Re: [v4 PATCH] crypto: arm64/sm4-gcm - Fix possible crash in GCM cryption

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

Hi Herbert,

On 2/2/23 4:33 PM, Herbert Xu wrote:
> On Wed, Feb 01, 2023 at 08:31:33PM +0800, Tianjia Zhang wrote:
>>
>> +	sm4_ce_pmull_gcm_crypt(ctx->key.rkey_enc, walk->dst.virt.addr,
>> +			       walk->src.virt.addr, iv, walk->nbytes, ghash,
>> +			       ctx->ghash_table, (const u8 *)&lengths);
> 
> I still think this is error-prone.  When walk->nbytes == 0,
> walk->src and walk->dst are undefined.  Sure you could argue
> that the underlying assembly code won't touch the values, but
> accessing uninitialised memory even if just to throw them away
> is still a bit icky.

You're right, whether used or not, accessing an undefined pointer is
always ugly. This benefited me a lot.

> 
> Anyway, here's my attempt at rewriting the gcm loop:
> 
> ---8<---
> 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 sm4 arm64 ccm code gets this wrong and ends up calling
> skcipher_walk_done even when walk->nbytes is zero.
> 
> This patch rewrites the loop in a form that resembles other callers.
> 
> Reported-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
> Fixes: ae1b83c7d572 ("crypto: arm64/sm4 - add CE implementation for GCM mode")
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Thanks for the fix, this patch works find to me, so

Tested-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>

> 
> diff --git a/arch/arm64/crypto/sm4-ce-gcm-glue.c b/arch/arm64/crypto/sm4-ce-gcm-glue.c
> index c450a2025ca9..73bfb6972d3a 100644
> --- a/arch/arm64/crypto/sm4-ce-gcm-glue.c
> +++ b/arch/arm64/crypto/sm4-ce-gcm-glue.c
> @@ -135,22 +135,23 @@ static void gcm_calculate_auth_mac(struct aead_request *req, u8 ghash[])
>   }
>   
>   static int gcm_crypt(struct aead_request *req, struct skcipher_walk *walk,
> -		     struct sm4_gcm_ctx *ctx, u8 ghash[],
> +		     u8 ghash[], int err,
>   		     void (*sm4_ce_pmull_gcm_crypt)(const u32 *rkey_enc,
>   				u8 *dst, const u8 *src, u8 *iv,
>   				unsigned int nbytes, u8 *ghash,
>   				const u8 *ghash_table, const u8 *lengths))
>   {
> +	struct crypto_aead *aead = crypto_aead_reqtfm(req);
> +	struct sm4_gcm_ctx *ctx = crypto_aead_ctx(aead);
>   	u8 __aligned(8) iv[SM4_BLOCK_SIZE];
>   	be128 __aligned(8) lengths;
> -	int err;
>   
>   	memset(ghash, 0, SM4_BLOCK_SIZE);
>   
>   	lengths.a = cpu_to_be64(req->assoclen * 8);
>   	lengths.b = cpu_to_be64(walk->total * 8);
>   
> -	memcpy(iv, walk->iv, GCM_IV_SIZE);
> +	memcpy(iv, req->iv, GCM_IV_SIZE);
>   	put_unaligned_be32(2, iv + GCM_IV_SIZE);
>   
>   	kernel_neon_begin();
> @@ -158,49 +159,51 @@ static int gcm_crypt(struct aead_request *req, struct skcipher_walk *walk,
>   	if (req->assoclen)
>   		gcm_calculate_auth_mac(req, ghash);
>   
> -	do {
> +	while (walk->nbytes) {
>   		unsigned int tail = walk->nbytes % SM4_BLOCK_SIZE;
>   		const u8 *src = walk->src.virt.addr;
>   		u8 *dst = walk->dst.virt.addr;
>   
>   		if (walk->nbytes == walk->total) {
> -			tail = 0;
> -
>   			sm4_ce_pmull_gcm_crypt(ctx->key.rkey_enc, dst, src, iv,
>   					       walk->nbytes, ghash,
>   					       ctx->ghash_table,
>   					       (const u8 *)&lengths);
> -		} else if (walk->nbytes - tail) {
> -			sm4_ce_pmull_gcm_crypt(ctx->key.rkey_enc, dst, src, iv,
> -					       walk->nbytes - tail, ghash,
> -					       ctx->ghash_table, NULL);
> +
> +			kernel_neon_end();
> +
> +			return skcipher_walk_done(walk, 0);
>   		}
>   
> +		sm4_ce_pmull_gcm_crypt(ctx->key.rkey_enc, dst, src, iv,
> +				       walk->nbytes - tail, ghash,
> +				       ctx->ghash_table, NULL);
> +
>   		kernel_neon_end();
>   
>   		err = skcipher_walk_done(walk, tail);
> -		if (err)
> -			return err;
> -		if (walk->nbytes)
> -			kernel_neon_begin();
> -	} while (walk->nbytes > 0);
>   
> -	return 0;
> +		kernel_neon_begin();
> +	}
> +
> +	sm4_ce_pmull_gcm_crypt(ctx->key.rkey_enc, NULL, NULL, iv,
> +			       walk->nbytes, ghash, ctx->ghash_table,
> +			       (const u8 *)&lengths);
> +
> +	kernel_neon_end();
> +
> +	return err;
>   }
>   
>   static int gcm_encrypt(struct aead_request *req)
>   {
>   	struct crypto_aead *aead = crypto_aead_reqtfm(req);
> -	struct sm4_gcm_ctx *ctx = crypto_aead_ctx(aead);
>   	u8 __aligned(8) ghash[SM4_BLOCK_SIZE];
>   	struct skcipher_walk walk;
>   	int err;
>   
>   	err = skcipher_walk_aead_encrypt(&walk, req, false);
> -	if (err)
> -		return err;
> -
> -	err = gcm_crypt(req, &walk, ctx, ghash, sm4_ce_pmull_gcm_enc);
> +	err = gcm_crypt(req, &walk, ghash, err, sm4_ce_pmull_gcm_enc);
>   	if (err)
>   		return err;
>   
> @@ -215,17 +218,13 @@ static int gcm_decrypt(struct aead_request *req)
>   {
>   	struct crypto_aead *aead = crypto_aead_reqtfm(req);
>   	unsigned int authsize = crypto_aead_authsize(aead);
> -	struct sm4_gcm_ctx *ctx = crypto_aead_ctx(aead);
>   	u8 __aligned(8) ghash[SM4_BLOCK_SIZE];
>   	u8 authtag[SM4_BLOCK_SIZE];
>   	struct skcipher_walk walk;
>   	int err;
>   
>   	err = skcipher_walk_aead_decrypt(&walk, req, false);
> -	if (err)
> -		return err;
> -
> -	err = gcm_crypt(req, &walk, ctx, ghash, sm4_ce_pmull_gcm_dec);
> +	err = gcm_crypt(req, &walk, ghash, err, sm4_ce_pmull_gcm_dec);
>   	if (err)
>   		return err;
>