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

From: Greg Kroah-Hartman <[email protected]>

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.

The Linux kernel CVE team has assigned CVE-2026-64496 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 3.15 with commit b91accafbb1031b80d22ad83576877ff2f8b4774 and fixed in 5.10.261 with commit 9dc84ba4be5bbeb29ee49efe6cea2cb32c461424
	Issue introduced in 3.15 with commit b91accafbb1031b80d22ad83576877ff2f8b4774 and fixed in 5.15.212 with commit d16a702ca7d29c0b7a9b509339d1b044a1cadb32
	Issue introduced in 3.15 with commit b91accafbb1031b80d22ad83576877ff2f8b4774 and fixed in 6.1.178 with commit a13ef1adbc62085b21b546b07b0be7e2fbf52150
	Issue introduced in 3.15 with commit b91accafbb1031b80d22ad83576877ff2f8b4774 and fixed in 6.6.145 with commit 0d4a646d7f87ea3625fafe387043fddc6a2f5e7f
	Issue introduced in 3.15 with commit b91accafbb1031b80d22ad83576877ff2f8b4774 and fixed in 6.12.96 with commit 72c6aa8e0d74eab91b8694cde97dec088c248fee
	Issue introduced in 3.15 with commit b91accafbb1031b80d22ad83576877ff2f8b4774 and fixed in 6.18.39 with commit 9edefd4c56bee3fe331e0355d1f10a533134999d
	Issue introduced in 3.15 with commit b91accafbb1031b80d22ad83576877ff2f8b4774 and fixed in 7.1.4 with commit f187dc5a4c4846ffa07d9bda6e760837ed005574
	Issue introduced in 3.15 with commit b91accafbb1031b80d22ad83576877ff2f8b4774 and fixed in 7.2-rc3 with commit af791d295737ea6b6ff2c8d8488462a49c14af01

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64496
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/iio/industrialio-event.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	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