Subject: Re: mmotm 2020-03-03-22-28 uploaded (warning: objtool:)

From: Peter Zijlstra <[email-masked]>
Date: Thu, 5 Mar 2020 10:54:36 +0100

On Thu, Mar 05, 2020 at 05:16:22PM +0800, Walter Wu wrote:
> On Thu, 2020-03-05 at 09:18 +0100, Peter Zijlstra wrote:
> > On Thu, Mar 05, 2020 at 09:17:17AM +0100, Peter Zijlstra wrote:
> > > On Wed, Mar 04, 2020 at 09:34:49AM -0800, Randy Dunlap wrote:
> > 
> > > > mm/kasan/common.o: warning: objtool: kasan_report()+0x13: call to report_enabled() with UACCESS enabled
> > > 
> > > I used next/master instead, and found the below broken commit
> > > responsible for this.
> > 
> > > @@ -634,12 +637,20 @@ void kasan_free_shadow(const struct vm_struct *vm)
> > >  #endif
> > >  
> > >  extern void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip);
> > > +extern bool report_enabled(void);
> > >  
> > > -void kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip)
> > > +bool kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip)
> > >  {
> > > -	unsigned long flags = user_access_save();
> > > +	unsigned long flags;
> > > +
> > > +	if (likely(!report_enabled()))
> > > +		return false;
> > 
> > This adds an explicit call before the user_access_save() and that is a
> > straight on bug.
> > 
> Hi Peter,
> 
> Thanks for your help. Unfortunately, I don't reproduce it in our
> environment, so I have asked Stephen, if I can reproduce it, then we
> will send new patch.

The patch is trivial; and all you need is an x86_64 (cross) compiler to
reproduce.

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index ad2dc0c9cc17..2906358e42f0 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -618,16 +618,17 @@ extern bool report_enabled(void);
 
 bool kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip)
 {
-	unsigned long flags;
+	unsigned long flags = user_access_save();
+	bool ret = false;
 
-	if (likely(!report_enabled()))
-		return false;
+	if (likely(report_enabled())) {
+		__kasan_report(addr, size, is_write, ip);
+		ret = true;
+	}
 
-	flags = user_access_save();
-	__kasan_report(addr, size, is_write, ip);
 	user_access_restore(flags);
 
-	return true;
+	return ret;
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG