CVE-2026-89771: ring-buffer: Fix subbuf resize race with ring buffer readers

CVE-2026-89771: ring-buffer: Fix subbuf resize race with ring buffer readers

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

ring-buffer: Fix subbuf resize race with ring buffer readers

trace_buffer subbuf_size is read lockless in ring_buffer_read_page() and
ring_buffer_read_start(), while it can simultaneously be resized with
ring_buffer_subbuf_order_set().

Instead of trace_buffer::subbuf_size, use bpage::order in
ring_buffer_read_start() and ring_buffer_read_page().

In ring_buffer_read_start(), even with resize_disabled, there is still a
possibility of a race with a buffer modification. Hold the trace_buffer
mutex to synchronise with any pending ring buffer order modification.

trace_buffer::subbuf_size is now actually useless, remove it. Also,
create accessors rb_subbuf_capacity() and rb_page_capacity() which
return the actual size available for storing events, while
rb_subbuf_size() returns the actual subbuf page-size.

The Linux kernel CVE team has assigned CVE-2026-89771 to this issue.


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

	Issue introduced in 6.8 with commit f9b94daa542a8d2532f0930f01cd9aec2d19621b and fixed in 6.12.109 with commit 6d666f0b8b36c0765cf19fbe6de5a7ba5a73aad9
	Issue introduced in 6.8 with commit f9b94daa542a8d2532f0930f01cd9aec2d19621b and fixed in 6.18.50 with commit 50f4a793c4ff24826efb0ac700989a5063cc53df
	Issue introduced in 6.8 with commit f9b94daa542a8d2532f0930f01cd9aec2d19621b and fixed in 7.2.4 with commit 0c7c517827a453128a8f58bfd849546a29813b27
	Issue introduced in 6.8 with commit f9b94daa542a8d2532f0930f01cd9aec2d19621b and fixed in 7.3-rc1 with commit 8a5f63637890f03177146efddaba5ec7a1b4d61f

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-2026-89771
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:
	kernel/trace/ring_buffer.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/6d666f0b8b36c0765cf19fbe6de5a7ba5a73aad9
	https://git.kernel.org/stable/c/50f4a793c4ff24826efb0ac700989a5063cc53df
	https://git.kernel.org/stable/c/0c7c517827a453128a8f58bfd849546a29813b27
	https://git.kernel.org/stable/c/8a5f63637890f03177146efddaba5ec7a1b4d61f