linux-kernel - Re: [PATCH] KEYS: encrypted: avoid encrypting/decrypting stack buffers

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Date:   Sat, 01 Apr 2017 22:23:57 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     Eric Biggers <ebiggers3@...il.com>, keyrings@...r.kernel.org
Cc:     David Howells <dhowells@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Eric Biggers <ebiggers@...gle.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] KEYS: encrypted: avoid encrypting/decrypting stack
 buffers

Hi Eric,

On Sat, 2017-04-01 at 12:17 -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> Since v4.9, the crypto API cannot (normally) be used to encrypt/decrypt
> stack buffers because the stack may be virtually mapped.  Fix this for
> the padding buffers in encrypted-keys by using ZERO_PAGE for the
> encryption padding and by allocating a temporary heap buffer for the
> decryption padding.
> 
> Tested with CONFIG_DEBUG_SG=y:
> 	keyctl new_session
> 	keyctl add user master "abcdefghijklmnop" @s
> 	keyid=$(keyctl add encrypted desc "new user:master 25" @s)
> 	datablob="$(keyctl pipe $keyid)"
> 	keyctl unlink $keyid
> 	keyid=$(keyctl add encrypted desc "load $datablob" @s)
> 	datablob2="$(keyctl pipe $keyid)"
> 	[ "$datablob" = "$datablob2" ] && echo "Success!"

Have you created an encrypted key on a kernel without this patch and
attempted to load that key on a kernel with this patch?  Does it still
work?

Mimi

> 
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Herbert Xu <herbert@...dor.apana.org.au>
> Cc: Mimi Zohar <zohar@...ux.vnet.ibm.com>
> Cc: stable@...r.kernel.org # 4.9+
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
>  security/keys/encrypted-keys/encrypted.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
> index 0010955d7876..1845d47474a0 100644
> --- a/security/keys/encrypted-keys/encrypted.c
> +++ b/security/keys/encrypted-keys/encrypted.c
> @@ -480,12 +480,9 @@ static int derived_key_encrypt(struct encrypted_key_payload *epayload,
>  	struct skcipher_request *req;
>  	unsigned int encrypted_datalen;
>  	u8 iv[AES_BLOCK_SIZE];
> -	unsigned int padlen;
> -	char pad[16];
>  	int ret;
> 
>  	encrypted_datalen = roundup(epayload->decrypted_datalen, blksize);
> -	padlen = encrypted_datalen - epayload->decrypted_datalen;
> 
>  	req = init_skcipher_req(derived_key, derived_keylen);
>  	ret = PTR_ERR(req);
> @@ -493,11 +490,10 @@ static int derived_key_encrypt(struct encrypted_key_payload *epayload,
>  		goto out;
>  	dump_decrypted_data(epayload);
> 
> -	memset(pad, 0, sizeof pad);
>  	sg_init_table(sg_in, 2);
>  	sg_set_buf(&sg_in[0], epayload->decrypted_data,
>  		   epayload->decrypted_datalen);
> -	sg_set_buf(&sg_in[1], pad, padlen);
> +	sg_set_page(&sg_in[1], ZERO_PAGE(0), AES_BLOCK_SIZE, 0);
> 
>  	sg_init_table(sg_out, 1);
>  	sg_set_buf(sg_out, epayload->encrypted_data, encrypted_datalen);
> @@ -584,9 +580,14 @@ static int derived_key_decrypt(struct encrypted_key_payload *epayload,
>  	struct skcipher_request *req;
>  	unsigned int encrypted_datalen;
>  	u8 iv[AES_BLOCK_SIZE];
> -	char pad[16];
> +	u8 *pad;
>  	int ret;
> 
> +	/* Throwaway buffer to hold the unused zero padding at the end */
> +	pad = kmalloc(AES_BLOCK_SIZE, GFP_KERNEL);
> +	if (!pad)
> +		return -ENOMEM;
> +
>  	encrypted_datalen = roundup(epayload->decrypted_datalen, blksize);
>  	req = init_skcipher_req(derived_key, derived_keylen);
>  	ret = PTR_ERR(req);
> @@ -594,13 +595,12 @@ static int derived_key_decrypt(struct encrypted_key_payload *epayload,
>  		goto out;
>  	dump_encrypted_data(epayload, encrypted_datalen);
> 
> -	memset(pad, 0, sizeof pad);
>  	sg_init_table(sg_in, 1);
>  	sg_init_table(sg_out, 2);
>  	sg_set_buf(sg_in, epayload->encrypted_data, encrypted_datalen);
>  	sg_set_buf(&sg_out[0], epayload->decrypted_data,
>  		   epayload->decrypted_datalen);
> -	sg_set_buf(&sg_out[1], pad, sizeof pad);
> +	sg_set_buf(&sg_out[1], pad, AES_BLOCK_SIZE);
> 
>  	memcpy(iv, epayload->iv, sizeof(iv));
>  	skcipher_request_set_crypt(req, sg_in, sg_out, encrypted_datalen, iv);
> @@ -612,6 +612,7 @@ static int derived_key_decrypt(struct encrypted_key_payload *epayload,
>  		goto out;
>  	dump_decrypted_data(epayload);
>  out:
> +	kfree(pad);
>  	return ret;
>  }
>