Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex

[Joe Lawrence]

On 09/14/2017 07:09 PM, Mikulas Patocka wrote:
> On Tue, 5 Sep 2017, Joe Lawrence wrote:
>> pipe_max_size is assigned directly via procfs sysctl:
>>
>>   static struct ctl_table fs_table[] = {
>>           ...
>>           {
>>                   .procname       = "pipe-max-size",
>>                   .data           = &pipe_max_size,
>>                   .maxlen         = sizeof(int),
>>                   .mode           = 0644,
>>                   .proc_handler   = &pipe_proc_fn,
>>                   .extra1         = &pipe_min_size,
>>           },
>>           ...
>>
>>   int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>>                    size_t *lenp, loff_t *ppos)
>>   {
>>           ...
>>           ret = proc_dointvec_minmax(table, write, buf, lenp, ppos)
>>           ...
>>
>> and then later rounded in-place a few statements later:
>>
>>           ...
>>           pipe_max_size = round_pipe_size(pipe_max_size);
>>           ...
>>
>> This leaves a window of time between initial assignment and rounding
>> that may be visible to other threads.  (For example, one thread sets a
>> non-rounded value to pipe_max_size while another reads its value.)
>>
>> Similar reads of pipe_max_size are potentially racey:
>>
>>   pipe.c :: alloc_pipe_info()
>>   pipe.c :: pipe_set_size()
>>
>> Protect them and the procfs sysctl assignment with a mutex.
>>
>> Reported-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
>> Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
>> ---
>>  fs/pipe.c | 28 ++++++++++++++++++++++++----
>>  1 file changed, 24 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/pipe.c b/fs/pipe.c
>> index fa28910b3c59..33bb11b0d78e 100644
>> --- a/fs/pipe.c
>> +++ b/fs/pipe.c
>> @@ -35,6 +35,11 @@
>>  unsigned int pipe_max_size = 1048576;
>>  
>>  /*
>> + * Provide mutual exclusion around access to pipe_max_size
>> + */
>> +static DEFINE_MUTEX(pipe_max_mutex);
>> +
>> +/*
>>   * Minimum pipe size, as required by POSIX
>>   */
>>  unsigned int pipe_min_size = PAGE_SIZE;
>> @@ -623,13 +628,18 @@ struct pipe_inode_info *alloc_pipe_info(void)
>>  	unsigned long pipe_bufs = PIPE_DEF_BUFFERS;
>>  	struct user_struct *user = get_current_user();
>>  	unsigned long user_bufs;
>> +	unsigned int max_size;
>>  
>>  	pipe = kzalloc(sizeof(struct pipe_inode_info), GFP_KERNEL_ACCOUNT);
>>  	if (pipe == NULL)
>>  		goto out_free_uid;
>>  
>> -	if (pipe_bufs * PAGE_SIZE > pipe_max_size && !capable(CAP_SYS_RESOURCE))
>> -		pipe_bufs = pipe_max_size >> PAGE_SHIFT;
>> +	mutex_lock(&pipe_max_mutex);
>> +	max_size = pipe_max_size;
>> +	mutex_unlock(&pipe_max_mutex);
>> +
>> +	if (pipe_bufs * PAGE_SIZE > max_size && !capable(CAP_SYS_RESOURCE))
>> +		pipe_bufs = max_size >> PAGE_SHIFT;
>>  
>>  	user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
>>  
>> @@ -1039,6 +1049,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>>  	struct pipe_buffer *bufs;
>>  	unsigned int size, nr_pages;
>>  	unsigned long user_bufs;
>> +	unsigned int max_size;
>>  	long ret = 0;
>>  
>>  	size = round_pipe_size(arg);
>> @@ -1056,8 +1067,11 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>>  	 * Decreasing the pipe capacity is always permitted, even
>>  	 * if the user is currently over a limit.
>>  	 */
>> +	mutex_lock(&pipe_max_mutex);
>> +	max_size = pipe_max_size;
>> +	mutex_unlock(&pipe_max_mutex);
>>  	if (nr_pages > pipe->buffers &&
>> -			size > pipe_max_size && !capable(CAP_SYS_RESOURCE))
>> +			size > max_size && !capable(CAP_SYS_RESOURCE))
>>  		return -EPERM;
>>  
>>  	user_bufs = account_pipe_buffers(pipe->user, pipe->buffers, nr_pages);
>> @@ -1131,18 +1145,24 @@ int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>>  	unsigned int rounded_pipe_max_size;
>>  	int ret;
>>  
>> +	mutex_lock(&pipe_max_mutex);
>>  	orig_pipe_max_size = pipe_max_size;
>>  	ret = proc_dointvec_minmax(table, write, buf, lenp, ppos);
>> -	if (ret < 0 || !write)
>> +	if (ret < 0 || !write) {
>> +		mutex_unlock(&pipe_max_mutex);
>>  		return ret;
>> +	}
>>  
>>  	rounded_pipe_max_size = round_pipe_size(pipe_max_size);
>>  	if (rounded_pipe_max_size == 0) {
>>  		pipe_max_size = orig_pipe_max_size;
>> +		mutex_unlock(&pipe_max_mutex);
>>  		return -EINVAL;
>>  	}
>>  
>>  	pipe_max_size = rounded_pipe_max_size;
>> +	mutex_unlock(&pipe_max_mutex);
>> +
>>  	return ret;
>>  }
>>  
>> -- 
>> 1.8.3.1
>>
> I think this mutex is too heavy - if multiple processes simultaneously
> create a pipe, the mutex would cause performance degradation.
>
> You can call do_proc_dointvec with a custom callback "conv" function that
> does the rounding of the pipe size value.
>
> Mikulas
>

Hi Mikulas,

I'm not strong when it comes to memory barriers, but one of the
side-effects of using the mutex is that pipe_set_size() and
alloc_pipe_info() should have a consistent view of pipe_max_size.

If I remove the mutex (and assume that I implement a custom
do_proc_dointvec "conv" callback), is it safe for these routines to
directly use pipe_max_size as they had done before?

If not, is it safe to alias through a temporary stack variable (ie,
could the compiler re-read pipe_max_size multiple times in the function)?

Would READ_ONCE() help in any way?

The mutex covered up some confusion on my part here.

OTOH, since pipe_max_size is read-only for pipe_set_size() and
alloc_pipe_info() and only updated occasionally by pipe_proc_fn(), would
rw_semaphore or RCU be a fit here?

Regards,

-- Joe