From: Ian Abbott abbotti@mev.co.uk
commit 3cd212e895ca2d58963fdc6422502b10dd3966bb upstream.
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.
Fixes: ed9eccbe8970 ("Staging: add comedi core") Reported-by: syzbot+a5e45f768aab5892da5d@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=a5e45f768aab5892da5d Reported-by: syzbot+fb4362a104d45ab09cf9@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=fb4362a104d45ab09cf9 Cc: stable stable@kernel.org # 5.13+ Cc: Arnaud Lecomte contact@arnaud-lcm.com Signed-off-by: Ian Abbott abbotti@mev.co.uk Link: https://lore.kernel.org/r/20250725125324.80276-1-abbotti@mev.co.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org [Andrey Troshin: backport fix from drivers/comedi/comedi_fops.c to drivers/staging/comedi/comedi_fops.c] Signed-off-by: Andrey Troshin drtrosh@yandex-team.ru --- Backport fix for CVE-2025-39684 Link: https://nvd.nist.gov/vuln/detail/CVE-2025-39684 --- drivers/staging/comedi/comedi_fops.c | 5 +++++ 1 file changed, 5 insertions(+)
diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c index 854b8bdc57a1..0af6e4a2fad9 100644 --- a/drivers/staging/comedi/comedi_fops.c +++ b/drivers/staging/comedi/comedi_fops.c @@ -1582,6 +1582,9 @@ static int do_insnlist_ioctl(struct comedi_device *dev, memset(&data[n], 0, (MIN_SAMPLES - n) * sizeof(unsigned int)); } + } else { + memset(data, 0, max_t(unsigned int, n, MIN_SAMPLES) * + sizeof(unsigned int)); } ret = parse_insn(dev, insns + i, data, file); if (ret < 0) @@ -1665,6 +1668,8 @@ static int do_insn_ioctl(struct comedi_device *dev, memset(&data[insn->n], 0, (MIN_SAMPLES - insn->n) * sizeof(unsigned int)); } + } else { + memset(data, 0, n_data * sizeof(unsigned int)); } ret = parse_insn(dev, insn, data, file); if (ret < 0)