CVE-2026-74633: tracing: Fix NULL pointer dereference in module event cache removal

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

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

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

tracing: Fix NULL pointer dereference in module event cache removal

A module-only event filter such as ":mod:foo" is cached with a NULL
event_mod->match when foo has not been loaded. If a later write tries to
remove a specific match from the same module, remove_cache_mod() passes
the NULL cached match to strcmp(), causing a NULL pointer dereference.

The issue can be reproduced from userspace:

  echo ':mod:trace_events_kunit_missing' > /sys/kernel/tracing/set_event
  echo '!foo_bar:mod:trace_events_kunit_missing' >> /sys/kernel/tracing/set_event

The second write must be a concatenation (">>") to not include O_TRUNC as
that would cause ftrace_clear_events() to clear the cached modules lines.

The crash was reproduced on x86_64 QEMU while KUnit workers contended on
the event tracing path:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  RIP: 0010:strcmp+0x10/0x30
  Call Trace:
   __ftrace_set_clr_event_nolock+0x373/0x4a0
   ftrace_set_clr_event+0xf0/0x180
   ftrace_event_write+0xdf/0x110
   vfs_write+0xf6/0x440
   ksys_write+0x68/0xe0
   do_syscall_64+0xf9/0x540
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Check event_mod->match before comparing it, consistent with the existing
NULL checks for the cached system and event fields. The mismatched removal
continues to return -EINVAL; a broad cached module filter is removed with
"!:mod:<module>".

The Linux kernel CVE team has assigned CVE-2026-74633 to this issue.


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

	Issue introduced in 6.14 with commit b355247df104ef6644288884afd2c08b7bf49897 and fixed in 6.18.45 with commit 152a00440dc6ef62c6e4cf9bd881a7e6bb81fda9
	Issue introduced in 6.14 with commit b355247df104ef6644288884afd2c08b7bf49897 and fixed in 7.1.9 with commit ad4e9dd5fec7a322a471ff36fb9c76214e3c6992
	Issue introduced in 6.14 with commit b355247df104ef6644288884afd2c08b7bf49897 and fixed in 7.2 with commit b69859204d4db3acd86c1c2dadcef0d77b451933

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-2026-74633
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/trace_events.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/152a00440dc6ef62c6e4cf9bd881a7e6bb81fda9
	https://git.kernel.org/stable/c/ad4e9dd5fec7a322a471ff36fb9c76214e3c6992
	https://git.kernel.org/stable/c/b69859204d4db3acd86c1c2dadcef0d77b451933