[PATCH] airo: replace deprecated strncpy with strscpy_pad

[PATCH] airo: replace deprecated strncpy with strscpy_pad

[Justin Stitt]

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

`extra` is clearly supposed to be NUL-terminated which is evident by the
manual NUL-byte assignment as well as its immediate usage with strlen().

Moreover, let's NUL-pad since there is deliberate effort (48 instances)
made elsewhere to zero-out buffers in these getters and setters:
6050 | memset(local->config.nodeName, 0, sizeof(local->config.nodeName));
6130 | memset(local->config.rates, 0, 8);
6139 | memset(local->config.rates, 0, 8);
6414 | memset(key.key, 0, MAX_KEY_SIZE);
6497 | memset(extra, 0, 16);
(to be clear, strncpy also NUL-padded -- we are matching that behavior)

Considering the above, a suitable replacement is `strscpy_pad` due to
the fact that it guarantees both NUL-termination and NUL-padding on the
destination buffer.

Technically, we can now copy one less byte into `extra` as we cannot
determine the sizeof `extra` at compile time and the hard-coded value of
16 means that strscpy_pad() will automatically truncate and set the byte
at offset 15 to NUL. However, the current code manually sets a
NUL-byte at offset 16. If this is an issue, the solution is to change
the hard-coded magic number to 17 instead of 16. I didn't do this in
this patch because a hard-coded 17 seems bad (even more so than 16).

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Note: build-tested only.

Found with: $ rg "strncpy\("
---
 drivers/net/wireless/cisco/airo.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/net/wireless/cisco/airo.c b/drivers/net/wireless/cisco/airo.c
index dbd13f7aa3e6..8cfb1de5933e 100644
--- a/drivers/net/wireless/cisco/airo.c
+++ b/drivers/net/wireless/cisco/airo.c
@@ -6067,8 +6067,7 @@ static int airo_get_nick(struct net_device *dev,
 	struct airo_info *local = dev->ml_priv;
 
 	readConfigRid(local, 1);
-	strncpy(extra, local->config.nodeName, 16);
-	extra[16] = '\0';
+	strscpy_pad(extra, local->config.nodeName, 16);
 	dwrq->length = strlen(extra);
 
 	return 0;

---
base-commit: 58720809f52779dc0f08e53e54b014209d13eebb
change-id: 20231017-strncpy-drivers-net-wireless-cisco-airo-c-d09cd0500a6e

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

Re: [PATCH] airo: replace deprecated strncpy with strscpy_pad

[Jeff Johnson]

On 10/17/2023 2:12 PM, Justin Stitt wrote:
> strncpy() is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
> 
> `extra` is clearly supposed to be NUL-terminated which is evident by the
> manual NUL-byte assignment as well as its immediate usage with strlen().
> 
> Moreover, let's NUL-pad since there is deliberate effort (48 instances)
> made elsewhere to zero-out buffers in these getters and setters:
> 6050 | memset(local->config.nodeName, 0, sizeof(local->config.nodeName));
> 6130 | memset(local->config.rates, 0, 8);
> 6139 | memset(local->config.rates, 0, 8);
> 6414 | memset(key.key, 0, MAX_KEY_SIZE);
> 6497 | memset(extra, 0, 16);
> (to be clear, strncpy also NUL-padded -- we are matching that behavior)
> 
> Considering the above, a suitable replacement is `strscpy_pad` due to
> the fact that it guarantees both NUL-termination and NUL-padding on the
> destination buffer.
> 
> Technically, we can now copy one less byte into `extra` as we cannot
> determine the sizeof `extra` at compile time and the hard-coded value of
> 16 means that strscpy_pad() will automatically truncate and set the byte
> at offset 15 to NUL. However, the current code manually sets a
> NUL-byte at offset 16. If this is an issue, the solution is to change
> the hard-coded magic number to 17 instead of 16. I didn't do this in
> this patch because a hard-coded 17 seems bad (even more so than 16).

