media: uvc: Avoid cyclic entity chains due to malformed USB descriptors

PATCH Superseded Will Deacon 2 participants 6 messages Oct 2, 2019 – Dec 18, 2019

Subsystems (3)

Stable branch Media (V4L/DVB) USB video class

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Will Deacon

author

2019-10-02 11:28 UTC #0 lore raw

PATCH 1/1 media: uvc: Avoid cyclic entity chains due to malformed USB descriptors

Way back in 2017, fuzzing the 4.14-rc2 USB stack with syzkaller kicked up the following WARNING from the UVC chain scanning code:

  | list_add double add: new=ffff880069084010, prev=ffff880069084010,
  | next=ffff880067d22298.
  | ------------[ cut here ]------------
  | WARNING: CPU: 1 PID: 1846 at lib/list_debug.c:31 __list_add_valid+0xbd/0xf0
  | Modules linked in:
  | CPU: 1 PID: 1846 Comm: kworker/1:2 Not tainted
  | 4.14.0-rc2-42613-g1488251d1a98 #238
  | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
  | Workqueue: usb_hub_wq hub_event
  | task: ffff88006b01ca40 task.stack: ffff880064358000
  | RIP: 0010:__list_add_valid+0xbd/0xf0 lib/list_debug.c:29
  | RSP: 0018:ffff88006435ddd0 EFLAGS: 00010286
  | RAX: 0000000000000058 RBX: ffff880067d22298 RCX: 0000000000000000
  | RDX: 0000000000000058 RSI: ffffffff85a58800 RDI: ffffed000c86bbac
  | RBP: ffff88006435dde8 R08: 1ffff1000c86ba52 R09: 0000000000000000
  | R10: 0000000000000002 R11: 0000000000000000 R12: ffff880069084010
  | R13: ffff880067d22298 R14: ffff880069084010 R15: ffff880067d222a0
  | FS:  0000000000000000(0000) GS:ffff88006c900000(0000) knlGS:0000000000000000
  | CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  | CR2: 0000000020004ff2 CR3: 000000006b447000 CR4: 00000000000006e0
  | Call Trace:
  |  __list_add ./include/linux/list.h:59
  |  list_add_tail+0x8c/0x1b0 ./include/linux/list.h:92
  |  uvc_scan_chain_forward.isra.8+0x373/0x416
  | drivers/media/usb/uvc/uvc_driver.c:1471
  |  uvc_scan_chain drivers/media/usb/uvc/uvc_driver.c:1585
  |  uvc_scan_device drivers/media/usb/uvc/uvc_driver.c:1769
  |  uvc_probe+0x77f2/0x8f00 drivers/media/usb/uvc/uvc_driver.c:2104

Looking into the output from usbmon, the interesting part is the following data packet:

ffff880069c63e00 30710169 C Ci:1:002:0 0 143 = 09028f00 01030080 00090403 00000e01 00000924 03000103 7c003328 010204db

If we drop the lead configuration and interface descriptors, we're left with an output terminal descriptor describing a generic display:

/* Output terminal descriptor */
buf[0] 09
buf[1] 24
buf[2] 03 /* UVC_VC_OUTPUT_TERMINAL */
buf[3] 00 /* ID */
buf[4] 01 /* type == 0x0301 (UVC_OTT_DISPLAY) */
buf[5] 03
buf[6] 7c
buf[7] 00 /* source ID refers to self! */
buf[8] 33

The problem with this descriptor is that it is self-referential: the source ID of 0 matches itself! This causes the 'struct uvc_entity' representing the display to be added to its chain list twice during 'uvc_scan_chain()': once via 'uvc_scan_chain_entity()' when it is processed directly from the 'dev->entities' list and then again immediately afterwards when trying to follow the source ID in 'uvc_scan_chain_forward()'

Add a check before adding an entity to a chain list to ensure that the entity is not already part of a chain.

