media: usb: siano: Fix undefined behavior bug in struct smsusb_urb_t

PATCH In review Gustavo A. R. Silva 5 participants 6 messages Sep 29, 2023 – Oct 1, 2023

Subsystems (3)

Media (V4L/DVB) USB Siano DVB driver

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Gustavo A. R. Silva

author

2023-09-29 15:42 UTC #0 lore raw

PATCH 1/1 media: usb: siano: Fix undefined behavior bug in struct smsusb_urb_t

struct urb is a flexible structure, which means that it contains a flexible-array member at the bottom. This could potentially lead to an overwrite of the object wq at run-time with the contents of urb.

Fix this by placing object urb at the end of struct smsusb_urb_t.

Fixes dd47fbd40e6e ("[media] smsusb: don't sleep while atomic")

Cc [email protected]

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
 drivers/media/usb/siano/smsusb.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

drivers/media/usb/siano/smsusb.c +2 -2

index 9d9e14c858e6..2c048f8e8371 100644
@@ -40,10 +40,10 @@ struct smsusb_urb_t {
 	struct smscore_buffer_t *cb;
 	struct smsusb_device_t *dev;
 
-	struct urb urb;
-
 	/* For the bottom half */
 	struct work_struct wq;
+
+	struct urb urb;
 };
 
 struct smsusb_device_t {

Signature · 1 line

2.34.1

Jann Horn

2023-09-29 16:20 UTC #1 lore raw

Quoted 5 lines · Gustavo A. R. Silva

struct urb is a flexible structure, which means that it contains a flexible-array member at the bottom. This could potentially lead to an overwrite of the object wq at run-time with the contents of urb.
Fix this by placing object urb at the end of struct smsusb_urb_t.

Does this really change the situation? "struct smsusb_device_t" contains an array of "struct smsusb_urb_t", so it seems to be like you're just shifting the "VLA inside a non-final member of a struct" thing around so that there is one more layer of abstraction in between.

Comments on "struct urb" say:

* Isochronous URBs have a different data transfer model, in part because
* the quality of service is only "best effort". Callers provide specially
* allocated URBs, with number_of_packets worth of iso_frame_desc structures
* at the end.

and:

/* (in) ISO ONLY */

And it looks like smsusb only uses that URB as a bulk URB, so the flex array is unused and we can't have an overflow here?

If this is intended to make it possible to enable some kinda compiler warning, it might be worth talking to the USB folks to figure out the right approach here.

Quoted 28 lines

Fixes dd47fbd40e6e ("[media] smsusb: don't sleep while atomic")

Cc [email protected]

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
 drivers/media/usb/siano/smsusb.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

drivers/media/usb/siano/smsusb.c +2 -3

index 9d9e14c858e6..2c048f8e8371 100644
@@ -40,10 +40,10 @@ struct smsusb_urb_t {
        struct smscore_buffer_t *cb;
        struct smsusb_device_t *dev;
 
-       struct urb urb;
-
        /* For the bottom half */
        struct work_struct wq;
+
+       struct urb urb;
 };
 
 struct smsusb_device_t {
--

2.34.1

Greg Kroah-Hartman

maintainer

2023-09-30 07:01 UTC #2 lore raw

Quoted 47 lines · Jann Horn

Quoted 5 lines · Gustavo A. R. Silva
struct urb is a flexible structure, which means that it contains a flexible-array member at the bottom. This could potentially lead to an overwrite of the object wq at run-time with the contents of urb.
Fix this by placing object urb at the end of struct smsusb_urb_t.
Does this really change the situation? "struct smsusb_device_t" contains an array of "struct smsusb_urb_t", so it seems to be like you're just shifting the "VLA inside a non-final member of a struct" thing around so that there is one more layer of abstraction in between.
Comments on "struct urb" say:
* Isochronous URBs have a different data transfer model, in part because
* the quality of service is only "best effort". Callers provide specially
* allocated URBs, with number_of_packets worth of iso_frame_desc structures
* at the end.
and:
/* (in) ISO ONLY */
And it looks like smsusb only uses that URB as a bulk URB, so the flex array is unused and we can't have an overflow here?
If this is intended to make it possible to enable some kinda compiler warning, it might be worth talking to the USB folks to figure out the right approach here.
Quoted 15 lines
Fixes dd47fbd40e6e ("[media] smsusb: don't sleep while atomic")
Cc [email protected]
Signed-off-by Gustavo A. R. Silva <[email protected]>
---
 drivers/media/usb/siano/smsusb.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
drivers/media/usb/siano/smsusb.c +0 -0
index 9d9e14c858e6..2c048f8e8371 100644
@@ -40,10 +40,10 @@ struct smsusb_urb_t {
        struct smscore_buffer_t *cb;
        struct smsusb_device_t *dev;
 

Quoted 7 lines

drivers/media/usb/siano/smsusb.c struct smsusb_urb_t { · line 47
Quoted 7 lines
drivers/media/usb/siano/smsusb.c +2 -2
-       struct urb urb;
-
        /* For the bottom half */
        struct work_struct wq;
+
+       struct urb urb;
 };