this function is a wext handler. In wext-core.c we have:
static const struct iw_ioctl_description standard_ioctl[] = {
...
	[IW_IOCTL_IDX(SIOCGIWNICKN)] = {
		.header_type	= IW_HEADER_TYPE_POINT,
		.token_size	= 1,
		.max_tokens	= IW_ESSID_MAX_SIZE,
	},

So the buffer size is (strangely) IW_ESSID_MAX_SIZE if you want to use 
that for the buffer size

> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@google.com>
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>   drivers/net/wireless/cisco/airo.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/net/wireless/cisco/airo.c b/drivers/net/wireless/cisco/airo.c
> index dbd13f7aa3e6..8cfb1de5933e 100644
> --- a/drivers/net/wireless/cisco/airo.c
> +++ b/drivers/net/wireless/cisco/airo.c
> @@ -6067,8 +6067,7 @@ static int airo_get_nick(struct net_device *dev,
>   	struct airo_info *local = dev->ml_priv;
>   
>   	readConfigRid(local, 1);
> -	strncpy(extra, local->config.nodeName, 16);
> -	extra[16] = '\0';
> +	strscpy_pad(extra, local->config.nodeName, 16);
>   	dwrq->length = strlen(extra);
>   
>   	return 0;
> 
> ---
> base-commit: 58720809f52779dc0f08e53e54b014209d13eebb
> change-id: 20231017-strncpy-drivers-net-wireless-cisco-airo-c-d09cd0500a6e
> 
> Best regards,
> --
> Justin Stitt <justinstitt@google.com>
> 

Re: [PATCH] airo: replace deprecated strncpy with strscpy_pad

[Kees Cook]

On Tue, Oct 17, 2023 at 03:51:58PM -0700, Jeff Johnson wrote:
> On 10/17/2023 2:12 PM, Justin Stitt wrote:
> > strncpy() is deprecated for use on NUL-terminated destination strings
> > [1] and as such we should prefer more robust and less ambiguous string
> > interfaces.
> > 
> > `extra` is clearly supposed to be NUL-terminated which is evident by the
> > manual NUL-byte assignment as well as its immediate usage with strlen().
> > 
> > Moreover, let's NUL-pad since there is deliberate effort (48 instances)
> > made elsewhere to zero-out buffers in these getters and setters:
> > 6050 | memset(local->config.nodeName, 0, sizeof(local->config.nodeName));
> > 6130 | memset(local->config.rates, 0, 8);
> > 6139 | memset(local->config.rates, 0, 8);
> > 6414 | memset(key.key, 0, MAX_KEY_SIZE);
> > 6497 | memset(extra, 0, 16);
> > (to be clear, strncpy also NUL-padded -- we are matching that behavior)
> > 
> > Considering the above, a suitable replacement is `strscpy_pad` due to
> > the fact that it guarantees both NUL-termination and NUL-padding on the
> > destination buffer.
> > 
> > Technically, we can now copy one less byte into `extra` as we cannot
> > determine the sizeof `extra` at compile time and the hard-coded value of
> > 16 means that strscpy_pad() will automatically truncate and set the byte
> > at offset 15 to NUL. However, the current code manually sets a
> > NUL-byte at offset 16. If this is an issue, the solution is to change
> > the hard-coded magic number to 17 instead of 16. I didn't do this in
> > this patch because a hard-coded 17 seems bad (even more so than 16).
> 
> this function is a wext handler. In wext-core.c we have:
> static const struct iw_ioctl_description standard_ioctl[] = {
> ...
> 	[IW_IOCTL_IDX(SIOCGIWNICKN)] = {
> 		.header_type	= IW_HEADER_TYPE_POINT,
> 		.token_size	= 1,
> 		.max_tokens	= IW_ESSID_MAX_SIZE,
> 	},
> 
> So the buffer size is (strangely) IW_ESSID_MAX_SIZE if you want to use that
> for the buffer size

Yeah, that seems like a good refactor to do at the same time.

> > -	strncpy(extra, local->config.nodeName, 16);
> > -	extra[16] = '\0';
> > +	strscpy_pad(extra, local->config.nodeName, 16);

Justin, can you respin this with the open-coded "16" updated?

-Kees

-- 
Kees Cook