linux-kernel - Re: [PATCH RFC 2/3] pipe: protect pipe_max

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Date:   Thu, 14 Sep 2017 19:09:10 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>
cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a
 mutex

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

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@...hat.com>
> Signed-off-by: Joe Lawrence <joe.lawrence@...hat.com>
> ---
>  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
>