potential underfow in crypto/lrw.c setkey() setkey

[Dan Carpenter <dan.carpenter@oracle.com>]

crypto/lrw.c
    72  static int setkey(struct crypto_skcipher *parent, const u8 *key,
    73                    unsigned int keylen)
    74  {
    75          struct priv *ctx = crypto_skcipher_ctx(parent);
    76          struct crypto_skcipher *child = ctx->child;
    77          int err, bsize = LRW_BLOCK_SIZE;
    78          const u8 *tweak = key + keylen - bsize;
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Smatch thinks that keylen is user controlled from zero to some upper
bound.  How do we know it's >= LRW_BLOCK_SIZE (16)?

I find the crypto code sort of hard to follow...  There are a bunch of
setkey pointers and they're sometimes called recursively.  Is there
some trick or hints?

    79          be128 tmp = { 0 };
    80          int i;
    81  
    82          crypto_skcipher_clear_flags(child, CRYPTO_TFM_REQ_MASK);
    83          crypto_skcipher_set_flags(child, crypto_skcipher_get_flags(parent) &
    84                                           CRYPTO_TFM_REQ_MASK);
    85          err = crypto_skcipher_setkey(child, key, keylen - bsize);
    86          crypto_skcipher_set_flags(parent, crypto_skcipher_get_flags(child) &
    87                                            CRYPTO_TFM_RES_MASK);
    88          if (err)
    89                  return err;
    90  
    91          if (ctx->table)
    92                  gf128mul_free_64k(ctx->table);
    93  
    94          /* initialize multiplication table for Key2 */
    95          ctx->table = gf128mul_init_64k_bbe((be128 *)tweak);
    96          if (!ctx->table)
    97                  return -ENOMEM;
    98  
    99          /* initialize optimization table */
   100          for (i = 0; i < 128; i++) {
   101                  setbit128_bbe(&tmp, i);
   102                  ctx->mulinc[i] = tmp;
   103                  gf128mul_64k_bbe(&ctx->mulinc[i], ctx->table);
   104          }
   105  
   106          return 0;
   107  }

regards,
dan carpenter