spi: spidev: fix possible arithmetic overflow for multi-transfer message

commit f20fbaad7620af2df36a1f9d1c9ecf48ead5b747 upstream.

`spidev_message()` sums the lengths of the individual SPI transfers to
determine the overall SPI message length.  It restricts the total
length, returning an error if too long, but it does not check for
arithmetic overflow.  For example, if the SPI message consisted of two
transfers and the first has a length of 10 and the second has a length
of (__u32)(-1), the total length would be seen as 9, even though the
second transfer is actually very long.  If the second transfer specifies
a null `rx_buf` and a non-null `tx_buf`, the `copy_from_user()` could
overrun the spidev's pre-allocated tx buffer before it reaches an
invalid user memory address.  Fix it by checking that neither the total
nor the individual transfer lengths exceed the maximum allowed value.

Thanks to Dan Carpenter for reporting the potential integer overflow.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>
[Ian Abbott: Note: original commit compares the lengths to INT_MAX
 instead of bufsiz due to changes in earlier commits.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
(cherry picked from commit 7499401e4a0b01ee43cff768de4ca630dcd0bc64)
Signed-off-by: Willy Tarreau <w@1wt.eu>

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
index 5d23983..4dd8e2a 100644
--- a/drivers/spi/spidev.c
+++ b/drivers/spi/spidev.c
@@ -241,7 +241,10 @@
 		k_tmp->len = u_tmp->len;
 		total += k_tmp->len;
-		if (total > bufsiz) {
+		/* Check total length of transfers.  Also check each
+		 * transfer length to avoid arithmetic overflow.
+		 */
+		if (total > bufsiz || k_tmp->len > bufsiz) {
 			status = -EMSGSIZE;
 			goto done;