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

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Date:   Thu, 19 Jan 2017 16:27:18 -0500
From:   "Frank Ch. Eigler" <fche@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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

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.

> I assume this is about systemtap modules. Can you please explain
> what you are trying to achieve? I guess you know that you actually
> access current, but then we need a seperate special function and not
> relaxing of the checks.

This part is used in a part of the runtime that is a userspace
analogue of probe_kernel_address(), where we're given a potential
userspace address.  We would like to quickly test whether it's even
plausible as a userspace address, before doing a (pagefault-disabled)
trial fetch/store to it.

- FChE