staging: comedi: avoid bad truncation of a size_t in comedi_write()

At one point in `comedi_write()`, the variable `n` gets assigned to the
minimum of the parameter `nbytes` and the amount of writeable buffer
space.  The way that is done currently is unsafe in the unlikely case
that `nbytes` exceeds `UINT_MAX`, so fix it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index 188a12a02ce731caf0de8e575f589a13e2b0817d..8c784c483c3e733bb82cbac124c971529c1db926 100644 (file)
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -2357,16 +2357,13 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
                        break;
                }
 
-               n = nbytes;
-
-               m = n;
+               /* Allocate all free buffer space. */
+               comedi_buf_write_alloc(s, async->prealloc_bufsz);
+               m = comedi_buf_write_n_allocated(s);
+               /* Avoid buffer wraparound. */
                if (async->buf_write_ptr + m > async->prealloc_bufsz)
                        m = async->prealloc_bufsz - async->buf_write_ptr;
-               comedi_buf_write_alloc(s, async->prealloc_bufsz);
-               if (m > comedi_buf_write_n_allocated(s))
-                       m = comedi_buf_write_n_allocated(s);
-               if (m < n)
-                       n = m;
+               n = min_t(size_t, m, nbytes);
 
                if (n == 0) {
                        if (file->f_flags & O_NONBLOCK) {