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

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

[Kees Cook]

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: 9fc9e278a5c0 ("panic: Introduce warn_limit")
Fixes: d4ccd54d28d3 ("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>
---
 kernel/exit.c  | 6 ++++--
 kernel/panic.c | 7 +++++--
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index deffb8e4b1b2..15dc2ec80c46 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -931,6 +931,7 @@ void __noreturn make_task_dead(int signr)
 	 * Then do everything else.
 	 */
 	struct task_struct *tsk = current;
+	unsigned int limit;
 
 	if (unlikely(in_interrupt()))
 		panic("Aiee, killing interrupt handler!");
@@ -954,8 +955,9 @@ 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);
 
 	/*
 	 * We're taking recursive faults here in make_task_dead. Safest is to just
diff --git a/kernel/panic.c b/kernel/panic.c
index 54deb743b2d5..7834c9854e02 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -231,12 +231,15 @@ static void panic_print_sys_info(bool console_flush)
 
 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);
 }
 
 /**
-- 
2.34.1

Re: [PATCH] exit: Use READ_ONCE() for all oops/warn limit reads

[Peter Zijlstra]

On Fri, Dec 16, 2022 at 12:30:28PM -0800, Kees Cook wrote:
> 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: 9fc9e278a5c0 ("panic: Introduce warn_limit")
> Fixes: d4ccd54d28d3 ("exit: Put an upper limit on how often we can oops")

Thanks!

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>