[PATCH] x86/process: Silence KASAN warnings in get_wchan()

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

get_wchan() is racy by design, it may access volatile stack
of running task, thus it may access redzone in a stack frame
and cause KASAN to warn about this.

Use kasan_disable_current()/kasan_enable_current() to silence
these warnings.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
---

 Perhaps it would be better to add something like this:
	READ_ONCE_NOCHECK()
	{
		kasan_disable_current();
		READ_ONCE();
		kasan_enable_current();
	}
  ?

 arch/x86/kernel/process.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 39e585a..0488eb9 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -13,6 +13,7 @@
 #include <linux/random.h>
 #include <linux/user-return-notifier.h>
 #include <linux/dmi.h>
+#include <linux/kasan.h>
 #include <linux/utsname.h>
 #include <linux/stackprotector.h>
 #include <linux/tick.h>
@@ -514,7 +515,7 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
  */
 unsigned long get_wchan(struct task_struct *p)
 {
-	unsigned long start, bottom, top, sp, fp, ip;
+	unsigned long start, bottom, top, sp, fp, ip, ret = 0;
 	int count = 0;
 
 	if (!p || p == current || p->state == TASK_RUNNING)
@@ -550,14 +551,21 @@ unsigned long get_wchan(struct task_struct *p)
 	if (sp < bottom || sp > top)
 		return 0;
 
+	kasan_disable_current();
 	fp = READ_ONCE(*(unsigned long *)sp);
 	do {
 		if (fp < bottom || fp > top)
-			return 0;
+			goto out;
+
 		ip = READ_ONCE(*(unsigned long *)(fp + sizeof(unsigned long)));
-		if (!in_sched_functions(ip))
-			return ip;
+		if (!in_sched_functions(ip)) {
+			ret = ip;
+			goto out;
+		}
 		fp = READ_ONCE(*(unsigned long *)fp);
 	} while (count++ < 16 && p->state != TASK_RUNNING);
-	return 0;
+
+out:
+	kasan_enable_current();
+	return ret;
 }
-- 
2.4.9