[PATCH 3.12 020/142] spi: spidev: fix possible arithmetic overflow for multi-transfer message

[Jiri Slaby]

From: Ian Abbott <abbotti@xxxxxxxxx>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

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@xxxxxxxxx>
Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
 drivers/spi/spidev.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
index ca5bcfe874d0..2466632c2c01 100644
--- a/drivers/spi/spidev.c
+++ b/drivers/spi/spidev.c
@@ -243,7 +243,10 @@ static int spidev_message(struct spidev_data *spidev,
 		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;
 		}
-- 
2.3.7

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