exit: Use READ_ONCE() for all oops/warn limit reads

commit 7535b83 upstream.

Use a temporary variable to take full advantage of READ_ONCE() behavior.
Without this, the report (and even the test) might be out of sync with
the initial test.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/lkml/Y5x7GXeluFmZ8E0E@hirez.programming.kicks-ass.net
Fixes: 9fc9e27 ("panic: Introduce warn_limit")
Fixes: d4ccd54 ("exit: Put an upper limit on how often we can oops")
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Jann Horn <jannh@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: tangmeng <tangmeng@uniontech.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/exit.c

@@ -973,6 +973,7 @@ void __noreturn make_task_dead(int signr)
 	 * Take the task off the cpu after something catastrophic has
 	 * happened.
 	 */
+	unsigned int limit;
 
 	/*
 	 * Every time the system oopses, if the oops happens while a reference
@@ -984,8 +985,9 @@ void __noreturn make_task_dead(int signr)
 	 * To make sure this can't happen, place an upper bound on how often the
 	 * kernel may oops without panic().
 	 */
-	if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit) && oops_limit)
-		panic("Oopsed too often (kernel.oops_limit is %d)", oops_limit);
+	limit = READ_ONCE(oops_limit);
+	if (atomic_inc_return(&oops_count) >= limit && limit)
+		panic("Oopsed too often (kernel.oops_limit is %d)", limit);
 
 	do_exit(signr);
 }

kernel/panic.c

@@ -165,12 +165,15 @@ EXPORT_SYMBOL(nmi_panic);
 
 void check_panic_on_warn(const char *origin)
 {
+	unsigned int limit;
+
 	if (panic_on_warn)
 		panic("%s: panic_on_warn set ...\n", origin);
 
-	if (atomic_inc_return(&warn_count) >= READ_ONCE(warn_limit) && warn_limit)
+	limit = READ_ONCE(warn_limit);
+	if (atomic_inc_return(&warn_count) >= limit && limit)
 		panic("%s: system warned too often (kernel.warn_limit is %d)",
-		      origin, warn_limit);
+		      origin, limit);
 }
 
 /**