linux-kernel - Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec

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Date:   Thu, 4 Jan 2018 13:43:13 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Julia Lawall <julia.lawall@...6.fr>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arch@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

On Thu, Jan 4, 2018 at 11:26 AM, Pavel Machek <pavel@....cz> wrote:
> Hi!
>
>> >> > What remains to be seen is if there are other patterns that affect
>> >> > different processors.
>> >> >
>> >> > In the longer term the compiler itself needs to know what is and isn't
>> >> > safe (ie you need to be able to write things like
>> >> >
>> >> > void foo(tainted __user int *x)
>> >> >
>> >> > and have the compiler figure out what level of speculation it can do and
>> >> > (on processors with those features like IA64) when it can and can't do
>> >> > various kinds of non-trapping loads.
>> >> >
>> >>
>> >> It would be great if coccinelle and/or smatch could be taught to catch
>> >> some of these case at least as a first pass "please audit this code
>> >> block" type of notification.
>> >>
>> >
>> > What should one be looking for.  Do you have a typical example?
>> >
>>
>> See "Exploiting Conditional Branch Misprediction" from the paper [1].
>>
>> The typical example is an attacker controlled index used to trigger a
>> dependent read near a branch. Where an example of "near" from the
>> paper is "up to 188 simple instructions inserted in the source code
>> between the ‘if’ statement and the line accessing array...".
>>
>> if (attacker_controlled_index < bound)
>>      val = array[attacker_controlled_index];
>> else
>>     return error;
>>
>> ...when the cpu speculates that the 'index < bound' branch is taken it
>> reads index and uses that value to read array[index]. The result of an
>> 'array' relative read is potentially observable in the cache.
>
> You still need
>
>         (void) array2[val];
>
> after that to get something observable, right?

As far as I understand the presence of array2[val] discloses more
information, but in terms of the cpu taking an action that it is
observable in the cache that's already occurred when "val =
array[attacker_controlled_index];" is speculated. Lets err on the side
of caution and shut down all the observable actions that are already
explicitly gated by an input validation check. In other words, a low
bandwidth information leak is still a leak.