linux-hardening - Re: [PATCH v4] EDAC/mc: Prefer strscpy over strcpy

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Message-ID: <YSU5wp/ZpsXuDgmu@zn.tnic>
Date:   Tue, 24 Aug 2021 20:26:10 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Len Baker <len.baker@....com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        James Morse <james.morse@....com>,
        Robert Richter <rric@...nel.org>,
        Joe Perches <joe@...ches.com>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] EDAC/mc: Prefer strscpy over strcpy

On Tue, Aug 24, 2021 at 12:28:07PM +0200, Len Baker wrote:
> This is a task of the KSPP [1] and the main reason is to clean up the
> proliferation of str*cpy functions in the kernel.

That I understood - you prominently explain where the patches stem from.

What I can't parse is that formulation "previous step". What previous
step do you mean?

> Yes, you are right. The same discussion happened in the v3 review [2] and
> I agree with the reasons that Robert Richter exposed. Using the strlen()
> implementation it is not necessary to check the return code of strcpy and
> we can assume a silent truncation.
> 
> [2] https://lore.kernel.org/linux-hardening/YRN+8u59lJ6MWsOL@rric.localdomain/

Ok, looking at the asm, gcc is actually smart enough not to call
strlen() twice on the same buffer.

But then there's this in the strscpy() kernel-doc comment:

"The destination buffer is always NUL terminated, unless it's
zero-sized."

so looking at the code, we're merrily decrementing len but nothing's
checking whether len can become 0. Because if it does, strscpy() will
do:

	if (count == 0 || WARN_ON_ONCE(count > INT_MAX))
		return -E2BIG;

so if p ends up pointing to something which is *not* '\0', strlen() will
go off into the weeds.

So I don't care if it doesn't look just as nice - it better be correct
in all cases first.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette