linux-kernel - Re: [PATCH] crypto: do not free algorithm before using

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Message-ID: <20181122144441.tkfmrq3lzibq2g3y@gondor.apana.org.au>
Date:   Thu, 22 Nov 2018 22:44:41 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Pan Bian <bianpan2016@....com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] crypto: do not free algorithm before using

On Thu, Nov 22, 2018 at 06:00:16PM +0800, Pan Bian wrote:
> In multiple functions, the algorithm fields are read after its reference
> is dropped through crypto_mod_put. In this case, the algorithm memory
> may be freed, resulting in use-after-free bugs. This patch delays the
> put operation until the algorithm is never used.
> 
> Signed-off-by: Pan Bian <bianpan2016@....com>

I don't think this patch is needed.

> ---
>  crypto/cbc.c  | 6 ++++--
>  crypto/cfb.c  | 6 ++++--
>  crypto/pcbc.c | 6 ++++--
>  3 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/crypto/cbc.c b/crypto/cbc.c
> index b761b1f..dd5f332 100644
> --- a/crypto/cbc.c
> +++ b/crypto/cbc.c
> @@ -140,9 +140,8 @@ static int crypto_cbc_create(struct crypto_template *tmpl, struct rtattr **tb)
>  	spawn = skcipher_instance_ctx(inst);
>  	err = crypto_init_spawn(spawn, alg, skcipher_crypto_instance(inst),
>  				CRYPTO_ALG_TYPE_MASK);
> -	crypto_mod_put(alg);
>  	if (err)
> -		goto err_free_inst;
> +		goto err_put_alg;

We can safely drop the reference to the algorithm because the spawn
is now meant to hold a reference to it.  As long as the spawn is
alive so will the algorithm.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt