CVE-2024-27405

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: ncm: Avoid dropping datagrams of properly parsed NTBs

It is observed sometimes when tethering is used over NCM with Windows 11 as host, at some instances, the gadget_giveback has one byte appended at the end of a proper NTB. When the NTB is parsed, unwrap call looks for any leftover bytes in SKB provided by u_ether and if there are any pending bytes, it treats them as a separate NTB and parses it. But in case the second NTB (as per unwrap call) is faulty/corrupt, all the datagrams that were parsed properly in the first NTB and saved in rx_list are dropped.

Adding a few custom traces showed the following: [002] d..1 7828.532866: dwc3_gadget_giveback: ep1out: req 000000003868811a length 1025/16384 zsI ==> 0 [002] d..1 7828.532867: ncm_unwrap_ntb: K: ncm_unwrap_ntb toprocess: 1025 [002] d..1 7828.532867: ncm_unwrap_ntb: K: ncm_unwrap_ntb nth: 1751999342 [002] d..1 7828.532868: ncm_unwrap_ntb: K: ncm_unwrap_ntb seq: 0xce67 [002] d..1 7828.532868: ncm_unwrap_ntb: K: ncm_unwrap_ntb blk_len: 0x400 [002] d..1 7828.532868: ncm_unwrap_ntb: K: ncm_unwrap_ntb ndp_len: 0x10 [002] d..1 7828.532869: ncm_unwrap_ntb: K: Parsed NTB with 1 frames

In this case, the giveback is of 1025 bytes and block length is 1024. The rest 1 byte (which is 0x00) wont be parsed resulting in drop of all datagrams in rx_list.

Same is case with packets of size 2048: [002] d..1 7828.557948: dwc3_gadget_giveback: ep1out: req 0000000011dfd96e length 2049/16384 zsI ==> 0 [002] d..1 7828.557949: ncm_unwrap_ntb: K: ncm_unwrap_ntb nth: 1751999342 [002] d..1 7828.557950: ncm_unwrap_ntb: K: ncm_unwrap_ntb blk_len: 0x800

Lecroy shows one byte coming in extra confirming that the byte is coming in from PC:

Transfer 2959 - Bytes Transferred(1025) Timestamp((18.524 843 590)

• Transaction 8391 - Data(1025 bytes) Timestamp(18.524 843 590) — Packet 4063861 Data(1024 bytes) Duration(2.117us) Idle(14.700ns) Timestamp(18.524 843 590) — Packet 4063863 Data(1 byte) Duration(66.160ns) Time(282.000ns) Timestamp(18.524 845 722)

According to Windows driver, no ZLP is needed if wBlockLength is non-zero, because the non-zero wBlockLength has already told the function side the size of transfer to be expected. However, there are in-market NCM devices that rely on ZLP as long as the wBlockLength is multiple of wMaxPacketSize. To deal with such devices, it pads an extra 0 at end so the transfer is no longer multiple of wMaxPacketSize.

Weakness

A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/059285e04ebb273d32323fbad5431c5b94f77e48
• https://git.kernel.org/stable/c/2b7ec68869d50ea998908af43b643bca7e54577e
• https://git.kernel.org/stable/c/2cb66b62a5d64ccf09b0591ab86fb085fa491fc5
• https://git.kernel.org/stable/c/35b604a37ec70d68b19dafd10bbacf1db505c9ca
• https://git.kernel.org/stable/c/57ca0e16f393bb21d69734e536e383a3a4c665fd
• https://git.kernel.org/stable/c/76c51146820c5dac629f21deafab0a7039bc3ccd
• https://git.kernel.org/stable/c/a31cf46d108dabce3df80b3e5c07661e24912151
• https://git.kernel.org/stable/c/c7f43900bc723203d7554d299a2ce844054fab8e
• https://lists.debian.org/debian-lts-announce/2024/06/msg00017.html
• https://lists.debian.org/debian-lts-announce/2024/06/msg00020.html