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

Peter Zijlstra
2020-03-05T09:54:48Z

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