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

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.

https://git.kernel.org/stable/c/0d4a646d7f87ea3625fafe387043fddc6a2f5e7f

https://git.kernel.org/stable/c/72c6aa8e0d74eab91b8694cde97dec088c248fee

https://git.kernel.org/stable/c/9dc84ba4be5bbeb29ee49efe6cea2cb32c461424

https://git.kernel.org/stable/c/9edefd4c56bee3fe331e0355d1f10a533134999d

https://git.kernel.org/stable/c/a13ef1adbc62085b21b546b07b0be7e2fbf52150

https://git.kernel.org/stable/c/af791d295737ea6b6ff2c8d8488462a49c14af01

https://git.kernel.org/stable/c/d16a702ca7d29c0b7a9b509339d1b044a1cadb32

https://git.kernel.org/stable/c/f187dc5a4c4846ffa07d9bda6e760837ed005574

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