s390/archrandom: add parameter check for s390_arch_random_generate

A review of the code showed, that this function which is exposed
within the whole kernel should do a parameter check for the
amount of bytes requested. If this requested bytes is too high
an unsigned int overflow could happen causing this function to
try to memcpy a really big memory chunk.

This is not a security issue as there are only two invocations
of this function from arch/s390/include/asm/archrandom.h and both
are not exposed to userland.
Reported-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/crypto/arch_random.c

@@ -54,6 +54,10 @@ static DECLARE_DELAYED_WORK(arch_rng_work, arch_rng_refill_buffer);
 
 bool s390_arch_random_generate(u8 *buf, unsigned int nbytes)
 {
+	/* max hunk is ARCH_RNG_BUF_SIZE */
+	if (nbytes > ARCH_RNG_BUF_SIZE)
+		return false;
+
 	/* lock rng buffer */
 	if (!spin_trylock(&arch_rng_lock))
 		return false;