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    Subject[RFC 2/3] seccomp: hoist out filter resolving logic
    Date
    Hoist out the nth filter resolving logic that ptrace uses into a new
    function. We'll use this in the next patch to implement the new
    PTRACE_SECCOMP_GET_FILTER_FLAGS command. This is based on an older patch
    that I had sent a while ago; it significantly revamps the get_nth_filter
    logic based on previous suggestions from Oleg.

    Signed-off-by: Tycho Andersen <tycho@tycho.ws>
    CC: Kees Cook <keescook@chromium.org>
    CC: Andy Lutomirski <luto@amacapital.net>
    CC: Oleg Nesterov <oleg@redhat.com>
    CC: Eric W. Biederman <ebiederm@xmission.com>
    CC: "Serge E. Hallyn" <serge@hallyn.com>
    CC: Christian Brauner <christian.brauner@ubuntu.com>
    CC: Tyler Hicks <tyhicks@canonical.com>
    CC: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
    ---
    kernel/seccomp.c | 77 +++++++++++++++++++++++++++++++++-----------------------
    1 file changed, 45 insertions(+), 32 deletions(-)

    diff --git a/kernel/seccomp.c b/kernel/seccomp.c
    index 9541eb379e74..800db3f2866f 100644
    --- a/kernel/seccomp.c
    +++ b/kernel/seccomp.c
    @@ -1179,49 +1179,68 @@ long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
    }

    #if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
    -long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
    - void __user *data)
    +static struct seccomp_filter *get_nth_filter(struct task_struct *task,
    + unsigned long filter_off)
    {
    - struct seccomp_filter *filter;
    - struct sock_fprog_kern *fprog;
    - long ret;
    - unsigned long count = 0;
    -
    - if (!capable(CAP_SYS_ADMIN) ||
    - current->seccomp.mode != SECCOMP_MODE_DISABLED) {
    - return -EACCES;
    - }
    + struct seccomp_filter *orig, *filter;
    + unsigned long count;

    + /*
    + * Note: this is only correct because the caller should be the (ptrace)
    + * tracer of the task, otherwise lock_task_sighand is needed.
    + */
    spin_lock_irq(&task->sighand->siglock);
    +
    if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
    - ret = -EINVAL;
    - goto out;
    + spin_unlock_irq(&task->sighand->siglock);
    + return ERR_PTR(-EINVAL);
    }

    - filter = task->seccomp.filter;
    - while (filter) {
    - filter = filter->prev;
    + orig = task->seccomp.filter;
    + __get_seccomp_filter(orig);
    + spin_unlock_irq(&task->sighand->siglock);
    +
    + count = 0;
    + for (filter = orig; filter; filter = filter->prev)
    count++;
    - }

    if (filter_off >= count) {
    - ret = -ENOENT;
    + filter = ERR_PTR(-ENOENT);
    goto out;
    }
    - count -= filter_off;

    - filter = task->seccomp.filter;
    - while (filter && count > 1) {
    - filter = filter->prev;
    + count -= filter_off;
    + for (filter = orig; filter && count > 1; filter = filter->prev)
    count--;
    - }

    if (WARN_ON(count != 1 || !filter)) {
    - /* The filter tree shouldn't shrink while we're using it. */
    - ret = -ENOENT;
    + filter = ERR_PTR(-ENOENT);
    goto out;
    }

    + __get_seccomp_filter(filter);
    +
    +out:
    + __put_seccomp_filter(orig);
    + return filter;
    +}
    +
    +long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
    + void __user *data)
    +{
    + struct seccomp_filter *filter;
    + struct sock_fprog_kern *fprog;
    + long ret;
    +
    + if (!capable(CAP_SYS_ADMIN) ||
    + current->seccomp.mode != SECCOMP_MODE_DISABLED) {
    + return -EACCES;
    + }
    +
    + filter = get_nth_filter(task, filter_off);
    + if (IS_ERR(filter))
    + return PTR_ERR(filter);
    +
    fprog = filter->prog->orig_prog;
    if (!fprog) {
    /* This must be a new non-cBPF filter, since we save
    @@ -1236,17 +1255,11 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
    if (!data)
    goto out;

    - __get_seccomp_filter(filter);
    - spin_unlock_irq(&task->sighand->siglock);
    -
    if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog)))
    ret = -EFAULT;

    - __put_seccomp_filter(filter);
    - return ret;
    -
    out:
    - spin_unlock_irq(&task->sighand->siglock);
    + __put_seccomp_filter(filter);
    return ret;
    }
    #endif
    --
    2.14.1
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