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

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Date: Wed, 18 Oct 2023 23:01:54 -0700
From: Kees Cook <keescook@...omium.org>
To: Christoph Hellwig <hch@....de>
Cc: Justin Stitt <justinstitt@...gle.com>, 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
Subject: Re: the nul-terminated string helper desk chair rearrangement

On Thu, Oct 19, 2023 at 07:46:42AM +0200, Christoph Hellwig wrote:
> On Wed, Oct 18, 2023 at 10:48:49PM +0000, Justin Stitt wrote:
> > strncpy() is deprecated for use on NUL-terminated destination strings
> > [1] and as such we should prefer more robust and less ambiguous string
> > interfaces.
> 
> If we want that we need to stop pretendening direct manipulation of
> nul-terminate strings is a good idea.  I suspect the churn of replacing
> one helper with another, maybe slightly better, one probably
> introduces more bugs than it fixes.
> 
> If we want to attack the issue for real we need to use something
> better.
> 
> lib/seq_buf.c is a good start for a lot of simple cases that just
> append to strings including creating complex ones.  Kent had a bunch
> of good ideas on how to improve it, but couldn't be convinced to
> contribute to it instead of duplicating the functionality which
> is a bit sad, but I think we need to switch to something like
> seq_buf that actually has a counted string instead of all this messing
> around with the null-terminated strings.

When doing more complex string creation, I agree. I spent some time
doing this while I was looking at removing strcat() and strlcat(); this
is where seq_buf shines. (And seq_buf is actually both: it maintains its
%NUL termination _and_ does the length counting.) The only thing clunky
about it was initialization, but all the conversions I experimented with
were way cleaner using seq_buf. I even added a comment to strlcat()'s
kern-doc to aim folks at seq_buf. :)

/**
 * strlcat - Append a string to an existing string
...
 * Do not use this function. While FORTIFY_SOURCE tries to avoid
 * read and write overflows, this is only possible when the sizes
 * of @p and @q are known to the compiler. Prefer building the
 * string with formatting, via scnprintf(), seq_buf, or similar.

Almost all of the remaining strncpy() usage is just string to string
copying, but the corner cases that are being spun out that aren't
strscpy() or strscpy_pad() are covered by strtomem(), kmemdup_nul(),
and memcpy(). Each of these are a clear improvement since they remove
the ambiguity of the intended behavior. Using seq_buf ends up being way
more overhead than is needed.

-Kees

-- 
Kees Cook