Cc Laurent Pinchart <[email protected]>

Cc Mauro Carvalho Chehab <[email protected]>

Cc Dmitry Vyukov <[email protected]>

Cc Kostya Serebryany <[email protected]>

Cc <[email protected]>

Fixes c0efd232929c ("V4L/DVB (8145a): USB Video Class driver")

Reported-by Andrey Konovalov <[email protected]>

Link https://lore.kernel.org/linux-media/CAAeHK+z+Si69jUR+N-SjN9q4O+o5KFiNManqEa-PjUta7EOb7A@mail.gmail.com/ open here

Signed-off-by Will Deacon <[email protected]>

---

I don't have a way to reproduce the original issue, so this change is based purely on inspection. Considering I'm not familiar with USB nor UVC, I may well have missed something!

 drivers/media/usb/uvc/uvc_driver.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

drivers/media/usb/uvc/uvc_driver.c +12 -0

index 66ee168ddc7e..e24420b1750a 100644
@@ -1493,6 +1493,11 @@ static int uvc_scan_chain_forward(struct uvc_video_chain *chain,
 			break;
 		if (forward == prev)
 			continue;
+		if (forward->chain.next || forward->chain.prev) {
+			uvc_trace(UVC_TRACE_DESCR, "Found reference to "
+				"entity %d already in chain.\n", forward->id);
+			return -EINVAL;
+		}
 
 		switch (UVC_ENTITY_TYPE(forward)) {
 		case UVC_VC_EXTENSION_UNIT:

@@ -1574,6 +1579,13 @@ static int uvc_scan_chain_backward(struct uvc_video_chain *chain,
 				return -1;
 			}
 
+			if (term->chain.next || term->chain.prev) {
+				uvc_trace(UVC_TRACE_DESCR, "Found reference to "
+					"entity %d already in chain.\n",
+					term->id);
+				return -EINVAL;
+			}
+
 			if (uvc_trace_param & UVC_TRACE_PROBE)
 				printk(KERN_CONT " %d", term->id);

Signature · 1 line

2.23.0.444.g18eeb5a265-goog

Laurent Pinchart

maintainer

2019-10-02 13:09 UTC #1 lore raw

Hi Will,

Thank you for the patch.

Quoted 72 lines · Will Deacon

Way back in 2017, fuzzing the 4.14-rc2 USB stack with syzkaller kicked up the following WARNING from the UVC chain scanning code:
  | list_add double add: new=ffff880069084010, prev=ffff880069084010,
  | next=ffff880067d22298.
  | ------------[ cut here ]------------
  | WARNING: CPU: 1 PID: 1846 at lib/list_debug.c:31 __list_add_valid+0xbd/0xf0
  | Modules linked in:
  | CPU: 1 PID: 1846 Comm: kworker/1:2 Not tainted
  | 4.14.0-rc2-42613-g1488251d1a98 #238
  | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
  | Workqueue: usb_hub_wq hub_event
  | task: ffff88006b01ca40 task.stack: ffff880064358000
  | RIP: 0010:__list_add_valid+0xbd/0xf0 lib/list_debug.c:29
  | RSP: 0018:ffff88006435ddd0 EFLAGS: 00010286
  | RAX: 0000000000000058 RBX: ffff880067d22298 RCX: 0000000000000000
  | RDX: 0000000000000058 RSI: ffffffff85a58800 RDI: ffffed000c86bbac
  | RBP: ffff88006435dde8 R08: 1ffff1000c86ba52 R09: 0000000000000000
  | R10: 0000000000000002 R11: 0000000000000000 R12: ffff880069084010
  | R13: ffff880067d22298 R14: ffff880069084010 R15: ffff880067d222a0
  | FS:  0000000000000000(0000) GS:ffff88006c900000(0000) knlGS:0000000000000000
  | CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  | CR2: 0000000020004ff2 CR3: 000000006b447000 CR4: 00000000000006e0
  | Call Trace:
  |  __list_add ./include/linux/list.h:59
  |  list_add_tail+0x8c/0x1b0 ./include/linux/list.h:92
  |  uvc_scan_chain_forward.isra.8+0x373/0x416
  | drivers/media/usb/uvc/uvc_driver.c:1471
  |  uvc_scan_chain drivers/media/usb/uvc/uvc_driver.c:1585
  |  uvc_scan_device drivers/media/usb/uvc/uvc_driver.c:1769
  |  uvc_probe+0x77f2/0x8f00 drivers/media/usb/uvc/uvc_driver.c:2104
Looking into the output from usbmon, the interesting part is the following data packet:
ffff880069c63e00 30710169 C Ci:1:002:0 0 143 = 09028f00 01030080 00090403 00000e01 00000924 03000103 7c003328 010204db
If we drop the lead configuration and interface descriptors, we're left with an output terminal descriptor describing a generic display:
/* Output terminal descriptor */
buf[0] 09
buf[1] 24
buf[2] 03 /* UVC_VC_OUTPUT_TERMINAL */
buf[3] 00 /* ID */
buf[4] 01 /* type == 0x0301 (UVC_OTT_DISPLAY) */
buf[5] 03
buf[6] 7c
buf[7] 00 /* source ID refers to self! */
buf[8] 33
The problem with this descriptor is that it is self-referential: the source ID of 0 matches itself! This causes the 'struct uvc_entity' representing the display to be added to its chain list twice during 'uvc_scan_chain()': once via 'uvc_scan_chain_entity()' when it is processed directly from the 'dev->entities' list and then again immediately afterwards when trying to follow the source ID in 'uvc_scan_chain_forward()'
Add a check before adding an entity to a chain list to ensure that the entity is not already part of a chain.
Cc Laurent Pinchart <[email protected]>
Cc Mauro Carvalho Chehab <[email protected]>
Cc Dmitry Vyukov <[email protected]>
Cc Kostya Serebryany <[email protected]>
Cc <[email protected]>
Fixes c0efd232929c ("V4L/DVB (8145a): USB Video Class driver")
Reported-by Andrey Konovalov <[email protected]>
Link https://lore.kernel.org/linux-media/CAAeHK+z+Si69jUR+N-SjN9q4O+o5KFiNManqEa-PjUta7EOb7A@mail.gmail.com/ open here
Signed-off-by Will Deacon <[email protected]>

Quoted 5 lines

---
I don't have a way to reproduce the original issue, so this change is based purely on inspection. Considering I'm not familiar with USB nor UVC, I may well have missed something!

I may also be missing something, I haven't touched this code for a long time :-)

uvc_scan_chain_entity(), at the end of the function, adds the entity to the list of entities in the chain with

	list_add_tail(&entity->chain, &chain->entities);

uvc_scan_chain_forward() is then called (from uvc_scan_chain()), and iterates over all entities connected to the entity being scanned.

	while (1) {
		forward = uvc_entity_by_reference(chain->dev, entity->id,
			forward);

At this point forward may be equal to entity, if entity references itself.

		if (forward == NULL)
			break;
		if (forward == prev)
			continue;
		if (forward->chain.next || forward->chain.prev) {
			uvc_trace(UVC_TRACE_DESCR, "Found reference to "
				"entity %d already in chain.\n", forward->id);
			return -EINVAL;
		}

But then this check should trigger, as forward == entity and entity is in the chain's list of entities.

Quoted 33 lines

 drivers/media/usb/uvc/uvc_driver.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

drivers/media/usb/uvc/uvc_driver.c +12 -0

index 66ee168ddc7e..e24420b1750a 100644
@@ -1493,6 +1493,11 @@ static int uvc_scan_chain_forward(struct uvc_video_chain *chain,
 			break;
 		if (forward == prev)
 			continue;
+		if (forward->chain.next || forward->chain.prev) {
+			uvc_trace(UVC_TRACE_DESCR, "Found reference to "
+				"entity %d already in chain.\n", forward->id);
+			return -EINVAL;
+		}
 
 		switch (UVC_ENTITY_TYPE(forward)) {
 		case UVC_VC_EXTENSION_UNIT:

@@ -1574,6 +1579,13 @@ static int uvc_scan_chain_backward(struct uvc_video_chain *chain,
 				return -1;
 			}
 
+			if (term->chain.next || term->chain.prev) {
+				uvc_trace(UVC_TRACE_DESCR, "Found reference to "
+					"entity %d already in chain.\n",
+					term->id);
+				return -EINVAL;
+			}
+
 			if (uvc_trace_param & UVC_TRACE_PROBE)
 				printk(KERN_CONT " %d", term->id);

Signature · 3 lines

Regards,

Laurent Pinchart

Will Deacon

author

2019-10-02 13:19 UTC #2 lore raw

Hi Laurent,

Quoted 1 line · Laurent Pinchart

Thank you for the patch.

And thank you for the quick response.

Quoted 7 lines · Will Deacon

Quoted 3 lines
I don't have a way to reproduce the original issue, so this change is based purely on inspection. Considering I'm not familiar with USB nor UVC, I may well have missed something!
I may also be missing something, I haven't touched this code for a long time :-)

Actually, that is pretty helpful because it will make backporting easier if we get to that :)

Quoted 4 lines

uvc_scan_chain_entity(), at the end of the function, adds the entity to the list of entities in the chain with
	list_add_tail(&entity->chain, &chain->entities);

Yes.

Quoted 6 lines

uvc_scan_chain_forward() is then called (from uvc_scan_chain()), and iterates over all entities connected to the entity being scanned.
	while (1) {
		forward = uvc_entity_by_reference(chain->dev, entity->id,
			forward);

Yes.

Quoted 2 lines

At this point forward may be equal to entity, if entity references itself.

