Re: [PATCH] arm64/sysreg: refactor deprecated strncpy

[Kees Cook <kees@kernel.org>]

On August 10, 2023 2:17:41 PM PDT, Justin Stitt <justinstitt@google.com> wrote:
>On Thu, Aug 10, 2023 at 12:58 PM Kees Cook <keescook@chromium.org> wrote:
>>
>> On Thu, Aug 10, 2023 at 12:25:37PM -0700, Justin Stitt wrote:
>> > On Thu, Aug 10, 2023 at 12:00 PM Kees Cook <keescook@chromium.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@vger.kernel.org
>> > > >
>> > > > Signed-off-by: Justin Stitt <justinstitt@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 | 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);
>Perhaps keeping the `...  - 1` is good because we then don't have to
>check strlen immediately after. This does still silently truncate but
>didn't the previous `strncpy` also do that?

Ah, actually there's no need to get too tricky here. This should be behaviorally identical:

len = strscpy(buf, cmdline, ARRAY_SIZE(buf));
if (len == -E2BIG)
    len = ARRAY_SIZE(buf) - 1;

-Kees


-- 
Kees Cook