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

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Date:   Thu, 10 Aug 2023 12:25:37 -0700
From:   Justin Stitt <justinstitt@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] arm64/sysreg: refactor deprecated strncpy

On Thu, Aug 10, 2023 at 12:00 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, Aug 10, 2023 at 06:39:03PM +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.
>
> Another note to include in these evaluations would be "does the
> destination expect to be %NUL padded?". Here, it looks like no, as all
> the routines "buf" is passed to expect a regular C string (padding
> doesn't matter).
>
> >
> > 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, there is some behavior being used in conjunction with
> > `strncpy` that `strscpy` already implements. This means we can drop some
> > of the extra stuff like `... -1` and `buf[len] = 0`
> >
> > This should have no functional change and yet uses a more robust and
> > less ambiguous interface whilst reducing code complexity.
> >
> > 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
> > Cc: linux-hardening@...r.kernel.org
> >
> > Signed-off-by: Justin Stitt <justinstitt@...gle.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 | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
> > index 2fe2491b692c..482dc5c71e90 100644
> > --- a/arch/arm64/kernel/idreg-override.c
> > +++ b/arch/arm64/kernel/idreg-override.c
> > @@ -262,9 +262,8 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
> >               if (!len)
> >                       return;
> >
> > -             len = min(len, ARRAY_SIZE(buf) - 1);
> > -             strncpy(buf, cmdline, len);
> > -             buf[len] = 0;
> > +             len = min(len, ARRAY_SIZE(buf));
> > +             strscpy(buf, cmdline, len);
>
> This, however, isn't correct: "cmdline" will be incremented by "leN"
> later, and we want a count of the characters copied into "buf", even if
> they're truncated. I think this should be:
>
>                 strscpy(buf, cmdline, ARRAY_SIZE(buf));
>                 len = strlen(buf);
>
Thoughts on using the return value from `strscpy` here?

> -Kees
>
> >
> >               if (strcmp(buf, "--") == 0)
> >                       return;
> >
> > ---
> > base-commit: 52a93d39b17dc7eb98b6aa3edb93943248e03b2f
> > change-id: 20230810-strncpy-arch-arm64-1f3d328bd9b8
> >
> > Best regards,
> > --
> > Justin Stitt <justinstitt@...gle.com>
> >
>
> --
> Kees Cook