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

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Date:   Wed, 18 Jan 2017 16:19:47 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     David Smith <dsmith@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [RFC][PATCH] x86: Verify access_ok() context

On Wed, Jan 18, 2017 at 2:16 PM, David Smith <dsmith@...hat.com> wrote:
> On 01/16/2017 03:14 PM, Thomas Gleixner wrote:
>> On Mon, 16 Jan 2017, David Smith wrote:
>>
>>> If you call access_ok() with page faulting disabled, you'll still see
>>> this new warning.
>>
>> And how so? It's just checking for task context. page fault disable/enable
>> has absolutely nothing to do with that.
>
> True, task context and page fault disable/enable have nothing to do with each other. However, the access_ok() comment states:
>
>  * Context: User context only. This function may sleep if pagefaults are
>  *          enabled.
>
> That seems to indicate that the function won't sleep if pagefaults are disabled, and thus there is no need for a CONFIG_DEBUG_ATOMIC_SLEEP warning if pagefaults are disabled.

ISTM even with pagefault_disable() in play, using access_ok() from,
say, interrupt context is dangerous unless you've first checked that
you're in a task.  But I guess that in_task() would still return
false, e.g. in perf.

>
>>> If you put that new access_ok() call in a module that gets
>>> loaded/unloaded, you see one warning for every module load, which gets a
>>> bit annoying.
>>
>> Can you please elaborate where this access_ok() is placed in the module
>> code?
>
> It doesn't really matter where you place the access_ok() call in the module code. If you call access_ok() in a module, then that module has its own WARN_ON_ONCE() static variable. If access_ok() was a function exported from the kernel, then there would be only one copy of the WARN_ON_ONCE() static variable.

That doesn't seem like such a big deal to me.