linux-kernel - Re: Is strncpy really less secure than strscpy ?

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Date:   Thu, 19 Oct 2023 10:40:02 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     Kees Cook <keescook@...omium.org>,
        Randy Dunlap <rdunlap@...radead.org>
Cc:     James Dutton <james.dutton@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Justin Stitt <justinstitt@...gle.com>,
        Calvince Otieno <calvncce@...il.com>,
        Azeem Shaikh <azeemshaikh38@...il.com>,
        Andy Shevchenko <andy@...nel.org>
Subject: Re: Is strncpy really less secure than strscpy ?

On Wed, Oct 18, 2023 at 07:56:36PM -0700, Kees Cook wrote:
> On Wed, Oct 18, 2023 at 07:27:20PM -0700, Randy Dunlap wrote:
> > 
> > 
> > On 10/18/23 18:49, Bagas Sanjaya wrote:
> > > [Disclaimer: I have little to no knowledge of C, so things may be wrong.
> > >  Please correct me if it is the case. Also Cc: recent people who work on
> > >  strscpy() conversion.]
> 
> Here are the current docs on the deprecated use of strncpy:
> https://docs.kernel.org/process/deprecated.html#strncpy-on-nul-terminated-strings
> which could probably be improved.
> 
> > Also Cc: the STRING maintainers.
> > 
> > > On Thu, Oct 19, 2023 at 12:22:33AM +0100, James Dutton wrote:
> > >> Is strncpy really less secure than strscpy ?
> 
> Very. :)
> 
> > >> If one uses strncpy and thus put a limit on the buffer size during the
> > >> copy, it is safe. There are no writes outside of the buffer.
> > >> If one uses strscpy and thus put a limit on the buffer size during the
> > >> copy, it is safe. There are no writes outside of the buffer.
> > > 
> > > Well, assuming that the string is NUL-terminated, the end result should
> > > be the same.
> 
> Note the use of "If" in the above sentences. :) This is what makes
> strncpy so dangerous -- it's only "correct" if the length argument is
> less than the size of the _source_ buffer. And by "correct", I mean
> that only then will strncpy produce a C-string. If not, it's a memcpy
> and leaves the buffer unterminated. This lack of %NUL-termination leads
> to all kinds of potential "downstream" problems with reading past the
> end of the buffer, etc.

Oh, that's what I mean by the same results.

> 
> One of the easiest ways to avoid bugs is to remove ambiguity, and
> strncpy is full of it. :P
> 
> Almost more important than the potential lack of %NUL-termination is
> the fact that strncpy() doesn't tell other maintainers _why_ it was used
> because it has several "effects" that aren't always intended:
> 
> - is the destination supposed to be %NUL terminated? (We covered this)
> - is the destination supposed to be %NUL padded?
> 
> strncpy pads the destination -- is this needed? Is it a waste of CPU
> time?
> 
> > > 
> > >> But, one can fit more characters in strncpy than strscpy because
> > >> strscpy enforces the final \0 on the end.
> > >> One could argue that strncpy is better because it might save the space
> > >> of one char at the end of a string array.
> 
> At the cost of creating non-C-strings. And this is a completely bonkers
> result for the "C String API" to produce. :P
> 
> > >> There are cases where strncpy might be unsafe. For example copying
> > >> between arrays of different sizes, and that is a case where strscpy
> > >> might be safer, but strncpy can be made safe if one ensures that the
> > >> size used in strncpy is the smallest of the two different array sizes.
> 
> The CONFIG_FORTIFY_SOURCE option in the kernel adds a bunch of
> sanity-checking to the APIs (some of which can be determined at compile
> time), but it doesn't remove the ambiguity of using strncpy. We want the
> kernel to have maintainable code, and when it's not clear which of a
> handful of side-effects are _intended_ from an API, that's a bad API. :)
> 
> > > 
> > > Code example on both cases?
> > > 
> > >>
> > >> If one blindly replaces strncpy with strscpy across all uses, one
> > >> could unintentionally be truncating the results and introduce new
> > >> bugs.
> 
> Yes, of course. That's why we're not blindly replacing them. :) And the
> diagnostic criteria has been carefully described:
> https://github.com/KSPP/linux/issues/90
> 

Thanks for the explanation!

-- 
An old man doll... just what I always wanted! - Clara

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