On 21/05/16 17:50, Dmitry Torokhov wrote:
On Mon, Mar 23, 2015 at 10:50 AM, Ian Abbott <abbotti@xxxxxxxxx> wrote:
`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>
Cc: <stable@xxxxxxxxxxxxxxx> # 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) {
What if total is INT_MAX - 2 and k_tmp->len is 3? What about total is
INT_MAX and k_tmp->len is INT_MAX as well? I think the proper check
In your questions, I assume you are referring to the values of 'total'
before the addition. I'll call the values 'old_total' and 'new_total'
(with the same type as 'total', i.e. 'unsigned int'). Note that total
(and old_total, and new_total) and 'k_tmp->len' have range UINT_MAX, or
2*INT_MAX+1.
Before the addition, we know that old_total <= INT_MAX (otherwise the
loop would have errored out already), but k_tmp->len can have any value
from 0 to UINT_MAX. After the addition, new_total can have any value
from 0 to UINT_MAX, and might be less than old_total. new_total can
only be less than old_total if old_total + k_tmp->len > UINT_MAX, and
here I am referring to proper addition, not addition modulo UINT_MAX+1.
Rearranging, new_total will be less than old_total if k_tmp->len >
UINT_MAX - old_total. Since the maximum value of old_total is INT_MAX,
the lowest possible value of k_tmp->len that could cause new_total to be
less than old_total is UINT_MAX - INT_MAX, or INT_MAX+1. That is what
the second part of the 'if' test is detecting.
should be:
if (total < k_tmp->len || total > INT_MAX) {
...
}
That also works.
--
-=( Ian Abbott @ MEV Ltd. E-mail: <abbotti@xxxxxxxxx> )=-
-=( Web: http://www.mev.co.uk/ )=-
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