linux-hardening - [PATCH] sched/core: Do not treat class list boundary markers as arrays

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Date:   Mon, 16 May 2022 12:42:41 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Christophe de Dinechin <dinechin@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH] sched/core: Do not treat class list boundary markers as arrays

GCC 12 is very sensitive about array checking, and views all negative
array accesses as unsafe (a not unreasonable position). Avoid the
warnings about __begin_sched_classes being accessed via negative bounds
by converting them to the pointers they actually are. Silences this
warning:

In file included from kernel/sched/core.c:81:
kernel/sched/core.c: In function ‘set_rq_online.part.0’:
kernel/sched/sched.h:2197:52: error: array subscript -1 is outside array bounds of ‘struct sched_class[44343134792571037]’
 [-Werror=array-bounds]
 2197 | #define sched_class_lowest  (__begin_sched_classes - 1)
      |                                                    ^
kernel/sched/sched.h:2200:41: note: in definition of macro ‘for_class_range’
 2200 |         for (class = (_from); class != (_to); class--)
      |                                         ^~~
kernel/sched/sched.h:2203:53: note: in expansion of macro ‘sched_class_lowest’
 2203 |for_class_range(class, sched_class_highest, sched_class_lowest)
      |                                            ^~~~~~~~~~~~~~~~~~
kernel/sched/core.c:9115:17: note: in expansion of macro ‘for_each_class’
 9115 |                 for_each_class(class) {
      |                 ^~~~~~~~~~~~~~
kernel/sched/sched.h:2193:27: note: at offset -208 into object ‘__begin_sched_classes’ of size [0, 9223372036854775807]
 2193 | extern struct sched_class __begin_sched_classes[];
      |                           ^~~~~~~~~~~~~~~~~~~~~

Reported-by: Christophe de Dinechin <dinechin@...hat.com>
Link: https://lore.kernel.org/lkml/20220414150855.2407137-2-dinechin@redhat.com/
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 kernel/sched/sched.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 8dccb34eb190..3d31ed9d33fa 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2190,8 +2190,8 @@ const struct sched_class name##_sched_class \
 	__section("__" #name "_sched_class")
 
 /* Defined in include/asm-generic/vmlinux.lds.h */
-extern struct sched_class __begin_sched_classes[];
-extern struct sched_class __end_sched_classes[];
+extern struct sched_class *__begin_sched_classes;
+extern struct sched_class *__end_sched_classes;
 
 #define sched_class_highest (__end_sched_classes - 1)
 #define sched_class_lowest  (__begin_sched_classes - 1)
-- 
2.32.0