CVE-2025-21655: io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period

CVE-2025-21655: io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period

[Greg Kroah-Hartman]

Description
===========

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

io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period

io_eventfd_do_signal() is invoked from an RCU callback, but when
dropping the reference to the io_ev_fd, it calls io_eventfd_free()
directly if the refcount drops to zero. This isn't correct, as any
potential freeing of the io_ev_fd should be deferred another RCU grace
period.

Just call io_eventfd_put() rather than open-code the dec-and-test and
free, which will correctly defer it another RCU grace period.

The Linux kernel CVE team has assigned CVE-2025-21655 to this issue.


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

	Issue introduced in 6.1 with commit 21a091b970cdbcf3e8ff829234b51be6f9192766 and fixed in 6.1.125 with commit 6b63308c28987c6010b1180c72a6db4df6c68033
	Issue introduced in 6.1 with commit 21a091b970cdbcf3e8ff829234b51be6f9192766 and fixed in 6.6.72 with commit 8efff2aa2d95dc437ab67c5b4a9f1d3f367baa10
	Issue introduced in 6.1 with commit 21a091b970cdbcf3e8ff829234b51be6f9192766 and fixed in 6.12.10 with commit a7085c3ae43b86d4b3d1b8275e6a67f14257e3b7
	Issue introduced in 6.1 with commit 21a091b970cdbcf3e8ff829234b51be6f9192766 and fixed in 6.13 with commit c9a40292a44e78f71258b8522655bffaf5753bdb

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-2025-21655
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:
	io_uring/eventfd.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/6b63308c28987c6010b1180c72a6db4df6c68033
	https://git.kernel.org/stable/c/8efff2aa2d95dc437ab67c5b4a9f1d3f367baa10
	https://git.kernel.org/stable/c/a7085c3ae43b86d4b3d1b8275e6a67f14257e3b7
	https://git.kernel.org/stable/c/c9a40292a44e78f71258b8522655bffaf5753bdb