From: Ian Abbott Date: Mon, 7 Jul 2025 15:33:54 +0000 (+0100) Subject: comedi: Fix use of uninitialized data in insn_rw_emulate_bits() X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=e9cb26291d009243a4478a7ffb37b3a9175bfce9;p=users%2Fdwmw2%2Flinux.git 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. 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 Link: https://lore.kernel.org/r/20250707153355.82474-1-abbotti@mev.co.uk Signed-off-by: Greg Kroah-Hartman --- diff --git a/drivers/comedi/drivers.c b/drivers/comedi/drivers.c index 922fe20738ef0..9e4b7c840a8f5 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;