Re: [PATCH v4 2/3] binder: use cred instead of task for getsecid

[Paul Moore <paul@paul-moore.com>]

On Wed, Oct 6, 2021 at 8:46 PM Todd Kjos <tkjos@google.com> wrote:
>
> Use the 'struct cred' saved at binder_open() to lookup
> the security ID via security_cred_getsecid(). This
> ensures that the security context that opened binder
> is the one used to generate the secctx.
>
> Fixes: ec74136ded79 ("binder: create node flag to request sender's
> security context")
> Signed-off-by: Todd Kjos <tkjos@google.com>
> Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Cc: stable@vger.kernel.org # 5.4+
> ---
> v3: added this patch to series
> v4: fix build-break for !CONFIG_SECURITY
>
>  drivers/android/binder.c | 11 +----------
>  include/linux/security.h |  4 ++++
>  2 files changed, 5 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index ca599ebdea4a..989afd0804ca 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -2722,16 +2722,7 @@ static void binder_transaction(struct binder_proc *proc,
>                 u32 secid;
>                 size_t added_size;
>
> -               /*
> -                * Arguably this should be the task's subjective LSM secid but
> -                * we can't reliably access the subjective creds of a task
> -                * other than our own so we must use the objective creds, which
> -                * are safe to access.  The downside is that if a task is
> -                * temporarily overriding it's creds it will not be reflected
> -                * here; however, it isn't clear that binder would handle that
> -                * case well anyway.
> -                */
> -               security_task_getsecid_obj(proc->tsk, &secid);
> +               security_cred_getsecid(proc->cred, &secid);
>                 ret = security_secid_to_secctx(secid, &secctx, &secctx_sz);
>                 if (ret) {
>                         return_error = BR_FAILED_REPLY;
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 6344d3362df7..f02cc0211b10 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -1041,6 +1041,10 @@ static inline void security_transfer_creds(struct cred *new,
>  {
>  }
>
> +static inline void security_cred_getsecid(const struct cred *c, u32 *secid)
> +{
> +}

Since security_cred_getsecid() doesn't return an error code we should
probably set the secid to 0 in this case, for example:

  static inline void security_cred_getsecid(...)
  {
    *secid = 0;
  }

-- 
paul moore
www.paul-moore.com