From: Lars-Peter Clausen Date: Tue, 7 Jul 2026 04:48:26 +0000 (-0700) Subject: iio: event: Fix event FIFO reset race X-Git-Tag: v7.2-rc3~3^2^2 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=af791d295737ea6b6ff2c8d8488462a49c14af01;p=thirdparty%2Fkernel%2Flinux.git iio: event: Fix event FIFO reset race `iio_event_getfd()` creates the event file descriptor with `anon_inode_getfd()`, which allocates a new fd, creates the anonymous file and installs it in the process fd table before returning to the caller. The IIO code resets the event FIFO after `anon_inode_getfd()` has returned, but before `IIO_GET_EVENT_FD_IOCTL` has copied the fd number to userspace. But since fd tables are shared between threads, another thread can guess the newly allocated fd number and issue a `read()` on it as soon as the fd has been installed. This means the `kfifo_to_user()` in `iio_event_chrdev_read()` can run in parallel with the `kfifo_reset_out()` in `iio_event_getfd()`. The kfifo documentation says that `kfifo_reset_out()` is only safe when it is called from the reader thread and there is only one concurrent reader. Otherwise it is dangerous and must be handled in the same way as `kfifo_reset()`. If that happens, `kfifo_to_user()` can advance the FIFO `out` index based on state from before the reset, after the reset has already moved the `out` index to the current `in` index. That can leave the FIFO with an `out` index past the `in` index. A later `read()` can then see an underflowed FIFO length and copy more data than the event FIFO buffer contains. This can result in an out-of-bounds read and leak adjacent kernel memory to userspace. Move the FIFO reset before `anon_inode_getfd()`. At that point the event fd is marked busy, but the new fd has not been installed yet, so userspace cannot access it while the FIFO is reset. Fixes: b91accafbb10 ("iio:event: Fix and cleanup locking") Reported-by: Codex:gpt-5.5 Signed-off-by: Lars-Peter Clausen Reviewed-by: Nuno Sá Cc: Signed-off-by: Jonathan Cameron --- diff --git a/drivers/iio/industrialio-event.c b/drivers/iio/industrialio-event.c index a0d6fcf2a9c9..e6730f52262a 100644 --- a/drivers/iio/industrialio-event.c +++ b/drivers/iio/industrialio-event.c @@ -207,6 +207,8 @@ static int iio_event_getfd(struct iio_dev *indio_dev) goto unlock; } + kfifo_reset_out(&ev_int->det_events); + iio_device_get(indio_dev); fd = anon_inode_getfd("iio:event", &iio_event_chrdev_fileops, @@ -214,10 +216,7 @@ static int iio_event_getfd(struct iio_dev *indio_dev) if (fd < 0) { clear_bit(IIO_BUSY_BIT_POS, &ev_int->flags); iio_device_put(indio_dev); - } else { - kfifo_reset_out(&ev_int->det_events); } - unlock: mutex_unlock(&iio_dev_opaque->mlock); return fd;