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SubjectRe: [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@redhat.com>
> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.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
>

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