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Date:   Fri, 21 Jun 2019 23:12:32 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Clark Williams <williams@redhat.com>,
        Juri Lelli <juri.lelli@gmail.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: WARN_ON: userstacktrace on irq events
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On Fri, 5 Apr 2019 09:32:09 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 5 Apr 2019 10:12:27 +0200 (CEST)
> Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > > BOOM! Warn on.
> > > 
> > > Can we make that access_ok() call in the copy_stack_frame not trigger
> > > the warning just if we are in an interrupt?  
> > 
> > You really want to have access_ok_atomic() or such which does not have the
> > WARN and use that in copy_stack_frame(). That's fine here because the
> > actual copy is inside a pagefault disabled region.
> 
> I was thinking the same.
> 
> Masami, did you post patches to do something like this?
> "access_ok_inatomic()" or something?

Yeah, last month I sent 
"x86/uaccess: Allow access_ok() in irq context if pagefault_disabled"

If you correctly disables the pagefault, access_ok() shouldn't warn it.
Ah, I see.

copy_stack_frame(const void __user *fp, struct stack_frame_user *frame)
{
        int ret;

        if (!access_ok(fp, sizeof(*frame))) <== this is out of pagefault_disable()!
                return 0;

        ret = 1;
        pagefault_disable();
        if (__copy_from_user_inatomic(frame, fp, sizeof(*frame)))
                ret = 0;
        pagefault_enable();

        return ret;
}

How is below patch?

---
diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
index 2abf27d7df6b..36ff77c801f7 100644
--- a/arch/x86/kernel/stacktrace.c
+++ b/arch/x86/kernel/stacktrace.c
@@ -98,14 +98,11 @@ struct stack_frame_user {
 static int
 copy_stack_frame(const void __user *fp, struct stack_frame_user *frame)
 {
-	int ret;
+	int ret = 1;
 
-	if (!access_ok(fp, sizeof(*frame)))
-		return 0;
-
-	ret = 1;
 	pagefault_disable();
-	if (__copy_from_user_inatomic(frame, fp, sizeof(*frame)))
+	if (!access_ok(fp, sizeof(*frame)) ||
+	    __copy_from_user_inatomic(frame, fp, sizeof(*frame)))
 		ret = 0;
 	pagefault_enable();
 

-- 
Masami Hiramatsu <mhiramat@kernel.org>