SUNRPC: Replace strlcpy() with strscpy()

strlcpy() reads the entire source buffer first. This read may exceed
the destination size limit. This is both inefficient and can lead
to linear read overflows if a source string is not NUL-terminated[1].
Additionally, it returns the size of the source string, not the
resulting size of the destination string. In an effort to remove strlcpy()
completely[2], replace strlcpy() here with strscpy().

Explicitly handle the truncation case by returning the size of the
resulting string.

If "nodename" was ever longer than sizeof(clnt->cl_nodename) - 1, this
change will fix a bug where clnt->cl_nodelen would end up thinking there
were more characters in clnt->cl_nodename than there actually were,
which might have lead to kernel memory content exposures.

Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna@kernel.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Neil Brown <neilb@suse.de>
Cc: Olga Kornievskaia <kolga@netapp.com>
Cc: Dai Ngo <Dai.Ngo@oracle.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-nfs@vger.kernel.org
Cc: netdev@vger.kernel.org
Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy [1]
Link: https://github.com/KSPP/linux/issues/89 [2]
Co-developed-by: Azeem Shaikh <azeemshaikh38@gmail.com>
Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20231114175407.work.410-kees@kernel.orgSigned-off-by: Kees Cook <keescook@chromium.org>

net/sunrpc/clnt.c

@@ -287,8 +287,14 @@ static struct rpc_xprt *rpc_clnt_set_transport(struct rpc_clnt *clnt,
 
 static void rpc_clnt_set_nodename(struct rpc_clnt *clnt, const char *nodename)
 {
-	clnt->cl_nodelen = strlcpy(clnt->cl_nodename,
+	ssize_t copied;
+
+	copied = strscpy(clnt->cl_nodename,
 			 nodename, sizeof(clnt->cl_nodename));
+
+	clnt->cl_nodelen = copied < 0
+				? sizeof(clnt->cl_nodename) - 1
+				: copied;
 }
 
 static int rpc_client_register(struct rpc_clnt *clnt,