CVE-2025-38480comedi: Fix use of uninitialized data in insn_rw_emulate_bits()

Official description

In the Linux kernel, the following vulnerability has been resolved: comedi: Fix use of uninitialized data in insn_rw_emulate_bits() For Comedi `INSN_READ` and `INSN_WRITE` instructions on "digital" subdevices (subdevice types `COMEDI_SUBD_DI`, `COMEDI_SUBD_DO`, and `COMEDI_SUBD_DIO`), it is common for the subdevice driver not to have `insn_read` and `insn_write` handler functions, but to have an `insn_bits` handler function for handling Comedi `INSN_BITS` instructions. In that case, the subdevice's `insn_read` and/or `insn_write` function handler pointers are set to point to the `insn_rw_emulate_bits()` function by `__comedi_device_postconfig()`. For `INSN_WRITE`, `insn_rw_emulate_bits()` currently assumes that the supplied `data[0]` value is a valid copy from user memory. It will at least exist because `do_insnlist_ioctl()` and `do_insn_ioctl()` in "comedi_fops.c" ensure at lease `MIN_SAMPLES` (16) elements are allocated. However, if `insn->n` is 0 (which is allowable for `INSN_READ` and `INSN_WRITE` instructions, then `data[0]` may contain uninitialized data, and certainly contains invalid data, possibly from a different instruction in the array of instructions handled by `do_insnlist_ioctl()`. This will result in an incorrect value being written to the digital output channel (or to the digital input/output channel if configured as an output), and may be reflected in the internal saved state of the channel. Fix it by returning 0 early if `insn->n` is 0, before reaching the code that accesses `data[0]`. Previously, the function always returned 1 on success, but it is supposed to be the number of data samples actually read or written up to `insn->n`, which is 0 in this case.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/4c2981bf30401adfcdbfece4ab6f411f7c5875a1
• https://git.kernel.org/stable/c/16256d7efcf7acc9f39abe21522c4c6b77f67c00
• https://git.kernel.org/stable/c/c53570e62b5b28bdb56bb563190227f8307817a5
• https://git.kernel.org/stable/c/3050d197d6bc9ef128944a70210f42d2430b3000
• https://git.kernel.org/stable/c/10f9024a8c824a41827fff1fefefb314c98e2c88
• https://git.kernel.org/stable/c/2af1e7d389c2619219171d23f5b96dbcbb7f9656
• https://git.kernel.org/stable/c/3ab55ffaaf75d0c7b68e332c1cdcc1b0e0044870
• https://git.kernel.org/stable/c/e9cb26291d009243a4478a7ffb37b3a9175bfce9
• https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html
• https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html