linux-kernel - Re: [RFC][PATCH] x86: Verify access

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Date:   Fri, 20 Jan 2017 00:04:17 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Frank Ch. Eigler" <fche@...hat.com>
cc:     David Smith <dsmith@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] x86: Verify access_ok() context

Frank.

On Thu, 19 Jan 2017, Frank Ch. Eigler wrote:
> > Well, if you are not in thread context then the check is pointless:
> > 	__range_not_ok(addr, size, user_addr_max())
> > and:
> > #define user_addr_max() (current->thread.addr_limit.seg)
> > 
> > So what guarantees when you are not in context of current, i.e. in thread
> > context, that the addr/size which is checked against the limits of current
> > actually belongs to current?
> 
> We're probably in task context in that there is a valid current(), but

current is always accessible no matter in which context you are - task,
softirq, hardirq, nmi ...

> running with preemption and/or interrupts and/or pagefaults disabled
> at that point, so in_task() objects.

As Peter explained, neither preempt disable nor interrupt disable not
pagefault disabled have any influence on in_task(). It merily checks the
context: !in_softirq() && !in_hardirq() && !in_nmi().

So that warning happens definitely not from task context.

Care to share the code?

Thanks,

	tglx