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.

Reported-by: syzbot+cb96ec476fb4914445c9@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=cb96ec476fb4914445c9 Fixes: ed9eccbe8970 ("Staging: add comedi core") Cc: stable@vger.kernel.org # 5.13+ Signed-off-by: Ian Abbott abbotti@mev.co.uk --- Patch does not apply cleanly to longterm kernels 5.4.x and 5.10.x. --- drivers/comedi/drivers.c | 3 +++ 1 file changed, 3 insertions(+)

diff --git a/drivers/comedi/drivers.c b/drivers/comedi/drivers.c index 376130bfba8a..98e565088289 100644 --- a/drivers/comedi/drivers.c +++ b/drivers/comedi/drivers.c @@ -615,6 +615,9 @@ static int insn_rw_emulate_bits(struct comedi_device *dev, unsigned int _data[2]; int ret;

+ if (insn->n == 0) + return 0; + memset(_data, 0, sizeof(_data)); memset(&_insn, 0, sizeof(_insn)); _insn.insn = INSN_BITS;