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

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Message-ID: <CA+55aFy-fxPm18RqtwpCJrWX0YvaQeT8HfcKNpF3qC+cxE8fqw@mail.gmail.com>
Date:   Tue, 18 Jul 2017 13:26:17 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...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>
Subject: Re: [PATCH] lib/strscpy: avoid KASAN false positive

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"

We have that for the dcache case too, although there the code does
that odd kasan_unpoison_shadow() instead.

                 Linus