linux-kernel - Re: [PATCH v2] arm64/sysreg: refactor deprecated strncpy

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Message-ID: <CAFhGd8qj+FOXjmBKUURR7fqwda-GVDUXtk8+xN65kYEoODP3VQ@mail.gmail.com>
Date:   Thu, 7 Sep 2023 17:04:18 -0700
From:   Justin Stitt <justinstitt@...gle.com>
To:     Bjorn Andersson <andersson@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org,
        Konrad Dybcio <konradybcio@...nel.org>
Subject: Re: [PATCH v2] arm64/sysreg: refactor deprecated strncpy

On Thu, Sep 7, 2023 at 2:23 PM Bjorn Andersson <andersson@...nel.org> wrote:
>
> On Fri, Aug 11, 2023 at 04:33:51PM +0000, Justin Stitt wrote:
> > `strncpy` is deprecated for use on NUL-terminated destination strings
> > [1]. Which seems to be the case here due to the forceful setting of `buf`'s
> > tail to 0.
> >
> > A suitable replacement is `strscpy` [2] due to the fact that it
> > guarantees NUL-termination on its destination buffer argument which is
> > _not_ the case for `strncpy`!
> >
> > In this case, we can simplify the logic and also check for any silent
> > truncation by using `strscpy`'s return value.
> >
> > This should have no functional change and yet uses a more robust and
> > less ambiguous interface whilst reducing code complexity.
> >
>
> I'm sorry, but this patch is wrong.
>
> __parse_cmdline() is supposed to match the command line against a set of
> keywords, one word at a time. The new implementation ignores the
> word-boundaries and matches the whole command line once and then breaks
> the loop, typically without having found a match. (See below)
>
> Can we please have this patch dropped, Will?
It has been, I believe. At any rate, a v4 is up [1] which prefers
memcpy over strncpy.

>
>
>
> Also, the commit message is a blanket statement about why strscpy is
> better than stncpy, but I don't see how this is applicable to the code
> it attempts to "fix". Afaict the code already handled these cases.
>
> > Link: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
> > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> > Link: https://github.com/KSPP/linux/issues/90
> > Suggested-by: Kees Cook <keescook@...omium.org>
> > Cc: linux-hardening@...r.kernel.org
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> > ---
> > Changes in v2:
> > - Utilize return value from strscpy and check for truncation (thanks Kees)
> > - Link to v1: https://lore.kernel.org/r/20230810-strncpy-arch-arm64-v1-1-f67f3685cd64@google.com
> > ---
> > For reference, see a part of `strscpy`'s implementation here:
> >
> > |     /* Hit buffer length without finding a NUL; force NUL-termination. */
> > |     if (res)
> > |             dest[res-1] = '\0';
> >
> > Note: compile tested
> > ---
> >  arch/arm64/kernel/idreg-override.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
> > index 2fe2491b692c..aee12c75b738 100644
> > --- a/arch/arm64/kernel/idreg-override.c
> > +++ b/arch/arm64/kernel/idreg-override.c
> > @@ -262,9 +262,9 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
> >               if (!len)
> >                       return;
> >
> > -             len = min(len, ARRAY_SIZE(buf) - 1);
>
> Here "len" was either the number of bytes to the first space, the end of
> the string, or the last byte in "buf".
>
> > -             strncpy(buf, cmdline, len);
>
> So this will copy one word, or the rest of the string.
>
> > -             buf[len] = 0;
>
> And it will NUL-terminate the word, which is then matched upon below.
>
> > +             len = strscpy(buf, cmdline, ARRAY_SIZE(buf));
>
> In this new implementation, the code copies the rest of the command line
> to "buf", makes an attempt to match with with the keywords, and then
> breaks the loop (as cmdline + len is the end of the string).
>
> Regards,
> Bjorn

[1]: https://lore.kernel.org/all/20230905-strncpy-arch-arm64-v4-1-bc4b14ddfaef@google.com/