linux-ext4 - Re: [PATCH 2/2] fscrypto: don't use on-stack buffer for key derivation

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Date:   Mon, 7 Nov 2016 14:22:16 +0100
From:   Richard Weinberger <richard@....at>
To:     Eric Biggers <ebiggers@...gle.com>, linux-fsdevel@...r.kernel.org
Cc:     linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-crypto@...r.kernel.org, tytso@....edu, jaegeuk@...nel.org,
        luto@...nel.org
Subject: Re: [PATCH 2/2] fscrypto: don't use on-stack buffer for key
 derivation

On 03.11.2016 23:03, Eric Biggers wrote:
> With the new (in 4.9) option to use a virtually-mapped stack
> (CONFIG_VMAP_STACK), stack buffers cannot be used as input/output for
> the scatterlist crypto API because they may not be directly mappable to
> struct page.  get_crypt_info() was using a stack buffer to hold the

See 1/2. :-)

> output from the encryption operation used to derive the per-file key.
> Fix it by using a heap buffer.
> 
> This bug could most easily be observed in a CONFIG_DEBUG_SG kernel
> because this allowed the BUG in sg_set_buf() to be triggered.
> 
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
>  fs/crypto/keyinfo.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
> index 82f0285..67fb6d8 100644
> --- a/fs/crypto/keyinfo.c
> +++ b/fs/crypto/keyinfo.c
> @@ -185,7 +185,7 @@ int get_crypt_info(struct inode *inode)
>  	struct crypto_skcipher *ctfm;
>  	const char *cipher_str;
>  	int keysize;
> -	u8 raw_key[FS_MAX_KEY_SIZE];
> +	u8 *raw_key = NULL;
>  	int res;
>  
>  	res = fscrypt_initialize();
> @@ -238,6 +238,15 @@ int get_crypt_info(struct inode *inode)
>  	if (res)
>  		goto out;
>  
> +	/*
> +	 * This cannot be a stack buffer because it is passed to the scatterlist
> +	 * crypto API as part of key derivation.
> +	 */
> +	res = -ENOMEM;
> +	raw_key = kmalloc(FS_MAX_KEY_SIZE, GFP_NOFS);
> +	if (!raw_key)
> +		goto out;
> +
>  	if (fscrypt_dummy_context_enabled(inode)) {
>  		memset(raw_key, 0x42, FS_AES_256_XTS_KEY_SIZE);
>  		goto got_key;
> @@ -276,7 +285,8 @@ int get_crypt_info(struct inode *inode)
>  	if (res)
>  		goto out;
>  
> -	memzero_explicit(raw_key, sizeof(raw_key));
> +	kzfree(raw_key);
> +	raw_key = NULL;
>  	if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) != NULL) {
>  		put_crypt_info(crypt_info);
>  		goto retry;
> @@ -287,7 +297,7 @@ int get_crypt_info(struct inode *inode)
>  	if (res == -ENOKEY)
>  		res = 0;
>  	put_crypt_info(crypt_info);
> -	memzero_explicit(raw_key, sizeof(raw_key));
> +	kzfree(raw_key);
>  	return res;
>  }

Reviewed-by: Richard Weinberger <richard@....at>

Thanks,
//richard

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