pipe: add proc_dopipe_max_size() to safely assign pipe_max_size

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 racy:

  pipe.c :: alloc_pipe_info()
  pipe.c :: pipe_set_size()

Add a new proc_dopipe_max_size() that consolidates reading the new value
from the user buffer, verifying bounds, and calling round_pipe_size()
with a single assignment to pipe_max_size.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Joe Lawrence <[email protected]>
Reported-by: Mikulas Patocka <[email protected]>
Reviewed-by: Mikulas Patocka <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/pipe.c

@@ -1020,7 +1020,7 @@ const struct file_operations pipefifo_fops = {
  * Currently we rely on the pipe array holding a power-of-2 number
  * of pages. Returns 0 on error.
  */
-static inline unsigned int round_pipe_size(unsigned int size)
+unsigned int round_pipe_size(unsigned int size)
 {
 	unsigned long nr_pages;
 
@@ -1125,25 +1125,13 @@ out_revert_acct:
 }
 
 /*
- * This should work even if CONFIG_PROC_FS isn't set, as proc_dointvec_minmax
+ * This should work even if CONFIG_PROC_FS isn't set, as proc_dopipe_max_size
  * will return an error.
  */
 int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
 		 size_t *lenp, loff_t *ppos)
 {
-	unsigned int rounded_pipe_max_size;
-	int ret;
-
-	ret = proc_douintvec_minmax(table, write, buf, lenp, ppos);
-	if (ret < 0 || !write)
-		return ret;
-
-	rounded_pipe_max_size = round_pipe_size(pipe_max_size);
-	if (rounded_pipe_max_size == 0)
-		return -EINVAL;
-
-	pipe_max_size = rounded_pipe_max_size;
-	return ret;
+	return proc_dopipe_max_size(table, write, buf, lenp, ppos);
 }
 
 /*