Correct -- that's indicative of a malformed entity which we want to reject, right?

Quoted 10 lines

		if (forward == NULL)
			break;
		if (forward == prev)
			continue;
		if (forward->chain.next || forward->chain.prev) {
			uvc_trace(UVC_TRACE_DESCR, "Found reference to "
				"entity %d already in chain.\n", forward->id);
			return -EINVAL;
		}

Quoted 2 lines

But then this check should trigger, as forward == entity and entity is in the chain's list of entities.

Right, but this code is added by my patch, no? In mainline, the code only has the first two checks, which both end up comparing against NULL the first time around:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/usb/uvc/uvc_driver.c#n1489

Or are you referring to somewhere else?

Will

Laurent Pinchart

maintainer

2019-10-02 18:56 UTC #3 lore raw

Hi Will,

Quoted 30 lines · Will Deacon

Quoted 1 line · Laurent Pinchart
Thank you for the patch.
And thank you for the quick response.
Quoted 7 lines · Will Deacon
Quoted 3 lines
I don't have a way to reproduce the original issue, so this change is based purely on inspection. Considering I'm not familiar with USB nor UVC, I may well have missed something!
I may also be missing something, I haven't touched this code for a long time :-)
Actually, that is pretty helpful because it will make backporting easier if we get to that :)
Quoted 4 lines
uvc_scan_chain_entity(), at the end of the function, adds the entity to the list of entities in the chain with
	list_add_tail(&entity->chain, &chain->entities);
Yes.
Quoted 6 lines
uvc_scan_chain_forward() is then called (from uvc_scan_chain()), and iterates over all entities connected to the entity being scanned.
	while (1) {
		forward = uvc_entity_by_reference(chain->dev, entity->id,
			forward);

Quoted 7 lines

Yes.
Quoted 2 lines
At this point forward may be equal to entity, if entity references itself.
Correct -- that's indicative of a malformed entity which we want to reject, right?

Right. We can reject the whole chain in that case. There's one case where we still want to succeed though, which is handled by uvc_scan_fallback().

Looking at the code, uvc_scan_device() does

                if (uvc_scan_chain(chain, term) < 0) {
                        kfree(chain);
                        continue;
                }

It seems that's missing removal of all entities that would have been successfully added to the chain. This prevents, I think, uvc_scan_fallback() from working properly in some cases.

Quoted 13 lines

Quoted 10 lines
		if (forward == NULL)
			break;
		if (forward == prev)
			continue;
		if (forward->chain.next || forward->chain.prev) {
			uvc_trace(UVC_TRACE_DESCR, "Found reference to "
				"entity %d already in chain.\n", forward->id);
			return -EINVAL;
		}
Quoted 2 lines
But then this check should trigger, as forward == entity and entity is in the chain's list of entities.

Quoted 7 lines

Right, but this code is added by my patch, no? In mainline, the code only has the first two checks, which both end up comparing against NULL the first time around:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/usb/uvc/uvc_driver.c#n1489
Or are you referring to somewhere else?

Oops. This is embarassing... :-) You're of course right. The second hunk seems fine too, even if I would have preferred centralising the check in a single place. That should be possible, but it would involve refactoring that isn't worth it at the moment.

Signature · 3 lines

Regards,

Laurent Pinchart

Will Deacon

author

2019-10-07 16:27 UTC #4 lore raw

Hi Laurent,

Sorry for the delay, I got tied up with other patches.

Quoted 22 lines · Laurent Pinchart

Quoted 7 lines · Will Deacon
Quoted 6 lines
uvc_scan_chain_forward() is then called (from uvc_scan_chain()), and iterates over all entities connected to the entity being scanned.
	while (1) {
		forward = uvc_entity_by_reference(chain->dev, entity->id,
			forward);
Quoted 7 lines
Yes.
Quoted 2 lines
At this point forward may be equal to entity, if entity references itself.
Correct -- that's indicative of a malformed entity which we want to reject, right?
Right. We can reject the whole chain in that case. There's one case where we still want to succeed though, which is handled by uvc_scan_fallback().
Looking at the code, uvc_scan_device() does

Quoted 8 lines

                if (uvc_scan_chain(chain, term) < 0) {
                        kfree(chain);
                        continue;
                }
It seems that's missing removal of all entities that would have been successfully added to the chain. This prevents, I think, uvc_scan_fallback() from working properly in some cases.

I started trying to hack something up here, but I'm actually not sure there's anything to do!

