linux-kernel - [PATCH] isdn: kcapi: replace deprecated strncpy with strscpy

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Message-ID: <20230922-strncpy-drivers-isdn-capi-kcapi-c-v1-1-55fcf8b075fb@google.com>
Date:   Fri, 22 Sep 2023 11:49:14 +0000
From:   Justin Stitt <justinstitt@...gle.com>
To:     Karsten Keil <isdn@...ux-pingi.de>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] isdn: kcapi: replace deprecated strncpy with strscpy_pad

`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@...r.kernel.org
Signed-off-by: Justin Stitt <justinstitt@...gle.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@...gle.com>