linux-kernel - Re: [PATCH -mm 4/5] fault-inject: simplify access check for fail-nth

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Message-ID: <CACT4Y+a_m50vQwuRtLvYTCWwdsT3Mt1JSc141r8UfJ3YCrcgSg@mail.gmail.com>
Date:   Fri, 7 Apr 2017 22:45:14 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Akinobu Mita <akinobu.mita@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 4/5] fault-inject: simplify access check for fail-nth

On Thu, Apr 6, 2017 at 4:56 PM, Akinobu Mita <akinobu.mita@...il.com> wrote:
> The fail-nth file is created with 0666 and the access is permitted if
> and only if the task is current.
>
> This file is owned by the currnet user.  So we can create it with 0644
> and allow the owner to write it.  This enables to watch the status of
> task->fail_nth from another processes.
>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
> ---
>  fs/proc/base.c | 22 ++++++++--------------
>  1 file changed, 8 insertions(+), 14 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 9d14215..14e7b73 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1366,16 +1366,16 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
>         int err;
>         unsigned int n;
>
> +       err = kstrtoint_from_user(buf, count, 0, &n);
> +       if (err)
> +               return err;
> +
>         task = get_proc_task(file_inode(file));
>         if (!task)
>                 return -ESRCH;
> +       task->fail_nth = n;
>         put_task_struct(task);
> -       if (task != current)
> -               return -EPERM;
> -       err = kstrtouint_from_user(buf, count, 0, &n);
> -       if (err)
> -               return err;
> -       current->fail_nth = n;
> +
>         return count;
>  }
>
> @@ -1389,11 +1389,9 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
>         task = get_proc_task(file_inode(file));
>         if (!task)
>                 return -ESRCH;
> -       put_task_struct(task);
> -       if (task != current)
> -               return -EPERM;
>         len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth);
>         len = simple_read_from_buffer(buf, count, ppos, numbuf, len);
> +       put_task_struct(task);
>
>         return len;
>  }
> @@ -3358,11 +3356,7 @@ static const struct pid_entry tid_base_stuff[] = {
>  #endif
>  #ifdef CONFIG_FAULT_INJECTION
>         REG("make-it-fail", S_IRUGO|S_IWUSR, proc_fault_inject_operations),
> -       /*
> -        * Operations on the file check that the task is current,
> -        * so we create it with 0666 to support testing under unprivileged user.
> -        */
> -       REG("fail-nth", 0666, proc_fail_nth_operations),
> +       REG("fail-nth", 0644, proc_fail_nth_operations),

/\/\/\/\/\/\

This breaks us.
Under setuid sandbox test threads can't open the file anymore. And we
can't pre-open the files before dropping privs as new threads can be
created afterwards.

I think the root cause of all these problems (permissions, parsing,
serialization, broken cat, symmetry) is that we are trying to fit a
programmatic API into reads and writes on textual files. We don't need
symmetry, we don't need read+write to reset injection, we don't need
parsing and serialization, it does not make sense to do this of
non-current task, it definitely does not make sense to cat this, etc.

What do you think of 2 ioctls on /sys/kernel/debug/fail_nth?

>  #endif
>  #ifdef CONFIG_TASK_IO_ACCOUNTING
>         ONE("io",       S_IRUSR, proc_tid_io_accounting),
> --
> 2.7.4
>