linux-hardening - [PATCH] um: vector: refactor deprecated strncpy

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Date:   Mon, 07 Aug 2023 18:22:30 +0000
From:   Justin Stitt <justinstitt@...gle.com>
To:     Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>
Cc:     linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org,
        Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] um: vector: refactor deprecated strncpy

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

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 are able to drop the now superfluous `... - 1`
instances because `strscpy` will automatically truncate the last byte by
setting it to a NUL byte if the source size exceeds the destination size
or if the source string is not NUL-terminated.

I've also opted to remove the seemingly useless char* casts. I'm not
sure why they're present at all since (after expanding the `ifr_name`
macro) `ifr.ifr_ifrn.ifrn_name` is a char* already.

All in all, `strscpy` is a more robust and less ambiguous interface
while also letting us remove some `... -1`'s which cleans things up a
bit.

[1]: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
[2]: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html

Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@...r.kernel.org
Signed-off-by: Justin Stitt <justinstitt@...gle.com>
---
 arch/um/drivers/vector_user.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/um/drivers/vector_user.c b/arch/um/drivers/vector_user.c
index c650e428432b..c719e1ec4645 100644
--- a/arch/um/drivers/vector_user.c
+++ b/arch/um/drivers/vector_user.c
@@ -141,7 +141,7 @@ static int create_tap_fd(char *iface)
 	}
 	memset(&ifr, 0, sizeof(ifr));
 	ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_VNET_HDR;
-	strncpy((char *)&ifr.ifr_name, iface, sizeof(ifr.ifr_name) - 1);
+	strscpy(ifr.ifr_name, iface, sizeof(ifr.ifr_name));
 
 	err = ioctl(fd, TUNSETIFF, (void *) &ifr);
 	if (err != 0) {
@@ -171,7 +171,7 @@ static int create_raw_fd(char *iface, int flags, int proto)
 		goto raw_fd_cleanup;
 	}
 	memset(&ifr, 0, sizeof(ifr));
-	strncpy((char *)&ifr.ifr_name, iface, sizeof(ifr.ifr_name) - 1);
+	strscpy(ifr.ifr_name, iface, sizeof(ifr.ifr_name));
 	if (ioctl(fd, SIOCGIFINDEX, (void *) &ifr) < 0) {
 		err = -errno;
 		goto raw_fd_cleanup;

---
base-commit: c1a515d3c0270628df8ae5f5118ba859b85464a2
change-id: 20230807-arch-um-drivers-ad44050885d0

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