[PATCH] isdn: kcapi: replace deprecated strncpy with strscpy_pad

[PATCH] isdn: kcapi: 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.

`buf` is used in this context as a data buffer with 64 bytes of memory
to be occupied by capi_manufakturer.

We see the caller capi20_get_manufacturer() passes data.manufacturer as
its `buf` argument which is then later passed over to user space. Due to
this, let's keep the NUL-padding that strncpy provided by using
strscpy_pad so as to not leak any stack data.
| 	cdev->errcode = capi20_get_manufacturer(data.contr, data.manufacturer);
| 	if (cdev->errcode)
| 		return -EIO;
|
| 	if (copy_to_user(argp, data.manufacturer,
| 			 sizeof(data.manufacturer)))
| 		return -EFAULT;

Perhaps this would also be a good instance to use `strtomem_pad` for but
in my testing the compiler was not able to determine the size of `buf`
-- even with all the hints.

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.
---
 drivers/isdn/capi/kcapi.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/isdn/capi/kcapi.c b/drivers/isdn/capi/kcapi.c
index ae24848af233..136ba9fe55e0 100644
--- a/drivers/isdn/capi/kcapi.c
+++ b/drivers/isdn/capi/kcapi.c
@@ -732,7 +732,7 @@ u16 capi20_get_manufacturer(u32 contr, u8 buf[CAPI_MANUFACTURER_LEN])
 	u16 ret;
 
 	if (contr == 0) {
-		strncpy(buf, capi_manufakturer, CAPI_MANUFACTURER_LEN);
+		strscpy_pad(buf, capi_manufakturer, CAPI_MANUFACTURER_LEN);
 		return CAPI_NOERROR;
 	}
 
@@ -740,7 +740,7 @@ u16 capi20_get_manufacturer(u32 contr, u8 buf[CAPI_MANUFACTURER_LEN])
 
 	ctr = get_capi_ctr_by_nr(contr);
 	if (ctr && ctr->state == CAPI_CTR_RUNNING) {
-		strncpy(buf, ctr->manu, CAPI_MANUFACTURER_LEN);
+		strscpy_pad(buf, ctr->manu, CAPI_MANUFACTURER_LEN);
 		ret = CAPI_NOERROR;
 	} else
 		ret = CAPI_REGNOTINSTALLED;

---
base-commit: 2cf0f715623872823a72e451243bbf555d10d032
change-id: 20230922-strncpy-drivers-isdn-capi-kcapi-c-516f17f59684

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

Re: [PATCH] isdn: kcapi: replace deprecated strncpy with strscpy_pad

[Kees Cook]

On Fri, Sep 22, 2023 at 11:49:14AM +0000, 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.
> 
> `buf` is used in this context as a data buffer with 64 bytes of memory
> to be occupied by capi_manufakturer.
> 
> We see the caller capi20_get_manufacturer() passes data.manufacturer as
> its `buf` argument which is then later passed over to user space. Due to
> this, let's keep the NUL-padding that strncpy provided by using
> strscpy_pad so as to not leak any stack data.
> | 	cdev->errcode = capi20_get_manufacturer(data.contr, data.manufacturer);
> | 	if (cdev->errcode)
> | 		return -EIO;
> |
> | 	if (copy_to_user(argp, data.manufacturer,
> | 			 sizeof(data.manufacturer)))
> | 		return -EFAULT;

Yup, strongly agreed: this needs the padding. I actually wonder if a
follow-up patch might be a good idea here, just for robustness:

 capi_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
         struct capidev *cdev = file->private_data;
-       capi_ioctl_struct data;
+       capi_ioctl_struct data = { };


> 
> Perhaps this would also be a good instance to use `strtomem_pad` for but
> in my testing the compiler was not able to determine the size of `buf`
> -- even with all the hints.
> 
> 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>

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

Re: [PATCH] isdn: kcapi: replace deprecated strncpy with strscpy_pad

[Kees Cook]

On Fri, 22 Sep 2023 11:49:14 +0000, 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.
> 
> `buf` is used in this context as a data buffer with 64 bytes of memory
> to be occupied by capi_manufakturer.
> 
> [...]

Applied to for-next/hardening, thanks!

[1/1] isdn: kcapi: replace deprecated strncpy with strscpy_pad
      https://git.kernel.org/kees/c/69cee158c9b0

Take care,

-- 
Kees Cook