Re: [PATCH 4/8] pipe: fix limit checking in pipe_set_size()

[Vegard Nossum]

On 08/19/2016 07:25 AM, Michael Kerrisk (man-pages) wrote:

The limit checking in pipe_set_size() (used by fcntl(F_SETPIPE_SZ))
has the following problems:

[...]

@@ -1030,6 +1030,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, 
unsigned long arg)
  {
        struct pipe_buffer *bufs;
        unsigned int size, nr_pages;
+       long ret = 0;

        size = round_pipe_size(arg);
        nr_pages = size >> PAGE_SHIFT;
@@ -1037,13 +1038,26 @@ static long pipe_set_size(struct pipe_inode_info *pipe, 
unsigned long arg)
        if (!nr_pages)
                return -EINVAL;

-       if (!capable(CAP_SYS_RESOURCE) && size > pipe_max_size)
-               return -EPERM;
+       account_pipe_buffers(pipe->user, pipe->buffers, nr_pages);

-       if ((too_many_pipe_buffers_hard(pipe->user) ||
-                       too_many_pipe_buffers_soft(pipe->user)) &&
-                       !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
-               return -EPERM;
+       /*
+        * If trying to increase the pipe capacity, check that an
+        * unprivileged user is not trying to exceed various limits.
+        * (Decreasing the pipe capacity is always permitted, even
+        * if the user is currently over a limit.)
+        */
+       if (nr_pages > pipe->buffers) {
+               if (!capable(CAP_SYS_RESOURCE) && size > pipe_max_size) {
+                       ret = -EPERM;
+                       goto out_revert_acct;
+               } else if ((too_many_pipe_buffers_hard(pipe->user) ||
+                               too_many_pipe_buffers_soft(pipe->user)) &&
+                               !capable(CAP_SYS_RESOURCE) &&
+                               !capable(CAP_SYS_ADMIN)) {
+                       ret = -EPERM;
+                       goto out_revert_acct;
+               }
+       }

I'm slightly worried about not checking arg/nr_pages before we pass it
on to account_pipe_buffers().

The potential problem happens if the user passes a very large number
which will overflow pipe->user->pipe_bufs.

On 32-bit, sizeof(int) == sizeof(long), so if they pass arg = INT_MAX
then round_pipe_size() returns INT_MAX. Although it's true that the
accounting is done in terms of pages and not bytes, so you'd need on the
order of (1 << 13) = 8192 processes hitting the limit at the same time
in order to make it overflow, which seems a bit unlikely.

(See https://lkml.org/lkml/2016/8/12/215 for another discussion on the
limit checking)

Is there any reason why we couldn't do the (size > pipe_max_size) check
before calling account_pipe_buffers()?


Vegard