CVE-2026-64496iio: event: Fix event FIFO reset race

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/9dc84ba4be5bbeb29ee49efe6cea2cb32c461424
• https://git.kernel.org/stable/c/d16a702ca7d29c0b7a9b509339d1b044a1cadb32
• https://git.kernel.org/stable/c/a13ef1adbc62085b21b546b07b0be7e2fbf52150
• https://git.kernel.org/stable/c/0d4a646d7f87ea3625fafe387043fddc6a2f5e7f
• https://git.kernel.org/stable/c/72c6aa8e0d74eab91b8694cde97dec088c248fee
• https://git.kernel.org/stable/c/9edefd4c56bee3fe331e0355d1f10a533134999d
• https://git.kernel.org/stable/c/f187dc5a4c4846ffa07d9bda6e760837ed005574
• https://git.kernel.org/stable/c/af791d295737ea6b6ff2c8d8488462a49c14af01