On 03/30/2018 03:20 AM, Jonathan Cameron wrote:
On Mon, 26 Mar 2018 14:27:52 -0700
Martin Kelly <mkelly@xxxxxxxx> wrote:
Currently, the following causes a kernel OOPS in memcpy:
echo 1073741825 > buffer/length
echo 1 > buffer/enable
Note that using 1073741824 instead of 1073741825 causes "write error:
Cannot allocate memory" but no OOPS.
This is because 1073741824 == 2^30 and 1073741825 == 2^30+1. Since kfifo
rounds up to the nearest power of 2, it will actually call kmalloc with
roundup_pow_of_two(length) * bytes_per_datum.
Using length == 1073741825 and bytes_per_datum == 2, we get:
kmalloc(roundup_pow_of_two(1073741825) * 2
or kmalloc(2147483648 * 2)
or kmalloc(4294967296)
or kmalloc(UINT_MAX + 1)
so this overflows to 0, causing kmalloc to return ZERO_SIZE_PTR and
subsequent memcpy to fail once the device is enabled.
Fix this by checking for overflow prior to allocating a kfifo. With this
check added, the above code returns -EINVAL when enabling the buffer,
rather than causing an OOPS.
Signed-off-by: Martin Kelly <mkelly@xxxxxxxx>
Applied to the fixes-togreg branch of iio.git and marked for stable.
I thought about this for a few mins. A 4 gig allocation on a 32bit
machine is going to fail anyway for obvious reasons, but good
to protect against overflow if someone were to write this value.
Yep, I found this by accident when my userspace had a bug, and then I
just got curious about what was happening. Such clean, isolated bugs
like this are rare and fun to find :).
Particularly good description btw!
Thanks,
Jonathan
---
drivers/iio/buffer/kfifo_buf.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/drivers/iio/buffer/kfifo_buf.c b/drivers/iio/buffer/kfifo_buf.c
index ac622edf2486..70c302a93d7f 100644
--- a/drivers/iio/buffer/kfifo_buf.c
+++ b/drivers/iio/buffer/kfifo_buf.c
@@ -27,6 +27,13 @@ static inline int __iio_allocate_kfifo(struct iio_kfifo *buf,
if ((length == 0) || (bytes_per_datum == 0))
return -EINVAL;
+ /*
+ * Make sure we don't overflow an unsigned int after kfifo rounds up to
+ * the next power of 2.
+ */
+ if (roundup_pow_of_two(length) > UINT_MAX / bytes_per_datum)
+ return -EINVAL;
+
return __kfifo_alloc((struct __kfifo *)&buf->kf, length,
bytes_per_datum, GFP_KERNEL);
}
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