Re: [PATCH 1/1] crypto: af_alg - add extra parameters for DRBG interface

[Eric Biggers <ebiggers@kernel.org>]

Just some bike-shedding:

On Mon, Jul 13, 2020 at 05:48:57PM +0100, Elena Petrova wrote:
> Extending the userspace RNG interface:
>   1. adding ALG_SET_DRBG_ENTROPY setsockopt option for entropy input;
>   2. using sendmsg syscall for specifying the additional data.
> 
> Signed-off-by: Elena Petrova <lenaptr@google.com>

A cover letter shouldn't really be used for a single patch.
Just put the details here in the commit message.

> diff --git a/crypto/algif_rng.c b/crypto/algif_rng.c
> index 087c0ad09d38..c3d1667db367 100644
> --- a/crypto/algif_rng.c
> +++ b/crypto/algif_rng.c
> @@ -53,8 +53,24 @@ struct rng_ctx {
>  #define MAXSIZE 128
>  	unsigned int len;
>  	struct crypto_rng *drng;
> +	u8 *addtl;
> +	size_t addtl_len;
>  };
>  
> +struct rng_parent_ctx {
> +	struct crypto_rng *drng;
> +	u8 *entropy;
> +};
> +
> +static void reset_addtl(struct rng_ctx *ctx)
> +{
> +	if (ctx->addtl) {
> +		kzfree(ctx->addtl);
> +		ctx->addtl = NULL;
> +	}
> +	ctx->addtl_len = 0;
> +}

It's recommended to prefix function names.  So, reset_addtl => rng_reset_addtl.

> +static int rng_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
> +{
> +	int err;
> +	struct alg_sock *ask = alg_sk(sock->sk);
> +	struct rng_ctx *ctx = ask->private;
> +
> +	reset_addtl(ctx);
> +	ctx->addtl = kzalloc(len, GFP_KERNEL);
> +	if (!ctx->addtl)
> +		return -ENOMEM;

Shouldn't the length be limited here?

Also, kmalloc would be sufficient since the memcpy_from_msg() immediately below
initializes the memory.

> +
> +	err = memcpy_from_msg(ctx->addtl, msg, len);
> +	if (err) {
> +		reset_addtl(ctx);
> +		return err;
> +	}
> +	ctx->addtl_len = len;
> +
> +	return 0;
> +}

>  static void *rng_bind(const char *name, u32 type, u32 mask)
>  {
> -	return crypto_alloc_rng(name, type, mask);
> +	struct rng_parent_ctx *pctx;
> +	void *err_ptr;
> +
> +	pctx = kzalloc(sizeof(*pctx), GFP_KERNEL);
> +	if (!pctx)
> +		return ERR_PTR(-ENOMEM);
> +
> +	pctx->drng = crypto_alloc_rng(name, type, mask);
> +	if (!IS_ERR(pctx->drng))
> +		return pctx;
> +
> +	err_ptr = pctx->drng;
> +	kfree(pctx);
> +	return err_ptr;
>  }

The error handling here is weird.  It would be more conventional to do something
like:

static void *rng_bind(const char *name, u32 type, u32 mask)
{
	struct rng_parent_ctx *pctx;
	struct crypto_rng *rng;

	pctx = kzalloc(sizeof(*pctx), GFP_KERNEL);
	if (!pctx)
		return ERR_PTR(-ENOMEM);

	rng = crypto_alloc_rng(name, type, mask);
	if (IS_ERR(rng)) {
		kfree(pctx);
		return ERR_CAST(rng);
	}

	pctx->drng = rng;
	return pctx;
}

>  
>  static void rng_release(void *private)
>  {
> -	crypto_free_rng(private);
> +	struct rng_parent_ctx *pctx = private;
> +	if (unlikely(!pctx))
> +		return;

There should be a blank line between declarations and statements.

> +	crypto_free_rng(pctx->drng);
> +	if (pctx->entropy)
> +		kzfree(pctx->entropy);

No need to check for NULL before calling kzfree().

> +static int rng_setentropy(void *private, const u8 *entropy, unsigned int len)
> +{
> +	struct rng_parent_ctx *pctx = private;
> +	u8 *kentropy = NULL;
> +
> +	if (pctx->entropy)
> +		return -EINVAL;
> +
> +	if (entropy && len) {

Best to check just 'len', so that users get an error as expected if they
accidentally pass entry=NULL len=nonzero.

> +		kentropy = kzalloc(len, GFP_KERNEL);
> +		if (!kentropy)
> +			return -ENOMEM;
> +		if (copy_from_user(kentropy, entropy, len)) {
> +			kzfree(kentropy);
> +			return -EFAULT;
> +		}

This can use memdup_user().  Also, should there be a length limit?

> +	}
> +
> +	crypto_rng_alg(pctx->drng)->set_ent(pctx->drng, kentropy, len);
> +	pctx->entropy = kentropy;

pctx->entropy could just be a bool 'has_entropy', right?  The actual value
doesn't need to be saved.

- Eric