.h            | 2 ++
>>>  security/security.c                 | 6 ++++++
>>>  security/selinux/hooks.c            | 9 +++++++++
>>>  security/selinux/include/classmap.h | 2 +-
>>>  7 files changed, 31 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
>>> index 9830848c8d25..9063db04826d 100644
>>> --- a/drivers/android/binder.c
>>> +++ b/drivers/android/binder.c
>>> @@ -2279,6 +2279,11 @@ static int binder_translate_fd(u32 fd, binder_size_t fd_offset, __u32 flags,
>>>       if (IS_ENABLED(CONFIG_MEMCG) && (flags & BINDER_FD_FLAG_XFER_CHARGE)) {
>>>               struct dma_buf *dmabuf;
>>>
>>> +             if (security_binder_transfer_charge(proc->cred, target_proc->cred)) {
>>> +                     ret = -EPERM;
>>> +                     goto err_security;
>>> +             }
>>> +
>>>               if (unlikely(!is_dma_buf_file(file))) {
>>>                       binder_user_error(
>>>                               "%d:%d got transaction with XFER_CHARGE for non-dmabuf fd, %d\n",
>>> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
>>> index ed6cb2ac55fa..8db2a958557e 100644
>>> --- a/include/linux/lsm_hook_defs.h
>>> +++ b/include/linux/lsm_hook_defs.h
>>> @@ -33,6 +33,8 @@ LSM_HOOK(int, 0, binder_transfer_binder, const struct cred *from,
>>>        const struct cred *to)
>>>  LSM_HOOK(int, 0, binder_transfer_file, const struct cred *from,
>>>        const struct cred *to, struct file *file)
>>> +LSM_HOOK(int, 0, binder_transfer_charge, const struct cred *from,
>>> +      const struct cred *to)
>>>  LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child,
>>>        unsigned int mode)
>>>  LSM_HOOK(int, 0, ptrace_traceme, struct task_struct *parent)
>>> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
>>> index 0a5ba81f7367..39c40c7bf519 100644
>>> --- a/include/linux/lsm_hooks.h
>>> +++ b/include/linux/lsm_hooks.h
>>> @@ -1385,6 +1385,12 @@
>>>   *   @file contains the struct file being transferred.
>>>   *   @to contains the struct cred for the receiving process.
>>>   *   Return 0 if permission is granted.
>>> + * @binder_transfer_charge:
>>> + *   Check whether @from is allowed to transfer the memory charge for a
>>> + *   buffer out of its cgroup to @to.
>>> + *   @from contains the struct cred for the sending process.
>>> + *   @to contains the struct cred for the receiving process.
>>> + *   Return 0 if permission is granted.
>>>   *
>>>   * @ptrace_access_check:
>>>   *   Check permission before allowing the current process to trace the
>>> diff --git a/include/linux/security.h b/include/linux/security.h
>>> index 5b67f208f7de..3b7472308430 100644
>>> --- a/include/linux/security.h
>>> +++ b/include/linux/security.h
>>> @@ -270,6 +270,8 @@ int security_binder_transfer_binder(const struct cred *from,
>>>                                   const struct cred *to);
>>>  int security_binder_transfer_file(const struct cred *from,
>>>                                 const struct cred *to, struct file *file);
>>> +int security_binder_transfer_charge(const struct cred *from,
>>> +                                 const struct cred *to);
>>>  int security_ptrace_access_check(struct task_struct *child, unsigned int mode);
>>>  int security_ptrace_traceme(struct task_struct *parent);
>>>  int security_capget(struct task_struct *target,
>>> diff --git a/security/security.c b/security/security.c
>>> index d1571900a8c7..97e1e74d1ff2 100644
>>> --- a/security/security.c
>>> +++ b/security/security.c
>>> @@ -801,6 +801,12 @@ int security_binder_transfer_file(const struct cred *from,
>>>       return call_int_hook(binder_transfer_file, 0, from, to, file);
>>>  }
>>>
>>> +int security_binder_transfer_charge(const struct cred *from,
>>> +                                 const struct cred *to)
>>> +{
>>> +     return call_int_hook(binder_transfer_charge, 0, from, to);
>>> +}
>>> +
>>>  int security_ptrace_access_check(struct task_struct *child, unsigned int mode)
>>>  {
>>>       return call_int_hook(ptrace_access_check, 0, child, mode);
>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>>> index 3c5be76a9199..823ef14924bd 100644
>>> --- a/security/selinux/hooks.c
>>> +++ b/security/selinux/hooks.c
>>> @@ -2066,6 +2066,14 @@ static int selinux_binder_transfer_file(const struct cred *from,
>>>                           &ad);
>>>  }
>>>
>>> +static int selinux_binder_transfer_charge(const struct cred *from, const struct cred *to)
>>> +{
>>> +     return avc_has_perm(&selinux_state,
>>> +                         cred_sid(from), cred_sid(to),
>>> +                         SECCLASS_BINDER, BINDER__TRANSFER_CHARGE,
>>> +                         NULL);
>>> +}
>>> +
>>>  static int selinux_ptrace_access_check(struct task_struct *child,
>>>                                      unsigned int mode)
>>>  {
>>> @@ -7052,6 +7060,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
>>>       LSM_HOOK_INIT(binder_transaction, selinux_binder_transaction),
>>>       LSM_HOOK_INIT(binder_transfer_binder, selinux_binder_transfer_binder),
>>>       LSM_HOOK_INIT(binder_transfer_file, selinux_binder_transfer_file),
>>> +     LSM_HOOK_INIT(binder_transfer_charge, selinux_binder_transfer_charge),
>>>
>>>       LSM_HOOK_INIT(ptrace_access_check, selinux_ptrace_access_check),
>>>       LSM_HOOK_INIT(ptrace_traceme, selinux_ptrace_traceme),
>>> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
>>> index a3c380775d41..2eef180d10d7 100644
>>> --- a/security/selinux/include/classmap.h
>>> +++ b/security/selinux/include/classmap.h
>>> @@ -172,7 +172,7 @@ const struct security_class_mapping secclass_map[] = {
>>>       { "tun_socket",
>>>         { COMMON_SOCK_PERMS, "attach_queue", NULL } },
>>>       { "binder", { "impersonate", "call", "set_context_mgr", "transfer",
>>> -                   NULL } },
>>> +                   "transfer_charge", NULL } },
>>>       { "cap_userns",
>>>         { COMMON_CAP_PERMS, NULL } },
>>>       { "cap2_userns",