CVE-2025-39684

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

comedi: Fix use of uninitialized memory in do_insn_ioctl() and do_insnlist_ioctl()

syzbot reports a KMSAN kernel-infoleak in `do_insn_ioctl()`. A kernel

buffer is allocated to hold `insn->n` samples (each of which is an

`unsigned int`). For some instruction types, `insn->n` samples are

copied back to user-space, unless an error code is being returned. The

problem is that not all the instruction handlers that need to return

data to userspace fill in the whole `insn->n` samples, so that there is

an information leak. There is a similar syzbot report for

`do_insnlist_ioctl()`, although it does not have a reproducer for it at

the time of writing.

One culprit is `insn_rw_emulate_bits()` which is used as the handler for

`INSN_READ` or `INSN_WRITE` instructions for subdevices that do not have

a specific handler for that instruction, but do have an `INSN_BITS`

handler. For `INSN_READ` it only fills in at most 1 sample, so if

`insn->n` is greater than 1, the remaining `insn->n - 1` samples copied

to userspace will be uninitialized kernel data.

Another culprit is `vm80xx_ai_insn_read()` in the "vm80xx" driver. It

never returns an error, even if it fails to fill the buffer.

Fix it in `do_insn_ioctl()` and `do_insnlist_ioctl()` by making sure

that uninitialized parts of the allocated buffer are zeroed before

handling each instruction.

Thanks to Arnaud Lecomte for their fix to `do_insn_ioctl()`. That fix

replaced the call to `kmalloc_array()` with `kcalloc()`, but it is not

always necessary to clear the whole buffer.