[PATCH v2] usb: gadget: uvc: align XU descriptor pointers to fix kmemleak reports

[Rituparna Warwatkar <rwarwatkar@gmail.com>]

kmemleak reports the baSourceID and bmControls arrays allocated by the
UVC extension-unit configfs attributes as leaked, e.g.:

  BUG: memory leak
  unreferenced object 0xffff888114fee2c0 (size 8):
    __kmalloc_noprof
    uvcg_extension_ba_source_id_store
    configfs_write_iter
    vfs_write
    ksys_write

The arrays are not actually leaked: they are reachable through
xu->desc.baSourceID / xu->desc.bmControls and are freed when the
extension unit is removed. The problem is that struct
uvcg_extension_unit_descriptor is marked __packed, so these two heap
pointers are stored at unaligned offsets. kmemleak only scans memory on
pointer-aligned boundaries, so it never sees the pointers and reports
the arrays as unreferenced.

Unlike the UAPI struct uvc_extension_unit_descriptor, this is an
in-memory staging structure: baSourceID and bmControls are pointers,
not inline arrays, and the wire descriptor is assembled field by field
in UVC_COPY_XU_DESCRIPTOR(). So __packed is not needed for layout
correctness and only serves to misalign the pointers.

Drop __packed and move the two remaining scalar members (bControlSize
and iExtension) ahead of the pointers so that baSourceID lands on a
natural 8-byte boundary. This keeps the pointers aligned and visible to
kmemleak while leaving the structure hole-free and the same size as
before (40 bytes on 64-bit). The bLength..bNrInPins prefix is unchanged,
so the "memcpy(dst, desc, 22)" in UVC_COPY_XU_DESCRIPTOR() and the wire
descriptor layout are unaffected.

Reported-by: syzbot+54927260acba030187a6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=54927260acba030187a6
Fixes: 0525210c9840 ("usb: gadget: uvc: Allow definition of XUs in configfs")
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rituparna Warwatkar <rwarwatkar@gmail.com>
---
Changes in v2:
 - Rather than only dropping __packed (which left padding holes and
   grew the struct), reorder the members so bControlSize and
   iExtension precede the two pointers, grouping baSourceID and
   bmControls at a natural 8-byte boundary. The struct stays 40 bytes
   with no padding, and the bLength..bNrInPins prefix (and thus
   UVC_COPY_XU_DESCRIPTOR() and the wire layout) is unchanged.

 drivers/usb/gadget/function/uvc_configfs.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/gadget/function/uvc_configfs.h b/drivers/usb/gadget/function/uvc_configfs.h
index 9391614135e..049fb9e14e4 100644
--- a/drivers/usb/gadget/function/uvc_configfs.h
+++ b/drivers/usb/gadget/function/uvc_configfs.h
@@ -172,11 +172,11 @@ struct uvcg_extension_unit_descriptor {
        u8 guidExtensionCode[16];
        u8 bNumControls;
        u8 bNrInPins;
-       u8 *baSourceID;
        u8 bControlSize;
-       u8 *bmControls;
        u8 iExtension;
-} __packed;
+       u8 *baSourceID;
+       u8 *bmControls;
+};

 struct uvcg_extension {
        struct config_item item;
--
2.47.3