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

["T.J. Mercier" <tjmercier@google.com>]

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.

> 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? 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

> 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 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",