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@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Link: https://lore.kernel.org/r/20230807-arch-um-drivers-v1-1-10d602c5577a@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/arch/um/drivers/vector_user.c b/arch/um/drivers/vector_user.c
index c650e42..c719e1e 100644 (file)
--- 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;