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

On 08/20/2016 08:56 AM, Michael Kerrisk (man-pages) wrote:

Hi Vegard,

On 08/19/2016 08:30 PM, Vegard Nossum wrote:

quoted

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

quoted

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

[...]

quoted

@@ -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()?

No reason that I can see. Just a little more work to be done in the
code, I think.

And, just so I make sure we're understanding each other... I assume you
mean changing the code here to something like:

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;
        long ret = 0;

        size = round_pipe_size(arg);
        nr_pages = size >> PAGE_SHIFT;

        if (!nr_pages)
                return -EINVAL;

        /*
         * If trying to increase the pipe capacity, check that an
         * unprivileged user is not trying to exceed various limits
         * (soft limit check here, hard limit check just below).
         * Decreasing the pipe capacity is always permitted, even
         * if the user is currently over a limit.
         */
        if (nr_pages > pipe->buffers &&
                        size > pipe_max_size && !capable(CAP_SYS_RESOURCE))
                return -EPERM;

        user_bufs = account_pipe_buffers(pipe->user, pipe->buffers, nr_pages);

        if (nr_pages > pipe->buffers &&
                        too_many_pipe_buffers_hard(user_bufs ||
                        too_many_pipe_buffers_soft(user_bufs)) &&
                        !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
                ret = -EPERM;
                goto out_revert_acct;
        }

Right?

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/