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

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Date:   Sat, 8 Apr 2017 17:25:41 +0900
From:   Akinobu Mita <akinobu.mita@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.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

2017-04-08 5:45 GMT+09:00 Dmitry Vyukov <dvyukov@...gle.com>:
> 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.

Could you provide a working example for this?  Because I'm not sure
I understand the problem you described here.

If we omit resetting tsk->fail_nth in dup_task_struct(), tsk->fail_nth
is inherited from parent to child process.  So the parent process can
pre-open and set fail-nth file and reset parent's own ->fail_nth after
fork by writing 0 to fail-nth file.  Does that fix your problem?

> 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?

I think the misc device is suitable than debugfs file for ioctl only
knob.  But I prefer read/write interface than ioctl if possible.

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