[PATCH 4.14 16/16] exit: Use READ_ONCE() for all oops/warn limit reads

[Eric Biggers]

From: Kees Cook <keescook@xxxxxxxxxxxx>

commit 7535b832c6399b5ebfc5b53af5c51dd915ee2538 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@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/Y5x7GXeluFmZ8E0E@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 9fc9e278a5c0 ("panic: Introduce warn_limit")
Fixes: d4ccd54d28d3 ("exit: Put an upper limit on how often we can oops")
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: tangmeng <tangmeng@xxxxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
 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 73103e008a627..c0ff4461643d0 100644
--- a/kernel/exit.c
+++ b/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);
 }
diff --git a/kernel/panic.c b/kernel/panic.c
index 8f7bf0a8cef1a..165fd747cebd0 100644
--- a/kernel/panic.c
+++ b/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);
 }
 
 /**
-- 
2.39.1