linux-kernel - Re: strlcpy() notes (was Re: [GIT PULL] smb3 client fixes)

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Message-ID: <Ywaf/CWYzmPx1lxa@arm.com>
Date:   Wed, 24 Aug 2022 23:02:36 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Steve French <smfrench@...il.com>,
        CIFS <linux-cifs@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: strlcpy() notes (was Re: [GIT PULL] smb3 client fixes)

On Tue, Aug 23, 2022 at 10:37:38AM -0700, Linus Torvalds wrote:
> On Tue, Aug 23, 2022 at 1:56 AM Catalin Marinas <catalin.marinas@....com> wrote:
> > With load_unaligned_zeropad(), the arm64 implementation disables tag
> > checking temporarily. We could do the same with read_word_at_a_time()
> > (there is a kasan_check_read() in this function but it wrongly uses a
> > size of 1).
> 
> The "size of 1" is not wrong, it's intentional, exactly because people
> do things like
> 
>     strscpy(dst, "string", sizeof(dst));
> 
> which is a bit unfortunate, but very understandable and intended to
> work. So that thing may over-read the string by up to a word. And
> KASAN ends up being unhappy.

Good point. We could attempt a single-byte checked read on arm64 as well
and then disable tag checking (the arm64 load_unaligned_zeropad()
doesn't bother with this).

For KASAN, if we want to be more precise, we could move the
kasan_check_read() (or add a new one) in the strscpy() implementation
that actually takes into account how much was copied (non-zero bytes).
Not sure it's worth it though. The check would be post-read though.

-- 
Catalin