Yeah, this is going to get messy. Ideally, just dynamically create the urb and change this to a "struct urb *urb;" instead.

thanks,

greg k-h

Gustavo A. R. Silva

2023-10-01 09:13 UTC #3 lore raw

Quoted 11 lines · Greg Kroah-Hartman

Quoted 10 lines · Jann Horn
Quoted 5 lines · Gustavo A. R. Silva
struct urb is a flexible structure, which means that it contains a flexible-array member at the bottom. This could potentially lead to an overwrite of the object wq at run-time with the contents of urb.
Fix this by placing object urb at the end of struct smsusb_urb_t.
Does this really change the situation? "struct smsusb_device_t" contains an array of "struct smsusb_urb_t", so it seems to be like

Yeah. I noticed that too.

Probably what Greg suggests (dynamically create the urb) can fix this, too.

I haven't taken a deep dive into this particular case. So, let me go and figure something out.

Quoted 37 lines

Quoted 37 lines
you're just shifting the "VLA inside a non-final member of a struct" thing around so that there is one more layer of abstraction in between.
Comments on "struct urb" say:
* Isochronous URBs have a different data transfer model, in part because
* the quality of service is only "best effort". Callers provide specially
* allocated URBs, with number_of_packets worth of iso_frame_desc structures
* at the end.
and:
/* (in) ISO ONLY */
And it looks like smsusb only uses that URB as a bulk URB, so the flex array is unused and we can't have an overflow here?
If this is intended to make it possible to enable some kinda compiler warning, it might be worth talking to the USB folks to figure out the right approach here.
Quoted 15 lines
Fixes dd47fbd40e6e ("[media] smsusb: don't sleep while atomic")
Cc [email protected]
Signed-off-by Gustavo A. R. Silva <[email protected]>
---
  drivers/media/usb/siano/smsusb.c | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)
drivers/media/usb/siano/smsusb.c +0 -0
index 9d9e14c858e6..2c048f8e8371 100644
@@ -40,10 +40,10 @@ struct smsusb_urb_t {
         struct smscore_buffer_t *cb;
         struct smsusb_device_t *dev;
 

Quoted 10 lines

Quoted 7 lines
drivers/media/usb/siano/smsusb.c struct smsusb_urb_t { · line 47
Quoted 7 lines
drivers/media/usb/siano/smsusb.c +2 -2
-       struct urb urb;
-
         /* For the bottom half */
         struct work_struct wq;
+
+       struct urb urb;
  };
Yeah, this is going to get messy. Ideally, just dynamically create the urb and change this to a "struct urb *urb;" instead.

Probably, yes.

Thanks
--
Gustavo

Kees Cook

Reviewed-by on v1 1/1

2023-09-29 17:28 UTC #4 lore raw

In reply to Gustavo A. R. Silva

Quoted 9 lines · Gustavo A. R. Silva

struct urb is a flexible structure, which means that it contains a flexible-array member at the bottom. This could potentially lead to an overwrite of the object wq at run-time with the contents of urb.
Fix this by placing object urb at the end of struct smsusb_urb_t.
Fixes dd47fbd40e6e ("[media] smsusb: don't sleep while atomic")
Cc [email protected]
Signed-off-by Gustavo A. R. Silva <[email protected]>

As Jann pointed out, it's unlikely there is a function bug here, but I still think it's right to make sure this is robust and clears the way for -Wflex-array-member-not-at-end.

Reviewed-by Kees Cook <[email protected]>

Signature · 1 line

Kees Cook

Jann Horn

2023-09-29 17:40 UTC #5 lore raw

Quoted 7 lines · Kees Cook

Quoted 6 lines · Gustavo A. R. Silva
struct urb is a flexible structure, which means that it contains a flexible-array member at the bottom. This could potentially lead to an overwrite of the object wq at run-time with the contents of urb.
Fix this by placing object urb at the end of struct smsusb_urb_t.

Quoted 7 lines

Quoted 3 lines
Fixes dd47fbd40e6e ("[media] smsusb: don't sleep while atomic")
Cc [email protected]
Signed-off-by Gustavo A. R. Silva <[email protected]>
As Jann pointed out, it's unlikely there is a function bug here, but I still think it's right to make sure this is robust and clears the way for -Wflex-array-member-not-at-end.

But if this change makes the warning go away, that just means the warning is implemented badly, right?

Like, before we had:

struct urb {
...
struct usb_iso_packet_descriptor iso_frame_desc[];
};
struct smsusb_urb_t {
...
struct urb urb; // not last element
...
};

whereas afterwards we have:

struct urb {
...
struct usb_iso_packet_descriptor iso_frame_desc[];
};
struct smsusb_urb_t {
...
struct urb urb;
};
struct smsusb_device_t {
...
struct smsusb_urb_t surbs[MAX_URBS]; // array, and not last element
...
};

That's basically the same pattern! Except that the new version has one more layer of indirection and there's an array involved.

And you can't address that by moving struct members around because of the involvement of the array.