crypto: arc4 - mark ecb(arc4) skcipher as obsolete

Cryptographic algorithms may have a lifespan that is significantly
shorter than Linux's, and so we need to start phasing out algorithms
that are known to be broken, and are no longer fit for general use.

RC4 (or arc4) is a good example here: there are a few areas where its
use is still somewhat acceptable, e.g., for interoperability with legacy
wifi hardware that can only use WEP or TKIP data encryption, but that
should not imply that, for instance, use of RC4 based EAP-TLS by the WPA
supplicant for negotiating TKIP keys is equally acceptable, or that RC4
should remain available as a general purpose cryptographic transform for
all in-kernel and user space clients.

Now that all in-kernel users that need to retain support have moved to
the arc4 library interface, and the known users of ecb(arc4) via the
socket API (iwd [0] and libell [1][2]) have been updated to switch to a
local implementation, we can take the next step, and mark the ecb(arc4)
skcipher as obsolete, and only provide it if the socket API is enabled in
the first place, as well as provide the option to disable all algorithms
that have been marked as obsolete.

[0] https://git.kernel.org/pub/scm/network/wireless/iwd.git/commit/?id=1db8a85a60c64523
[1] https://git.kernel.org/pub/scm/libs/ell/ell.git/commit/?id=53482ce421b727c2
[2] https://git.kernel.org/pub/scm/libs/ell/ell.git/commit/?id=7f6a137809d42f6b

Signed-off-by: Ard Biesheuvel <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

crypto/Kconfig

···
 
 config CRYPTO_ARC4
 	tristate "ARC4 cipher algorithm"
+	depends on CRYPTO_USER_API_ENABLE_OBSOLETE
 	select CRYPTO_SKCIPHER
 	select CRYPTO_LIB_ARC4
 	help
···
 	help
 	  This option enables the user-spaces interface for AEAD
 	  cipher algorithms.
+
+config CRYPTO_USER_API_ENABLE_OBSOLETE
+	bool "Enable obsolete cryptographic algorithms for userspace"
+	depends on CRYPTO_USER_API
+	default y
+	help
+	  Allow obsolete cryptographic algorithms to be selected that have
+	  already been phased out from internal use by the kernel, and are
+	  only useful for userspace clients that still rely on them.
 
 config CRYPTO_STATS
 	bool "Crypto usage statistics for User-space"

crypto/arc4.c

···
 #include <crypto/arc4.h>
 #include <crypto/internal/skcipher.h>
 #include <linux/init.h>
+#include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/sched.h>
 
 static int crypto_arc4_setkey(struct crypto_skcipher *tfm, const u8 *in_key,
 			      unsigned int key_len)
···
 	return err;
 }
 
+static int crypto_arc4_init(struct crypto_skcipher *tfm)
+{
+	pr_warn_ratelimited("\"%s\" (%ld) uses obsolete ecb(arc4) skcipher\n",
+			    current->comm, (unsigned long)current->pid);
+
+	return 0;
+}
+
 static struct skcipher_alg arc4_alg = {
 	/*
 	 * For legacy reasons, this is named "ecb(arc4)", not "arc4".
···
 	.setkey			=	crypto_arc4_setkey,
 	.encrypt		=	crypto_arc4_crypt,
 	.decrypt		=	crypto_arc4_crypt,
+	.init			=	crypto_arc4_init,
 };
 
 static int __init arc4_init(void)