Re: [PATCH v2 1/2] evm: Alloc evm_digest in evm_verify_hmac() if CONFIG_VMAP_STACK=y

[Eric Biggers]

On Thu, Dec 01, 2022 at 11:06:24AM +0100, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> 
> Commit ac4e97abce9b8 ("scatterlist: sg_set_buf() argument must be in linear
> mapping") checks that both the signature and the digest reside in the
> linear mapping area.
> 
> However, more recently commit ba14a194a434c ("fork: Add generic vmalloced
> stack support"), made it possible to move the stack in the vmalloc area,
> which is not contiguous, and thus not suitable for sg_set_buf() which needs
> adjacent pages.
> 
> Fix this by checking if CONFIG_VMAP_STACK is enabled. If yes, allocate an
> evm_digest structure, and use that instead of the in-stack counterpart.
> 
> Cc: stable@xxxxxxxxxxxxxxx # 4.9.x
> Fixes: ba14a194a434 ("fork: Add generic vmalloced stack support")
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---
>  security/integrity/evm/evm_main.c | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 23d484e05e6f..7f76d6103f2e 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -174,6 +174,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  	struct signature_v2_hdr *hdr;
>  	enum integrity_status evm_status = INTEGRITY_PASS;
>  	struct evm_digest digest;
> +	struct evm_digest *digest_ptr = &digest;
>  	struct inode *inode;
>  	int rc, xattr_len, evm_immutable = 0;
>  
> @@ -231,14 +232,26 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  		}
>  
>  		hdr = (struct signature_v2_hdr *)xattr_data;
> -		digest.hdr.algo = hdr->hash_algo;
> +
> +		if (IS_ENABLED(CONFIG_VMAP_STACK)) {
> +			digest_ptr = kmalloc(sizeof(*digest_ptr), GFP_NOFS);
> +			if (!digest_ptr) {
> +				rc = -ENOMEM;
> +				break;
> +			}
> +		}
> +
> +		digest_ptr->hdr.algo = hdr->hash_algo;
> +
>  		rc = evm_calc_hash(dentry, xattr_name, xattr_value,
> -				   xattr_value_len, xattr_data->type, &digest);
> +				   xattr_value_len, xattr_data->type,
> +				   digest_ptr);
>  		if (rc)
>  			break;
>  		rc = integrity_digsig_verify(INTEGRITY_KEYRING_EVM,
>  					(const char *)xattr_data, xattr_len,
> -					digest.digest, digest.hdr.length);
> +					digest_ptr->digest,
> +					digest_ptr->hdr.length);
>  		if (!rc) {
>  			inode = d_backing_inode(dentry);
>  
> @@ -268,8 +281,11 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  		else
>  			evm_status = INTEGRITY_FAIL;
>  	}
> -	pr_debug("digest: (%d) [%*phN]\n", digest.hdr.length, digest.hdr.length,
> -		  digest.digest);
> +	pr_debug("digest: (%d) [%*phN]\n", digest_ptr->hdr.length,
> +		 digest_ptr->hdr.length, digest_ptr->digest);
> +
> +	if (digest_ptr && digest_ptr != &digest)
> +		kfree(digest_ptr);

What is the actual problem here?  Where is a scatterlist being created from this
buffer?  AFAICS it never happens.

- Eric