I agree that 'uvc_scan_chain()' can fail after adding entities to the chain, however, 'uvc_scan_fallback()' allocates a new chain and calls only 'uvc_scan_chain_entity()' to add entities to it. This doesn't fail on pre-existing 'list_head' structures, so the dangling pointers shouldn't pose a problem there. My patch only adds the checks to 'uvc_scan_chain_forward()' and 'uvc_scan_chain_backward()', neither of which are invoked on the fallback path.

The fallback also seems like a best-effort thing, since it isn't even invoked if we managed to initialise *any* chains successfully.

Does that make sense, or did you have another failure case in mind?

Quoted 17 lines

Quoted 17 lines
Quoted 10 lines
		if (forward == NULL)
			break;
		if (forward == prev)
			continue;
		if (forward->chain.next || forward->chain.prev) {
			uvc_trace(UVC_TRACE_DESCR, "Found reference to "
				"entity %d already in chain.\n", forward->id);
			return -EINVAL;
		}
Quoted 2 lines
But then this check should trigger, as forward == entity and entity is in the chain's list of entities.
Right, but this code is added by my patch, no? In mainline, the code only has the first two checks, which both end up comparing against NULL the first time around:

Quoted 8 lines

Quoted 3 lines
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/usb/uvc/uvc_driver.c#n1489
Or are you referring to somewhere else?
Oops. This is embarassing... :-) You're of course right. The second hunk seems fine too, even if I would have preferred centralising the check in a single place. That should be possible, but it would involve refactoring that isn't worth it at the moment.

Agreed, thanks.

Will

Laurent Pinchart

maintainer

2019-12-18 16:45 UTC #5 lore raw

Hi Will,

Quoted 43 lines · Will Deacon

Quoted 22 lines · Laurent Pinchart
Quoted 7 lines · Will Deacon
Quoted 6 lines
uvc_scan_chain_forward() is then called (from uvc_scan_chain()), and iterates over all entities connected to the entity being scanned.
	while (1) {
		forward = uvc_entity_by_reference(chain->dev, entity->id,
			forward);
Quoted 7 lines
Yes.
Quoted 2 lines
At this point forward may be equal to entity, if entity references itself.
Correct -- that's indicative of a malformed entity which we want to reject, right?
Right. We can reject the whole chain in that case. There's one case where we still want to succeed though, which is handled by uvc_scan_fallback().
Looking at the code, uvc_scan_device() does
Quoted 8 lines
                if (uvc_scan_chain(chain, term) < 0) {
                        kfree(chain);
                        continue;
                }
It seems that's missing removal of all entities that would have been successfully added to the chain. This prevents, I think, uvc_scan_fallback() from working properly in some cases.
I started trying to hack something up here, but I'm actually not sure there's anything to do!
I agree that 'uvc_scan_chain()' can fail after adding entities to the chain, however, 'uvc_scan_fallback()' allocates a new chain and calls only 'uvc_scan_chain_entity()' to add entities to it. This doesn't fail on pre-existing 'list_head' structures, so the dangling pointers shouldn't pose a problem there. My patch only adds the checks to 'uvc_scan_chain_forward()' and 'uvc_scan_chain_backward()', neither of which are invoked on the fallback path.

Quoted 4 lines

The fallback also seems like a best-effort thing, since it isn't even invoked if we managed to initialise *any* chains successfully.
Does that make sense, or did you have another failure case in mind?

No, I think you're right. It may still be good to remove the entities from the chain before freeing it to avoid dangling pointers, but that's not handled properly anywhere in the driver anyway, so your patch doesn't introduce any issue.

Quoted 27 lines

Quoted 17 lines
Quoted 17 lines
Quoted 10 lines
		if (forward == NULL)
			break;
		if (forward == prev)
			continue;
		if (forward->chain.next || forward->chain.prev) {
			uvc_trace(UVC_TRACE_DESCR, "Found reference to "
				"entity %d already in chain.\n", forward->id);
			return -EINVAL;
		}
Quoted 2 lines
But then this check should trigger, as forward == entity and entity is in the chain's list of entities.
Right, but this code is added by my patch, no? In mainline, the code only has the first two checks, which both end up comparing against NULL the first time around:
Quoted 8 lines
Quoted 3 lines
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/usb/uvc/uvc_driver.c#n1489
Or are you referring to somewhere else?
Oops. This is embarassing... :-) You're of course right. The second hunk seems fine too, even if I would have preferred centralising the check in a single place. That should be possible, but it would involve refactoring that isn't worth it at the moment.
Agreed, thanks.

Signature · 3 lines

Regards,

Laurent Pinchart