linux-kernel - Re: [PATCH v2] seccomp: fix the usage of get/put_seccomp_filter() in seccomp_get

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Date:   Wed, 27 Sep 2017 17:42:35 +0200
From:   Kees Cook <keescook@...omium.org>
To:     Tycho Andersen <tycho@...ker.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        "security@...nel.org" <security@...nel.org>,
        Chris Salls <chrissalls5@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] seccomp: fix the usage of get/put_seccomp_filter() in seccomp_get_filter()

On Wed, Sep 27, 2017 at 5:25 PM, Tycho Andersen <tycho@...ker.com> wrote:
> From: Oleg Nesterov <oleg@...hat.com>
>
> As Chris explains, get_seccomp_filter() and put_seccomp_filter() can
> use the different filters, once we drop ->siglock task->seccomp.filter
> can be replaced by SECCOMP_FILTER_FLAG_TSYNC.
>
> v2: add __get_seccomp_filter vs. open coding refcount_inc()
>
> Fixes: f8e529ed941b ("seccomp, ptrace: add support for dumping seccomp filters")
> Reported-by: Chris Salls <chrissalls5@...il.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Tycho Andersen <tycho@...ker.com>

Thanks, applied!

-Kees

> ---
>  kernel/seccomp.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index c24579dfa7a1..bb3a38005b9c 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -473,14 +473,19 @@ static long seccomp_attach_filter(unsigned int flags,
>         return 0;
>  }
>
> +void __get_seccomp_filter(struct seccomp_filter *filter)
> +{
> +       /* Reference count is bounded by the number of total processes. */
> +       refcount_inc(&filter->usage);
> +}
> +
>  /* get_seccomp_filter - increments the reference count of the filter on @tsk */
>  void get_seccomp_filter(struct task_struct *tsk)
>  {
>         struct seccomp_filter *orig = tsk->seccomp.filter;
>         if (!orig)
>                 return;
> -       /* Reference count is bounded by the number of total processes. */
> -       refcount_inc(&orig->usage);
> +       __get_seccomp_filter(orig);
>  }
>
>  static inline void seccomp_filter_free(struct seccomp_filter *filter)
> @@ -491,10 +496,8 @@ static inline void seccomp_filter_free(struct seccomp_filter *filter)
>         }
>  }
>
> -/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
> -void put_seccomp_filter(struct task_struct *tsk)
> +static void __put_seccomp_filter(struct seccomp_filter *orig)
>  {
> -       struct seccomp_filter *orig = tsk->seccomp.filter;
>         /* Clean up single-reference branches iteratively. */
>         while (orig && refcount_dec_and_test(&orig->usage)) {
>                 struct seccomp_filter *freeme = orig;
> @@ -503,6 +506,12 @@ void put_seccomp_filter(struct task_struct *tsk)
>         }
>  }
>
> +/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
> +void put_seccomp_filter(struct task_struct *tsk)
> +{
> +       __put_seccomp_filter(tsk->seccomp.filter);
> +}
> +
>  static void seccomp_init_siginfo(siginfo_t *info, int syscall, int reason)
>  {
>         memset(info, 0, sizeof(*info));
> @@ -1025,13 +1034,13 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
>         if (!data)
>                 goto out;
>
> -       get_seccomp_filter(task);
> +       __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(task);
> +       __put_seccomp_filter(filter);
>         return ret;
>
>  out:
> --
> 2.11.0
>



-- 
Kees Cook
Pixel Security