crypto: arm64/aes-neonbs - fix returning final keystream block

The arm64 NEON bit-sliced implementation of AES-CTR fails the improved
skcipher tests because it sometimes produces the wrong ciphertext.  The
bug is that the final keystream block isn't returned from the assembly
code when the number of non-final blocks is zero.  This can happen if
the input data ends a few bytes after a page boundary.  In this case the
last bytes get "encrypted" by XOR'ing them with uninitialized memory.

Fix the assembly code to return the final keystream block when needed.

Fixes: 88a3f582 ("crypto: arm64/aes - don't use IV buffer to return final keystream block")
Cc: <stable@vger.kernel.org> # v4.11+
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

arch/arm64/crypto/aes-neonbs-core.S

@@ -971,18 +971,22 @@ CPU_LE(	rev		x8, x8		)
 
 8:	next_ctr	v0
 	st1		{v0.16b}, [x24]
-	cbz		x23, 0f
+	cbz		x23, .Lctr_done
 
 	cond_yield_neon	98b
 	b		99b
 
-0:	frame_pop
+.Lctr_done:
+	frame_pop
 	ret
 
 	/*
 	 * If we are handling the tail of the input (x6 != NULL), return the
 	 * final keystream block back to the caller.
 	 */
+0:	cbz		x25, 8b
+	st1		{v0.16b}, [x25]
+	b		8b
 1:	cbz		x25, 8b
 	st1		{v1.16b}, [x25]
 	b		8b