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

[Ian Abbott <abbotti-GUHe90Wt2aFaa/9Udqfwiw@public.gmane.org>]

`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-GUHe90Wt2aFaa/9Udqfwiw@public.gmane.org>
Cc: <stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org> # 4.0+
---
This could be backported to kernels prior to 4.0, but the total and
individual lengths would need to be checked against `bufsiz` instead of
`INT_MAX`.
---
 drivers/spi/spidev.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
index bb6b3ab..23ad978 100644
--- a/drivers/spi/spidev.c
+++ b/drivers/spi/spidev.c
@@ -249,9 +249,10 @@ static int spidev_message(struct spidev_data *spidev,
 		total += k_tmp->len;
 		/* Since the function returns the total length of transfers
 		 * on success, restrict the total to positive int values to
-		 * avoid the return value looking like an error.
+		 * avoid the return value looking like an error.  Also check
+		 * each transfer length to avoid arithmetic overflow.
 		 */
-		if (total > INT_MAX) {
+		if (total > INT_MAX || k_tmp->len > INT_MAX) {
 			status = -EMSGSIZE;
 			goto done;
 		}
-- 
2.1.4

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