Re: [PATCH] HID: hiddev: fix potential Spectre v1

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Hi Gustavo,

On 10/17/2018 05:30 PM, Gustavo A. R. Silva wrote:
> 
> Hi Breno,
> 
> On 10/17/18 9:47 PM, Breno Leitao wrote:
>> uref->usage_index can be indirectly controlled by userspace, hence leading
>> to a potential exploitation of the Spectre variant 1 vulnerability.
>>
>> This problem might show up in the cmd = HIDIOCGCOLLECTIONINDEX flow at function
>> hiddev_ioctl_usage(), where uref->usage_index is compared to field->maxusage
>> and then used as an index to dereference field->usage array.
>>
>> This is a summary of the current flow, which matches the traditional
>> Spectre V1 issue:
>>
>> 	copy_from_user(uref, user_arg, sizeof(*uref))
>> 	if (uref->usage_index >= field->maxusage)
>> 		goto inval;
>> 	i = field->usage[uref->usage_index].collection_index;
>> 	return i;
>>
>> This patch fixes this by sanitizing field uref->usage_index before using it to
>> index field->usage, thus, avoiding speculation in the first load.
>>
>> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
>> ---
>>  drivers/hid/usbhid/hiddev.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/hid/usbhid/hiddev.c b/drivers/hid/usbhid/hiddev.c
>> index 23872d08308c..8829cbc1f6b1 100644
>> --- a/drivers/hid/usbhid/hiddev.c
>> +++ b/drivers/hid/usbhid/hiddev.c
>> @@ -512,6 +512,9 @@ static noinline int hiddev_ioctl_usage(struct hiddev *hiddev, unsigned int cmd,
>>  			if (cmd == HIDIOCGCOLLECTIONINDEX) {
>>  				if (uref->usage_index >= field->maxusage)
>>  					goto inval;
>> +				uref->usage_index =
>> +					array_index_nospec(uref->usage_index,
>> +							   field->maxusage);
> 
> Good catch.
> 
>>  			} else if (uref->usage_index >= field->report_count)
>>  				goto inval;
> 
> Although, notice that this is the same index, and it can be used to index field->value[]
> at lines 526 and 532.

Right, this seems to be a possible problem also, when 'cmd' = HIDIOC{G,S}USAGES.

I am reworking the patch to cover both issues. What do you think of the draft
below?

Thank you for reviewing it!

---

Subject: [PATCH] HID: hiddev: fix potential Spectre v1

uref->usage_index can be indirectly controlled by userspace, hence leading
to a potential exploitation of the Spectre variant 1 vulnerability.

This field is used as an array index by the hiddev_ioctl_usage() function,
when 'cmd' is HIDIOCGCOLLECTIONINDEX, HIDIOCGUSAGES or HIDIOCSUSAGES.

For cmd == HIDIOCGCOLLECTIONINDEX case, uref->usage_index is compared to
field->maxusage and then used as an index to dereference field->usage
array.  The very same thing happens to the cmd == HIDIOC{G,S}USAGES cases,
where uref->usage_index is checked against an array maximum value and then
it is used as an index in this array.

This is a summary of the HIDIOCGCOLLECTIONINDEX case, which matches the
traditional Spectre V1 first load:

	copy_from_user(uref, user_arg, sizeof(*uref))
	if (uref->usage_index >= field->maxusage)
		goto inval;
	i = field->usage[uref->usage_index].collection_index;
	return i;

This patch fixes this by sanitizing field uref->usage_index before using it
to index field->usage, thus, avoiding speculation in the first load.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 drivers/hid/usbhid/hiddev.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/hid/usbhid/hiddev.c b/drivers/hid/usbhid/hiddev.c
index 23872d08308c..053f93bdee72 100644
--- a/drivers/hid/usbhid/hiddev.c
+++ b/drivers/hid/usbhid/hiddev.c
@@ -512,14 +512,24 @@ static noinline int hiddev_ioctl_usage(struct hiddev
*hiddev, unsigned int cmd,
 			if (cmd == HIDIOCGCOLLECTIONINDEX) {
 				if (uref->usage_index >= field->maxusage)
 					goto inval;
+				uref->usage_index =
+					array_index_nospec(uref->usage_index,
+							   field->maxusage);
 			} else if (uref->usage_index >= field->report_count)
 				goto inval;
 		}

-		if ((cmd == HIDIOCGUSAGES || cmd == HIDIOCSUSAGES) &&
-		    (uref_multi->num_values > HID_MAX_MULTI_USAGES ||
-		     uref->usage_index + uref_multi->num_values > field->report_count))
-			goto inval;
+		if (cmd == HIDIOCGUSAGES || cmd == HIDIOCSUSAGES) {
+			if (uref_multi->num_values > HID_MAX_MULTI_USAGES ||
+			    uref->usage_index + uref_multi->num_values >
+				 field->report_count)
+				goto inval;
+
+			uref->usage_index =
+				array_index_nospec(uref->usage_index,
+						   field->report_count -
+						   uref_multi->num_values);
+		}

 		switch (cmd) {
 		case HIDIOCGUSAGE:




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