[PATCH v2] arm64/sysreg: refactor deprecated strncpy

[PATCH v2] arm64/sysreg: refactor deprecated strncpy

[Justin Stitt]

`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.

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@chromium.org>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.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);
-		strncpy(buf, cmdline, len);
-		buf[len] = 0;
+		len = strscpy(buf, cmdline, ARRAY_SIZE(buf));
+		if (len == -E2BIG)
+			len = ARRAY_SIZE(buf) - 1;
 
 		if (strcmp(buf, "--") == 0)
 			return;

---
base-commit: 52a93d39b17dc7eb98b6aa3edb93943248e03b2f
change-id: 20230810-strncpy-arch-arm64-1f3d328bd9b8

Best regards,
--
Justin Stitt <justinstitt@google.com>

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

[Will Deacon]

On Fri, 11 Aug 2023 16:33:51 +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`!
> 
> [...]

Applied to arm64 (for-next/misc), thanks!

[1/1] arm64/sysreg: refactor deprecated strncpy
      https://git.kernel.org/arm64/c/d232606773a0

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

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

[Bjorn Andersson]

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?



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@chromium.org>
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@google.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

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

[Justin Stitt]

On Thu, Sep 7, 2023 at 2:23 PM Bjorn Andersson <andersson@kernel.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@chromium.org>
> > Cc: linux-hardening@vger.kernel.org
> > Signed-off-by: Justin Stitt <justinstitt@google.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/

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

[Will Deacon]

On Thu, Sep 07, 2023 at 02:28:05PM -0700, Bjorn Andersson 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?

Yup, this was fixed yesterday so please take linux-next 20230908 for a
spin and let us know how you get on.

Cheers,

Will