An Extension Unit descriptor is 24 + bNrInPins + bControlSize bytes long
(UVC_DT_EXTENSION_UNIT_SIZE(p, n)), where bNrInPins and bControlSize are
configfs attributes each accepted over the full 0..255 range by
kstrtou8().  uvc_configfs stores the computed size in the u8 bLength
field of the descriptor, so once 24 + p + n exceeds 255 it silently
wraps: p = n = 255 describes a 534-byte descriptor with bLength = 22.

uvc_copy_descriptors() reserves xu->desc.bLength bytes per Extension Unit
in the descriptor buffer it allocates at bind time, but
UVC_COPY_XU_DESCRIPTOR() copies the real descriptor contents, i.e.
22 + bNrInPins + 1 + bControlSize + 1 bytes.  With the wrapped length the
copy overruns the allocation by up to 512 bytes.

Attack chain (write access to a UVC gadget's configfs attributes; no
race, no USB traffic, a single bind triggers it):

    echo 255 > .../functions/uvc.0/extensions/ext.0/b_nr_in_pins
    echo 255 > .../functions/uvc.0/extensions/ext.0/b_control_size
      -> uvcg_extension_b_nr_in_pins_store() /
         uvcg_extension_b_control_size_store()
      -> bLength = UVC_DT_EXTENSION_UNIT_SIZE(255, 255) wraps to 22
    bind the gadget to a UDC
      -> uvc_function_bind() -> uvc_copy_descriptors()
      -> kmalloc() sized using the wrapped bLength: 22 bytes for the XU
      -> UVC_COPY_XU_DESCRIPTOR() writes the real 534 bytes into that
         slot (22-byte head, 255 baSourceID, bControlSize, 255
         bmControls, iExtension)
      -> heap out-of-bounds write during bind

Reproduced on 7.2.0+: a stock build (FORTIFY on) can derive the remaining
allocation size at the bmControls memcpy and BUGs in __fortify_panic()
during bind - a deterministic kernel crash; with FORTIFY disabled for the
file, KASAN reports "slab-out-of-bounds Write of size 255" in
uvc_copy_descriptors() against the kmalloc-192 descriptor buffer.

Reject combinations whose descriptor does not fit into bLength at all four
configfs entry points that can grow an Extension Unit (b_nr_in_pins,
b_control_size, ba_source_id, bm_controls), and make uvc_copy_descriptors()
refuse an Extension Unit whose bLength does not match its contents instead
of overflowing the buffer.

Fixes: 0525210c9840 ("usb: gadget: uvc: Allow definition of XUs in configfs")
Signed-off-by: Haofeng Li <lihaofeng@kylinos.cn>
Assisted-by: opencode:deepseek-v4-flash-free
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
---
Changes in v2:
- reword a comment per Randy Dunlap: "The descriptor length must fit
  into the one-byte bLength field".

Changes in v3:
- Add a warning message when the descriptor size is rejected, as suggested
  by Dan Scally.

Changes in v4:
- Remove the blank line inside the sign-off block.
 drivers/usb/gadget/function/f_uvc.c        |  6 +++++
 drivers/usb/gadget/function/uvc_configfs.c | 28 ++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/drivers/usb/gadget/function/f_uvc.c b/drivers/usb/gadget/function/f_uvc.c
index 73dc7e42875f..0d3432f80cae 100644
--- a/drivers/usb/gadget/function/f_uvc.c
+++ b/drivers/usb/gadget/function/f_uvc.c
@@ -598,6 +598,12 @@ uvc_copy_descriptors(struct uvc_device *uvc, enum usb_device_speed speed)
 	}
 
 	list_for_each_entry(xu, uvc->desc.extension_units, list) {
+		/* Mismatched bLength would overflow the buffer sized after it */
+		if (xu->desc.bLength !=
+		    UVC_DT_EXTENSION_UNIT_SIZE(xu->desc.bNrInPins,
+					       xu->desc.bControlSize))
+			return ERR_PTR(-EINVAL);
+
 		control_size += xu->desc.bLength;
 		bytes += xu->desc.bLength;
 		n_desc++;
diff --git a/drivers/usb/gadget/function/uvc_configfs.c b/drivers/usb/gadget/function/uvc_configfs.c
index 70a1415ea401..b52c9cf2b715 100644
--- a/drivers/usb/gadget/function/uvc_configfs.c
+++ b/drivers/usb/gadget/function/uvc_configfs.c
@@ -845,6 +845,18 @@ static ssize_t uvcg_extension_b_num_controls_store(struct config_item *item,
 }
 UVCG_EXTENSION_ATTR(b_num_controls, bNumControls);
 
+/* The descriptor length must fit into the one-byte bLength field */
+static int uvcg_extension_check_size(u8 nr_in_pins, u8 control_size)
+{
+	if (UVC_DT_EXTENSION_UNIT_SIZE(nr_in_pins, control_size) > 255) {
+		pr_warn("uvc: Extension Unit descriptor size (%u + %u) does not fit into bLength\n",
+			nr_in_pins, control_size);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 /*
  * In addition to storing bNrInPins, this function needs to realloc the
  * memory for the baSourceID array and additionally expand bLength.
@@ -877,6 +889,10 @@ static ssize_t uvcg_extension_b_nr_in_pins_store(struct config_item *item,
 		goto unlock;
 	}
 
+	ret = uvcg_extension_check_size(num, xu->desc.bControlSize);
+	if (ret)
+		goto unlock;
+
 	tmp_buf = krealloc_array(xu->desc.baSourceID, num, sizeof(u8),
 				 GFP_KERNEL | __GFP_ZERO);
 	if (!tmp_buf) {
@@ -930,6 +946,10 @@ static ssize_t uvcg_extension_b_control_size_store(struct config_item *item,
 		goto unlock;
 	}
 
+	ret = uvcg_extension_check_size(xu->desc.bNrInPins, num);
+	if (ret)
+		goto unlock;
+
 	tmp_buf = krealloc_array(xu->desc.bmControls, num, sizeof(u8),
 				 GFP_KERNEL | __GFP_ZERO);
 	if (!tmp_buf) {
@@ -1055,6 +1075,10 @@ static ssize_t uvcg_extension_ba_source_id_store(struct config_item *item,
 	if (ret)
 		goto unlock;
 
+	ret = uvcg_extension_check_size(n, xu->desc.bControlSize);
+	if (ret)
+		goto unlock;
+
 	iter = source_ids = kcalloc(n, sizeof(u8), GFP_KERNEL);
 	if (!source_ids) {
 		ret = -ENOMEM;
@@ -1134,6 +1158,10 @@ static ssize_t uvcg_extension_bm_controls_store(struct config_item *item,
 	if (ret)
 		goto unlock;
 
+	ret = uvcg_extension_check_size(xu->desc.bNrInPins, n);
+	if (ret)
+		goto unlock;
+
 	iter = bm_controls = kcalloc(n, sizeof(u8), GFP_KERNEL);
 	if (!bm_controls) {
 		ret = -ENOMEM;
-- 
2.25.1