On 1/31/25 23:12, Kees Cook wrote: > On Fri, Jan 31, 2025 at 06:15:58PM +0300, Nikita Zhandarovich wrote: >> Syzbot reports [1] a reemerging out-of-bounds bug regarding hid >> descriptors supposedly having unpredictable bNumDescriptors values in >> usbhid_parse(). >> >> The issue stems from the fact that hid_class_descriptor is supposed >> to be a flexible array, however it was sized as desc[1], using only >> one element. Therefore, going beyond 1 element, courtesy of >> bNumDescriptors, may lead to an error. >> >> Modify struct hid_descriptor by employing __counted_by macro, tying >> together struct hid_class_descriptor desc[] and number of descriptors >> bNumDescriptors. Also, fix places where this change affects work with >> newly updated struct. >> >> [1] Syzbot report: >> >> UBSAN: array-index-out-of-bounds in drivers/hid/usbhid/hid-core.c:1024:7 >> index 1 is out of range for type 'struct hid_class_descriptor[1]' >> ... >> Workqueue: usb_hub_wq hub_event >> Call Trace: >> <TASK> >> __dump_stack lib/dump_stack.c:88 [inline] >> dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114 >> ubsan_epilogue lib/ubsan.c:231 [inline] >> __ubsan_handle_out_of_bounds+0x121/0x150 lib/ubsan.c:429 >> usbhid_parse+0x5a7/0xc80 drivers/hid/usbhid/hid-core.c:1024 >> hid_add_device+0x132/0x520 drivers/hid/hid-core.c:2790 >> usbhid_probe+0xb38/0xea0 drivers/hid/usbhid/hid-core.c:1429 >> usb_probe_interface+0x645/0xbb0 drivers/usb/core/driver.c:399 >> really_probe+0x2b8/0xad0 drivers/base/dd.c:656 >> __driver_probe_device+0x1a2/0x390 drivers/base/dd.c:798 >> driver_probe_device+0x50/0x430 drivers/base/dd.c:828 >> __device_attach_driver+0x2d6/0x530 drivers/base/dd.c:956 >> bus_for_each_drv+0x24e/0x2e0 drivers/base/bus.c:457 >> __device_attach+0x333/0x520 drivers/base/dd.c:1028 >> bus_probe_device+0x189/0x260 drivers/base/bus.c:532 >> device_add+0x8ff/0xca0 drivers/base/core.c:3720 >> usb_set_configuration+0x1976/0x1fb0 drivers/usb/core/message.c:2210 >> usb_generic_driver_probe+0x88/0x140 drivers/usb/core/generic.c:254 >> usb_probe_device+0x1b8/0x380 drivers/usb/core/driver.c:294 >> >> Reported-by: syzbot+c52569baf0c843f35495@xxxxxxxxxxxxxxxxxxxxxxxxx >> Closes: https://syzkaller.appspot.com/bug?extid=c52569baf0c843f35495 >> Fixes: f043bfc98c19 ("HID: usbhid: fix out-of-bounds bug") >> Cc: stable@xxxxxxxxxxxxxxx >> Signed-off-by: Nikita Zhandarovich <n.zhandarovich@xxxxxxxxxx> >> --- >> v1: https://lore.kernel.org/all/20240524120112.28076-1-n.zhandarovich@xxxxxxxxxx/ >> >> v2: Instead of essentially forcing usbhid_parse() to only check >> the first descriptor, modify hid_descriptor struct to anticipate >> multiple hid_class_descriptor in desc[] by utilizing __counted_by >> macro. This change, in turn, requires several other ones wherever >> that struct is used. Adjust commit description accordingly. >> >> P.S. Since syzbot currently breaks trying to reproduce the issue, >> with or without this patch, I only managed to test it locally with >> syz repros. Would greatly appreciate other people's effort to test >> it as well. >> >> P.P.S. Terry Junge <linuxhid@xxxxxxxxxxxxxxxxxxxxxx> suggested a >> different approach to this issue, see: >> Link: https://lore.kernel.org/all/f7963a1d-e069-4ec9-82a1-e17fd324a44f@xxxxxxxxxxxxxxxxxxxxxx/ >> >> drivers/hid/usbhid/hid-core.c | 2 +- >> drivers/usb/gadget/function/f_fs.c | 3 ++- >> drivers/usb/gadget/function/f_hid.c | 22 ++++++++++++++-------- >> include/linux/hid.h | 2 +- >> 4 files changed, 18 insertions(+), 11 deletions(-) >> >> diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c >> index a6eb6fe6130d..eb4807785d6d 100644 >> --- a/drivers/hid/usbhid/hid-core.c >> +++ b/drivers/hid/usbhid/hid-core.c >> @@ -1010,7 +1010,7 @@ static int usbhid_parse(struct hid_device *hid) >> return -ENODEV; >> } >> >> - if (hdesc->bLength < sizeof(struct hid_descriptor)) { >> + if (hdesc->bLength < struct_size(hdesc, desc, hdesc->bNumDescriptors)) { >> dbg_hid("hid descriptor is too short\n"); >> return -EINVAL; >> } > > I don't think you want this hunk in the patch. The existing logic will > correctly adapt num_descriptors to a size that fits within > hdesc->bLength. In theory, the above change could break a weird but > working device that reported too high bNumDescriptors but still worked > with what num_descriptors walks. > Thank you for mentioning this and you are right about possibly breaking a working device. However, sizeof(struct hid_descriptor) doesn't count flex array sizes. So, leaving this check as is will miss cases when hdesc->bLength >= sizeof(struct hid_descriptor) but short enough to have a desc[0] element. Maybe doing struct_size(hdesc, desc, 1) is better? Then we make sure that at least one mandatory hid_class_desriptor is there. >> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c >> index 2dea9e42a0f8..a4b6d7cbf56d 100644 >> --- a/drivers/usb/gadget/function/f_fs.c >> +++ b/drivers/usb/gadget/function/f_fs.c >> @@ -2550,7 +2550,8 @@ static int __must_check ffs_do_single_desc(char *data, unsigned len, >> case USB_TYPE_CLASS | 0x01: >> if (*current_class == USB_INTERFACE_CLASS_HID) { >> pr_vdebug("hid descriptor\n"); >> - if (length != sizeof(struct hid_descriptor)) >> + if (length < sizeof(struct hid_descriptor) + >> + sizeof(struct hid_class_descriptor)) >> goto inv_length; >> break; >> } else if (*current_class == USB_INTERFACE_CLASS_CCID) { > > Same here, I think? This isn't needed unless I'm misunderstanding > something about the fix. Once again, sizeof(struct hid_descriptor) will not count struct hid_class_descriptor desc[] __counted_by(...) so even correct and predictable lengths will fail the check. We need to test length against hid_descriptor with at least one element of its flex array. I would prefer struct_size() here as well but it's not optimal in this case. > >> diff --git a/drivers/usb/gadget/function/f_hid.c b/drivers/usb/gadget/function/f_hid.c >> index 740311c4fa24..ec8c2e2d6812 100644 >> --- a/drivers/usb/gadget/function/f_hid.c >> +++ b/drivers/usb/gadget/function/f_hid.c >> @@ -139,13 +139,17 @@ static struct usb_interface_descriptor hidg_interface_desc = { >> }; >> >> static struct hid_descriptor hidg_desc = { >> - .bLength = sizeof hidg_desc, >> + .bLength = struct_size(&hidg_desc, desc, 1), >> .bDescriptorType = HID_DT_HID, >> .bcdHID = cpu_to_le16(0x0101), >> .bCountryCode = 0x00, >> .bNumDescriptors = 0x1, >> - /*.desc[0].bDescriptorType = DYNAMIC */ >> - /*.desc[0].wDescriptorLenght = DYNAMIC */ >> + .desc = { >> + { >> + .bDescriptorType = 0, /* DYNAMIC */ >> + .wDescriptorLength = 0, /* DYNAMIC */ >> + } >> + } >> }; >> >> /* Super-Speed Support */ >> @@ -936,16 +940,18 @@ static int hidg_setup(struct usb_function *f, >> switch (value >> 8) { >> case HID_DT_HID: >> { >> - struct hid_descriptor hidg_desc_copy = hidg_desc; >> + DEFINE_FLEX(struct hid_descriptor, hidg_desc_copy, >> + desc, bNumDescriptors, 1); >> + *hidg_desc_copy = hidg_desc; >> >> VDBG(cdev, "USB_REQ_GET_DESCRIPTOR: HID\n"); >> - hidg_desc_copy.desc[0].bDescriptorType = HID_DT_REPORT; >> - hidg_desc_copy.desc[0].wDescriptorLength = >> + hidg_desc_copy->desc[0].bDescriptorType = HID_DT_REPORT; >> + hidg_desc_copy->desc[0].wDescriptorLength = >> cpu_to_le16(hidg->report_desc_length); >> >> length = min_t(unsigned short, length, >> - hidg_desc_copy.bLength); >> - memcpy(req->buf, &hidg_desc_copy, length); >> + hidg_desc_copy->bLength); >> + memcpy(req->buf, hidg_desc_copy, length); >> goto respond; >> break; >> } >> diff --git a/include/linux/hid.h b/include/linux/hid.h >> index cdc0dc13c87f..85a58ae2c4a0 100644 >> --- a/include/linux/hid.h >> +++ b/include/linux/hid.h >> @@ -739,7 +739,7 @@ struct hid_descriptor { >> __u8 bCountryCode; >> __u8 bNumDescriptors; >> >> - struct hid_class_descriptor desc[1]; >> + struct hid_class_descriptor desc[] __counted_by(bNumDescriptors); >> } __attribute__ ((packed)); >> >> #define HID_DEVICE(b, g, ven, prod) \ > > Otherwise, this looks correct to me. > Regards, Nikita
