v4l2: handle multiplication overflow

From: Márton Németh <[email protected]>

When a V4L2 ioctl is executed the memory provided by the caller in user space is copied to the kernel space in video_usercopy() function. To find out how many bytes has to be copied the check_array_args() helper function is used.

This patch adds a check for multiplication overflow. Without this check the multiplication may overflow resulting array_size to be zero. This means that later on uninitialized value can trigger NULL pointer exception.

The overflow happens because ctrls->count is an __u32 multiplied by a constant coming from sizeof() operator. Multiplication result of two 32bit value may be a 64bit value, thus overflow can happen. Similarly buf->length is an __u32 multiplied by a constant coming from sizeof() operator.

The chosen error value is -ENOMEM because we are just about to allocate kernel memory to copy data from userspace. We cannot even store the required number of bytes in the variable of size_t type.

To trigger the error from user space use the v4l-test 0.19 [1] or essentially the following code fragment:

	struct v4l2_ext_controls controls;
	memset(&controls, 0, sizeof(controls));
	controls.ctrl_class = V4L2_CTRL_CLASS_USER;
	controls.count = 0x40000000;
	controls.controls = NULL;
	ret = ioctl(f, VIDIOC_G_EXT_CTRLS, &controls);

References:
[1] v4l-test: Test environment for Video For Linux Two (V4L2) API
http://v4l-test.sourceforge.net/

Signed-off-by Márton Németh <[email protected]>

---

I'm a little bit in doubt whether this is the correct way to check for the multiplication overflow. In case of x86 architecture the MUL insruction [2] can be used to multiply EAX with an other 32bit register, and the 64bit result is placed to EDX:EAX. In case of EDX != 0 the carry and overflow flags are set.

This means that executing the MUL instruction on x86 already provides information whether the result fits to 32bit or not. I might use __u64 as a result of the multiplication and check whether the upper 32bit is still zero but the optimal would be to check for the carry or the overflow flag.

Still, there could be a special case when the constant from sizeof() operator is a power of 2, in this case the compiler might optimize the multiplication using a shift left. In this case something else is needed.

I couldn't really find information about the number of bits for size_t on different architectures. If size_t happens to be at least 64bit on some architecture then this overflow will not happen at all and the array_size will contain the correct value.

What do you think?

Em 22-12-2011 07:42, Németh Márton escreveu:

Quoted 56 lines

From: Márton Németh <[email protected]>

When a V4L2 ioctl is executed the memory provided by the caller in user space is copied to the kernel space in video_usercopy() function. To find out how many bytes has to be copied the check_array_args() helper function is used.

This patch adds a check for multiplication overflow. Without this check the multiplication may overflow resulting array_size to be zero. This means that later on uninitialized value can trigger NULL pointer exception.

The overflow happens because ctrls->count is an __u32 multiplied by a constant coming from sizeof() operator. Multiplication result of two 32bit value may be a 64bit value, thus overflow can happen. Similarly buf->length is an __u32 multiplied by a constant coming from sizeof() operator.

The chosen error value is -ENOMEM because we are just about to allocate kernel memory to copy data from userspace. We cannot even store the required number of bytes in the variable of size_t type.

To trigger the error from user space use the v4l-test 0.19 [1] or essentially the following code fragment:

	struct v4l2_ext_controls controls;
	memset(&controls, 0, sizeof(controls));
	controls.ctrl_class = V4L2_CTRL_CLASS_USER;
	controls.count = 0x40000000;
	controls.controls = NULL;
	ret = ioctl(f, VIDIOC_G_EXT_CTRLS, &controls);

References:
[1] v4l-test: Test environment for Video For Linux Two (V4L2) API
http://v4l-test.sourceforge.net/

Signed-off-by Márton Németh <[email protected]>

---

I'm a little bit in doubt whether this is the correct way to check for the multiplication overflow. In case of x86 architecture the MUL insruction [2] can be used to multiply EAX with an other 32bit register, and the 64bit result is placed to EDX:EAX. In case of EDX != 0 the carry and overflow flags are set.

This means that executing the MUL instruction on x86 already provides information whether the result fits to 32bit or not. I might use __u64 as a result of the multiplication and check whether the upper 32bit is still zero but the optimal would be to check for the carry or the overflow flag.

Still, there could be a special case when the constant from sizeof() operator is a power of 2, in this case the compiler might optimize the multiplication using a shift left. In this case something else is needed.

I couldn't really find information about the number of bits for size_t on different architectures. If size_t happens to be at least 64bit on some architecture then this overflow will not happen at all and the array_size will contain the correct value.

Quoted 1 line

What do you think?

Hi Németh,

IMO, the check should, instead, use a max hard limit for the array size. There's no sense on allowing very large arrays there.

I think that this patch become obsoleted by this one, already merged:

commit 6c06108be53ca5e94d8b0e93883d534dd9079646

Author: Dan Carpenter <[email protected]>
Date:   Thu Jan 5 02:27:57 2012 -0300

[media] V4L/DVB: v4l2-ioctl: integer overflow in video_usercopy()

If ctrls->count is too high the multiplication could overflow and array_size would be lower than expected. Mauro and Hans Verkuil suggested that we cap it at 1024. That comes from the maximum number of controls with lots of room for expantion.

$ grep V4L2_CID include/linux/videodev2.h | wc -l 211

Cc stable <[email protected]>

Signed-off-by Dan Carpenter <[email protected]>

Signed-off-by Mauro Carvalho Chehab <[email protected]>

drivers/media/video/v4l2-ioctl.c +4 -0

index e1da8fc..639abee 100644
@@ -2226,6 +2226,10 @@ static int check_array_args(unsigned int cmd, void *parg, size_t *array_size,
 		struct v4l2_ext_controls *ctrls = parg;
 
 		if (ctrls->count != 0) {
+			if (ctrls->count > V4L2_CID_MAX_CTRLS) {
+				ret = -EINVAL;
+				break;
+			}
 			*user_ptr = (void __user *)ctrls->controls;
 			*kernel_ptr = (void *)&ctrls->controls;
 			*array_size = sizeof(struct v4l2_ext_control)

include/linux/videodev2.h +1 -0

index 6bfaa76..b2e1331 100644
@@ -1132,6 +1132,7 @@ struct v4l2_querymenu {
 #define V4L2_CTRL_FLAG_NEXT_CTRL	0x80000000
 
 /*  User-class control IDs defined by V4L2 */
+#define V4L2_CID_MAX_CTRLS		1024
 #define V4L2_CID_BASE			(V4L2_CTRL_CLASS_USER | 0x900)
 #define V4L2_CID_USER_BASE 		V4L2_CID_BASE
 /*  IDs reserved for driver specific controls */

Regards,
Mauro