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    Subject[PATCH 5.4 003/355] binder: use euid from cred instead of using task
    Date
    From: Todd Kjos <tkjos@google.com>

    commit 29bc22ac5e5bc63275e850f0c8fc549e3d0e306b upstream.

    Save the 'struct cred' associated with a binder process
    at initial open to avoid potential race conditions
    when converting to an euid.

    Set a transaction's sender_euid from the 'struct cred'
    saved at binder_open() instead of looking up the euid
    from the binder proc's 'struct task'. This ensures
    the euid is associated with the security context that
    of the task that opened binder.

    Cc: stable@vger.kernel.org # 4.4+
    Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
    Signed-off-by: Todd Kjos <tkjos@google.com>
    Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
    Suggested-by: Jann Horn <jannh@google.com>
    Acked-by: Casey Schaufler <casey@schaufler-ca.com>
    Signed-off-by: Paul Moore <paul@paul-moore.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/android/binder.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    --- a/drivers/android/binder.c
    +++ b/drivers/android/binder.c
    @@ -424,6 +424,9 @@ enum binder_deferred_state {
    * (invariant after initialized)
    * @tsk task_struct for group_leader of process
    * (invariant after initialized)
    + * @cred struct cred associated with the `struct file`
    + * in binder_open()
    + * (invariant after initialized)
    * @deferred_work_node: element for binder_deferred_list
    * (protected by binder_deferred_lock)
    * @deferred_work: bitmap of deferred work to perform
    @@ -469,6 +472,7 @@ struct binder_proc {
    struct list_head waiting_threads;
    int pid;
    struct task_struct *tsk;
    + const struct cred *cred;
    struct hlist_node deferred_work_node;
    int deferred_work;
    bool is_dead;
    @@ -3091,7 +3095,7 @@ static void binder_transaction(struct bi
    t->from = thread;
    else
    t->from = NULL;
    - t->sender_euid = task_euid(proc->tsk);
    + t->sender_euid = proc->cred->euid;
    t->to_proc = target_proc;
    t->to_thread = target_thread;
    t->code = tr->code;
    @@ -4707,6 +4711,7 @@ static void binder_free_proc(struct bind
    }
    binder_alloc_deferred_release(&proc->alloc);
    put_task_struct(proc->tsk);
    + put_cred(proc->cred);
    binder_stats_deleted(BINDER_STAT_PROC);
    kfree(proc);
    }
    @@ -5234,6 +5239,7 @@ static int binder_open(struct inode *nod
    spin_lock_init(&proc->outer_lock);
    get_task_struct(current->group_leader);
    proc->tsk = current->group_leader;
    + proc->cred = get_cred(filp->f_cred);
    INIT_LIST_HEAD(&proc->todo);
    proc->default_priority = task_nice(current);
    /* binderfs stashes devices in i_private */

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