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

[Joe Lawrence <joe.lawrence@redhat.com>]

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