linux-hardening - Re: the nul-terminated string helper desk chair rearrangement

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Date: Fri, 20 Oct 2023 11:22:27 -0700
From: Kees Cook <keescook@...omium.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Justin Stitt <justinstitt@...gle.com>, Christoph Hellwig <hch@....de>,
	Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Sagi Grimberg <sagi@...mberg.me>, linux-nvme@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	ksummit@...ts.linux.dev, Azeem Shaikh <azeemshaikh38@...il.com>
Subject: Re: the nul-terminated string helper desk chair rearrangement

On Fri, Oct 20, 2023 at 10:56:31AM -0700, Linus Torvalds wrote:
> On Fri, 20 Oct 2023 at 10:40, Justin Stitt <justinstitt@...gle.com> wrote:
> >
> > There's some docs at [1]. Perhaps there could be more?
> >
> > [1]: https://elixir.bootlin.com/linux/v6.6-rc6/source/include/linux/fortify-string.h#L292
> 
> Note that we have so few 'strlcpy()' calls that we really should
> remove that horrid horrid interface. It's a buggy piece of sh*t.

Yup, that's on-going. There's just a few left; Azeem has been chipping
away at strlcpy.

> It does mean that if you used to have
> 
>     dst[4];
>     strlcpy(dst, "abc", 8);
> 
> then that *used* to work (because it would copy four bytes: "abc\0"
> and that fits in 'dst[]'). But
> 
>    dst[4];
>    strscpy(dst, "abc", 8);
> 
> will overflow dst[], because it will do a word-copy and you told
> 'strscpy()' that you had a 8-byte buffer, and it will try to write
> "abc\0\0\0\0\0" into the destination.

Luckily, we already have checks for these mismatched sizes at compile
time (i.e. CONFIG_FORTIFY_SOURCE will already check for pathological
cases like above where 8 > sizeof(dst)).

> The above is insane code, but it's an example of why a blind
> strlcpy->strscpy conversion might change semantics.

Totally agreed. All of the recent string conversions have been paying
close attention to the behavioral differences.

-- 
Kees Cook