linux-kernel - Re: [PATCH] drbd: dynamically allocate shash descriptor

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Date:   Mon, 17 Jun 2019 16:36:35 +0200
From:   Roland Kammerer <roland.kammerer@...bit.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Philipp Reisner <philipp.reisner@...bit.com>,
        Lars Ellenberg <lars.ellenberg@...bit.com>,
        Jens Axboe <axboe@...nel.dk>,
        Roland Kammerer <roland.kammerer@...bit.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Kees Cook <keescook@...omium.org>, drbd-dev@...ts.linbit.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drbd: dynamically allocate shash descriptor

On Mon, Jun 17, 2019 at 03:24:13PM +0200, Arnd Bergmann wrote:
> Building with clang and KASAN, we get a warning about an overly large
> stack frame on 32-bit architectures:
> 
> drivers/block/drbd/drbd_receiver.c:921:31: error: stack frame size of 1280 bytes in function 'conn_connect'
>       [-Werror,-Wframe-larger-than=]
> 
> We already allocate other data dynamically in this function, so
> just do the same for the shash descriptor, which makes up most of
> this memory.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/block/drbd/drbd_receiver.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
> index 90ebfcae0ce6..10fb26e862d7 100644
> --- a/drivers/block/drbd/drbd_receiver.c
> +++ b/drivers/block/drbd/drbd_receiver.c
> @@ -5417,7 +5417,7 @@ static int drbd_do_auth(struct drbd_connection *connection)
>  	unsigned int key_len;
>  	char secret[SHARED_SECRET_MAX]; /* 64 byte */
>  	unsigned int resp_size;
> -	SHASH_DESC_ON_STACK(desc, connection->cram_hmac_tfm);
> +	struct shash_desc *desc;
>  	struct packet_info pi;
>  	struct net_conf *nc;
>  	int err, rv;
> @@ -5430,6 +5430,13 @@ static int drbd_do_auth(struct drbd_connection *connection)
>  	memcpy(secret, nc->shared_secret, key_len);
>  	rcu_read_unlock();
>  
> +	desc = kmalloc(sizeof(struct shash_desc) +
> +			crypto_shash_descsize(connection->cram_hmac_tfm),
> +		       GFP_KERNEL);
> +	if (!desc) {
> +		rv = -1;
> +		goto fail;
> +	}
>  	desc->tfm = connection->cram_hmac_tfm;
>  
>  	rv = crypto_shash_setkey(connection->cram_hmac_tfm, (u8 *)secret, key_len);
> @@ -5572,6 +5579,7 @@ static int drbd_do_auth(struct drbd_connection *connection)
>  	kfree(response);
>  	kfree(right_response);
>  	shash_desc_zero(desc);
> +	kfree(desc);
>  
>  	return rv;
>  }

Hi Arnd,

are you sure your cleanup is okay?

>  	shash_desc_zero(desc);
> +	kfree(desc);

You shash_desc_zero() a potential NULL pointer. memzero_expicit() in the
function then dereferences it:

memzero_explicit(desc,
	sizeof(*desc) + crypto_shash_descsize(desc->tfm));

Maybe some if (desc) guard?

Best, rck