crypto: doc - improve the skcipher API example code

Rewrite the skcipher API example, changing it to encrypt a buffer with
AES-256-XTS.  This addresses various problems with the previous example:

- It requests a specific driver "cbc-aes-aesni", which is unusual.
  Normally users ask for "cbc(aes)", not a specific driver.

- It encrypts only a single AES block.  For the reader, that doesn't
  clearly distinguish the "skcipher" API from the "cipher" API.

- Showing how to encrypt something with bare CBC is arguably a poor
  choice of example, as it doesn't follow modern crypto trends.  Now,
  usually authenticated encryption is recommended, in which case the
  user would use the AEAD API, not skcipher.  Disk encryption is still a
  legitimate use for skcipher, but for that usually XTS is recommended.

- Many other bugs and poor coding practices, such as not setting
  CRYPTO_TFM_REQ_MAY_SLEEP, unnecessarily allocating a heap buffer for
  the IV, unnecessary NULL checks, using a pointless wrapper struct, and
  forgetting to set an error code in one case.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/crypto/api-samples.rst

@@ -4,111 +4,89 @@ Code Examples
 Code Example For Symmetric Key Cipher Operation
 -----------------------------------------------
 
-::
-
-
-    /* tie all data structures together */
-    struct skcipher_def {
-        struct scatterlist sg;
-        struct crypto_skcipher *tfm;
-        struct skcipher_request *req;
-        struct crypto_wait wait;
-    };
-
-    /* Perform cipher operation */
-    static unsigned int test_skcipher_encdec(struct skcipher_def *sk,
-                         int enc)
-    {
-        int rc;
-
-        if (enc)
-            rc = crypto_wait_req(crypto_skcipher_encrypt(sk->req), &sk->wait);
-        else
-            rc = crypto_wait_req(crypto_skcipher_decrypt(sk->req), &sk->wait);
+This code encrypts some data with AES-256-XTS.  For sake of example,
+all inputs are random bytes, the encryption is done in-place, and it's
+assumed the code is running in a context where it can sleep.
 
-	if (rc)
-		pr_info("skcipher encrypt returned with result %d\n", rc);
-
-        return rc;
-    }
+::
 
-    /* Initialize and trigger cipher operation */
     static int test_skcipher(void)
     {
-        struct skcipher_def sk;
-        struct crypto_skcipher *skcipher = NULL;
+            struct crypto_skcipher *tfm = NULL;
             struct skcipher_request *req = NULL;
-        char *scratchpad = NULL;
-        char *ivdata = NULL;
-        unsigned char key[32];
-        int ret = -EFAULT;
-
-        skcipher = crypto_alloc_skcipher("cbc-aes-aesni", 0, 0);
-        if (IS_ERR(skcipher)) {
-            pr_info("could not allocate skcipher handle\n");
-            return PTR_ERR(skcipher);
+            u8 *data = NULL;
+            const size_t datasize = 512; /* data size in bytes */
+            struct scatterlist sg;
+            DECLARE_CRYPTO_WAIT(wait);
+            u8 iv[16];  /* AES-256-XTS takes a 16-byte IV */
+            u8 key[64]; /* AES-256-XTS takes a 64-byte key */
+            int err;
+
+            /*
+             * Allocate a tfm (a transformation object) and set the key.
+             *
+             * In real-world use, a tfm and key are typically used for many
+             * encryption/decryption operations.  But in this example, we'll just do a
+             * single encryption operation with it (which is not very efficient).
+             */
+
+            tfm = crypto_alloc_skcipher("xts(aes)", 0, 0);
+            if (IS_ERR(tfm)) {
+                    pr_err("Error allocating xts(aes) handle: %ld\n", PTR_ERR(tfm));
+                    return PTR_ERR(tfm);
             }
 
-        req = skcipher_request_alloc(skcipher, GFP_KERNEL);
-        if (!req) {
-            pr_info("could not allocate skcipher request\n");
-            ret = -ENOMEM;
+            get_random_bytes(key, sizeof(key));
+            err = crypto_skcipher_setkey(tfm, key, sizeof(key));
+            if (err) {
+                    pr_err("Error setting key: %d\n", err);
                     goto out;
             }
 
-        skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG,
-                          crypto_req_done,
-                          &sk.wait);
-
-        /* AES 256 with random key */
-        get_random_bytes(&key, 32);
-        if (crypto_skcipher_setkey(skcipher, key, 32)) {
-            pr_info("key could not be set\n");
-            ret = -EAGAIN;
+            /* Allocate a request object */
+            req = skcipher_request_alloc(tfm, GFP_KERNEL);
+            if (!req) {
+                    err = -ENOMEM;
                     goto out;
             }
 
-        /* IV will be random */
-        ivdata = kmalloc(16, GFP_KERNEL);
-        if (!ivdata) {
-            pr_info("could not allocate ivdata\n");
+            /* Prepare the input data */
+            data = kmalloc(datasize, GFP_KERNEL);
+            if (!data) {
+                    err = -ENOMEM;
                     goto out;
             }
-        get_random_bytes(ivdata, 16);
-
-        /* Input data will be random */
-        scratchpad = kmalloc(16, GFP_KERNEL);
-        if (!scratchpad) {
-            pr_info("could not allocate scratchpad\n");
+            get_random_bytes(data, datasize);
+
+            /* Initialize the IV */
+            get_random_bytes(iv, sizeof(iv));
+
+            /*
+             * Encrypt the data in-place.
+             *
+             * For simplicity, in this example we wait for the request to complete
+             * before proceeding, even if the underlying implementation is asynchronous.
+             *
+             * To decrypt instead of encrypt, just change crypto_skcipher_encrypt() to
+             * crypto_skcipher_decrypt().
+             */
+            sg_init_one(&sg, data, datasize);
+            skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG |
+                                               CRYPTO_TFM_REQ_MAY_SLEEP,
+                                          crypto_req_done, &wait);
+            skcipher_request_set_crypt(req, &sg, &sg, datasize, iv);
+            err = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
+            if (err) {
+                    pr_err("Error encrypting data: %d\n", err);
                     goto out;
             }
-        get_random_bytes(scratchpad, 16);
-
-        sk.tfm = skcipher;
-        sk.req = req;
-
-        /* We encrypt one block */
-        sg_init_one(&sk.sg, scratchpad, 16);
-        skcipher_request_set_crypt(req, &sk.sg, &sk.sg, 16, ivdata);
-        crypto_init_wait(&sk.wait);
-
-        /* encrypt data */
-        ret = test_skcipher_encdec(&sk, 1);
-        if (ret)
-            goto out;
-
-        pr_info("Encryption triggered successfully\n");
 
+            pr_debug("Encryption was successful\n");
     out:
-        if (skcipher)
-            crypto_free_skcipher(skcipher);
-        if (req)
+            crypto_free_skcipher(tfm);
             skcipher_request_free(req);
-        if (ivdata)
-            kfree(ivdata);
-        if (scratchpad)
-            kfree(scratchpad);
-        return ret;
+            kfree(data);
+            return err;
     }