linux-kernel - Re: [PATCH] lib/strscpy: avoid KASAN false positive

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Date:   Wed, 19 Jul 2017 11:39:32 -0400
From:   Chris Metcalf <cmetcalf@...lanox.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Jones <davej@...emonkey.org.uk>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Chris Metcalf <cmetcalf@...hip.com>
Subject: Re: [PATCH] lib/strscpy: avoid KASAN false positive

On 7/18/2017 6:04 PM, Andrew Morton wrote:
> On Wed, 19 Jul 2017 00:31:36 +0300 Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
>
>> On 07/18/2017 11:26 PM, Linus Torvalds wrote:
>>> On Tue, Jul 18, 2017 at 1:15 PM, Andrey Ryabinin
>>> <aryabinin@...tuozzo.com> wrote:
>>>> No, it does warn about valid users. The report that Dave posted wasn't about wrong strscpy() usage
>>>> it was about reading 8-bytes from 5-bytes source string. It wasn't about buggy 'count' at all.
>>>> So KASAN will warn for perfectly valid code like this:
>>>>          char dest[16];
>>>>          strscpy(dest, "12345", sizeof(dest)):
>>> Ugh, ok, yes.
>>>
>>>> For strscpy() that would mean making the *whole* read from 'src' buffer unchecked by KASAN.
>>> So we do have that READ_ONCE_NOCHECK(), but could we perhaps have
>>> something that doesn't do a NOCHECK but a partial check and is simply
>>> ok with "this is an optimistc longer access"
>>>
>> This can be dont, I think.
>>
>> Something like this:
>> static inline unsigned long read_partial_nocheck(unsigned long *x)
>> {
>> 	unsigned long ret = READ_ONCE_NOCHECK(x);
>> 	kasan_check_partial(x, sizeof(unsigned long));
>> 	return ret;
>> }
>>
> (Cc Chris)
>
> We could just remove all that word-at-a-time logic.  Do we have any
> evidence that this would harm anything?

The word-at-a-time logic was part of the initial commit since I wanted
to ensure that strscpy could be used to replace strlcpy or strncpy without
serious concerns about performance.  It seems unfortunate to remove it
unconditionally to support KASAN, but I haven't looked deeply at the
tradeoffs here.

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com