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]>

2 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 6a80cfc63e0c..2dc5e9870fcd 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -191,5 +191,6 @@ long pipe_fcntl(struct file *, unsigned int, unsigned long arg);
 struct pipe_inode_info *get_pipe_info(struct file *file);
 
 int create_pipe_files(struct file **, int);
+unsigned int round_pipe_size(unsigned int size);
 
 #endif
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index b769ecfcc3bd..992bc9948232 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -51,6 +51,9 @@ extern int proc_dointvec_minmax(struct ctl_table *, int,
 extern int proc_douintvec_minmax(struct ctl_table *table, int write,
 				 void __user *buffer, size_t *lenp,
 				 loff_t *ppos);
+extern int proc_dopipe_max_size(struct ctl_table *table, int write,
+				void __user *buffer, size_t *lenp,
+				loff_t *ppos);
 extern int proc_dointvec_jiffies(struct ctl_table *, int,
 				 void __user *, size_t *, loff_t *);
 extern int proc_dointvec_userhz_jiffies(struct ctl_table *, int,