Re: [PATCH] crypto: arm64/aes-ccm - Rewrite skcipher walker loop

[Tianjia Zhang <tianjia.zhang@linux.alibaba.com>]

On 1/30/23 4:58 PM, Herbert Xu wrote:
> An often overlooked aspect of the skcipher walker API is that an
> error is not just indicated by a non-zero return value, but by the
> fact that walk->nbytes is zero.
> 
> Thus it is an error to call skcipher_walk_done after getting back
> walk->nbytes == 0 from the previous interaction with the walker.
> 
> This is because when walk->nbytes is zero the walker is left in
> an undefined state and any further calls to it may try to free
> uninitialised stack memory.
> 
> The arm64 ccm code has to deal with zero-length messages, and
> it needs to process data even when walk->nbytes == 0 is returned.
> It doesn't have this bug because there is an explicit check for
> walk->nbytes != 0 prior to the skcipher_walk_done call.
> 
> However, the loop is still sufficiently different from the usual
> layout and it appears to have been copied into other code which
> then ended up with this bug.  This patch rewrites it to follow the
> usual convention of checking walk->nbytes.
> 
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
> 
> diff --git a/arch/arm64/crypto/aes-ce-ccm-glue.c b/arch/arm64/crypto/aes-ce-ccm-glue.c
> index c4f14415f5f0..25cd3808ecbe 100644
> --- a/arch/arm64/crypto/aes-ce-ccm-glue.c
> +++ b/arch/arm64/crypto/aes-ce-ccm-glue.c
> @@ -161,43 +161,39 @@ static int ccm_encrypt(struct aead_request *req)
>   	memcpy(buf, req->iv, AES_BLOCK_SIZE);
>   
>   	err = skcipher_walk_aead_encrypt(&walk, req, false);
> -	if (unlikely(err))
> -		return err;
>   
>   	kernel_neon_begin();
>   
>   	if (req->assoclen)
>   		ccm_calculate_auth_mac(req, mac);
>   
> -	do {
> +	while (walk.nbytes) {
>   		u32 tail = walk.nbytes % AES_BLOCK_SIZE;
> +		bool final = walk.nbytes == walk.total;
>   
> -		if (walk.nbytes == walk.total)
> +		if (final)
>   			tail = 0;
>   
>   		ce_aes_ccm_encrypt(walk.dst.virt.addr, walk.src.virt.addr,
>   				   walk.nbytes - tail, ctx->key_enc,
>   				   num_rounds(ctx), mac, walk.iv);
>   
> -		if (walk.nbytes == walk.total)
> -			ce_aes_ccm_final(mac, buf, ctx->key_enc, num_rounds(ctx));
> +		if (!final)
> +			kernel_neon_end();
> +		err = skcipher_walk_done(&walk, tail);
> +		if (!final)
> +			kernel_neon_begin();
> +	}
>   
> -		kernel_neon_end();
> +	ce_aes_ccm_final(mac, buf, ctx->key_enc, num_rounds(ctx));
>   
> -		if (walk.nbytes) {
> -			err = skcipher_walk_done(&walk, tail);
> -			if (unlikely(err))
> -				return err;
> -			if (unlikely(walk.nbytes))
> -				kernel_neon_begin();
> -		}
> -	} while (walk.nbytes);
> +	kernel_neon_end();
>   
>   	/* copy authtag to end of dst */
>   	scatterwalk_map_and_copy(mac, req->dst, req->assoclen + req->cryptlen,
>   				 crypto_aead_authsize(aead), 1);
>   
> -	return 0;
> +	return err;
>   }

I think the following is a more cleaner rewriting form of the loop,
which handles the last chunk separately, and both gcm and ccm can be
handled similarly.

	while (walk.nbytes != walk.total) {
		u32 tail = walk.nbytes % AES_BLOCK_SIZE;

		ce_aes_ccm_encrypt(walk.dst.virt.addr, walk.src.virt.addr,
				   walk.nbytes - tail, ctx->key_enc,
				   num_rounds(ctx), mac, walk.iv);

		kernel_neon_end();

		err = skcipher_walk_done(&walk, tail);

		kernel_neon_begin();
	}

	if (walk.nbytes) {
		ce_aes_ccm_encrypt(walk.dst.virt.addr, walk.src.virt.addr,
				   walk.nbytes, ctx->key_enc,
				   num_rounds(ctx), mac, walk.iv);

		err = skcipher_walk_done(&walk, 0);
	}

	ce_aes_ccm_final(mac, buf, ctx->key_enc, num_rounds(ctx));

	kernel_neon_end();


I have tested it initially. What are your opinions？

>   
>   static int ccm_decrypt(struct aead_request *req)
> @@ -219,37 +215,36 @@ static int ccm_decrypt(struct aead_request *req)
>   	memcpy(buf, req->iv, AES_BLOCK_SIZE);
>   
>   	err = skcipher_walk_aead_decrypt(&walk, req, false);
> -	if (unlikely(err))
> -		return err;
>   
>   	kernel_neon_begin();
>   
>   	if (req->assoclen)
>   		ccm_calculate_auth_mac(req, mac);
>   
> -	do {
> +	while (walk.nbytes) {
>   		u32 tail = walk.nbytes % AES_BLOCK_SIZE;
> +		bool final = walk.nbytes == walk.total;
>   
> -		if (walk.nbytes == walk.total)
> +		if (final)
>   			tail = 0;
>   
>   		ce_aes_ccm_decrypt(walk.dst.virt.addr, walk.src.virt.addr,
>   				   walk.nbytes - tail, ctx->key_enc,
>   				   num_rounds(ctx), mac, walk.iv);
>   
> -		if (walk.nbytes == walk.total)
> -			ce_aes_ccm_final(mac, buf, ctx->key_enc, num_rounds(ctx));
> +		if (!final)
> +			kernel_neon_end();
> +		err = skcipher_walk_done(&walk, tail);
> +		if (!final)
> +			kernel_neon_begin();
> +	}
>   
> -		kernel_neon_end();
> +	ce_aes_ccm_final(mac, buf, ctx->key_enc, num_rounds(ctx));
>   
> -		if (walk.nbytes) {
> -			err = skcipher_walk_done(&walk, tail);
> -			if (unlikely(err))
> -				return err;
> -			if (unlikely(walk.nbytes))
> -				kernel_neon_begin();
> -		}
> -	} while (walk.nbytes);
> +	kernel_neon_end();
> +
> +	if (unlikely(err))
> +		return err;
>   
>   	/* compare calculated auth tag with the stored one */
>   	scatterwalk_map_and_copy(buf, req->src,