linux-kernel - Re: [RFC PATCH v6 1/6] x86/uaccess: Allow access_ok() in irq context if pagefault

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Message-ID: <20190506113923.4fbd56ae@gandalf.local.home>
Date:   Mon, 6 May 2019 11:39:23 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Changbin Du <changbin.du@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Nadav Amit <namit@...are.com>,
        Joel Fernandes <joel@...lfernandes.org>, yhs@...com
Subject: Re: [RFC PATCH v6 1/6] x86/uaccess: Allow access_ok() in irq
 context if pagefault_disabled

On Tue, 7 May 2019 00:22:03 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Hi Steve,
> 
> It seems I missed this message...

No problem. The number of times I missed messages... ugh.

> 
> > 
> > I'm curious, what interrupt handler are kprobes executing in that needs
> > random user space addresses?  
> 
> Sorry for confusion. Kprobes is using an exception (of course!). So the
> title can mislead, it should be "in exception" instead of "in irq context",
> However, current code checks it by "!in_task()", which includes both of
> IRQ and exception. A better solution might change it to "in_irq()".

That makes sense.

> 
> However, I could not find a way to distinguish the "exception" and
> "external IRQ" by the execution context (based on the preempt count)
> because exception is treated as a kind of IRQ.
> Thus, in this patch, I changed it as not only checking what the context
> is, but also whether it is appropriately called.
> 

As exceptions typically disable interrupts, we treat them as their own
context. Especially for looking at recursion detection algorithms,
which allow for different contexts to recurse.

Normal-context -> softirq -> exception / IRQ -> NMI


Anyway, that WARN_ON_IN_IRQ() should come with a big comment about why
we allow it if we have pagefault_disable() set.

This will need to go through the x86 maintainers. I'll go and review
the tracing patches of this series and give an ack / reviewed-by if
there's no issues.

-- Steve