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

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Message-ID: <20170119222008.GI6515@twins.programming.kicks-ass.net>
Date:   Thu, 19 Jan 2017 23:20:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Frank Ch. Eigler" <fche@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        David Smith <dsmith@...hat.com>,
        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

On Thu, Jan 19, 2017 at 04:27:18PM -0500, Frank Ch. Eigler wrote:
> Hi, Thomas -
> 
> > 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
> running with preemption and/or interrupts and/or pagefaults disabled
> at that point, so in_task() objects.  Think of it like from a kprobes
> handler callback, except maybe more temporary preemption blocking.

#define in_task()               (!(preempt_count() & \ 
                                   (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET))) 

So it doesn't care about preempt_disable(), and it doesn't care about
local_irq_disable(), it also doesn't care about local_bh_disable().

What it does care about are nmi_enter(), irq_enter() and __do_softirq().