[PATCH v3] evm: terminate and bound the evm_xattrs read buffer

[Roberto Sassu]

On Thu, 2026-04-23 at 23:30 +0800, Pengpeng Hou wrote:
> evm_read_xattrs() allocates size + 1 bytes, fills them from the list of
> enabled xattrs, and then passes strlen(temp) to
> simple_read_from_buffer(). When no configured xattrs are enabled, the
> fill loop stores nothing and temp[0] remains uninitialized, so strlen()
> reads beyond initialized memory.
> 
> Explicitly terminate the buffer after allocation, use snprintf() for
> each formatted line, and pass the accumulated length, without risk of
> truncation, to simple_read_from_buffer().
> 
> Fixes: fa516b66a1bf ("EVM: Allow runtime modification of the set of verified xattrs")
> Signed-off-by: Pengpeng Hou <pengpeng at iscas.ac.cn>

Reviewed-by: Roberto Sassu <roberto.sassu at huawei.com>

Thanks

Roberto

> ---
> Changes since v2:
> - adjust the changelog wording to mention why the accumulated length is
>   safe
> - add the blank line after the allocation error path
> - add a comment explaining why snprintf() cannot truncate in the fill loop
> 
> diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
> index acd840461902..4baf5e23bc97 100644
> --- a/security/integrity/evm/evm_secfs.c
> +++ b/security/integrity/evm/evm_secfs.c
> @@ -127,8 +127,8 @@ static ssize_t evm_read_xattrs(struct file *filp, char __user *buf,
>  			       size_t count, loff_t *ppos)
>  {
>  	char *temp;
> -	int offset = 0;
> -	ssize_t rc, size = 0;
> +	size_t offset = 0, size = 0;
> +	ssize_t rc;
>  	struct xattr_list *xattr;
>  
>  	if (*ppos != 0)
> @@ -151,16 +151,22 @@ static ssize_t evm_read_xattrs(struct file *filp, char __user *buf,
>  		return -ENOMEM;
>  	}
>  
> +	temp[size] = '\0';
> +
> +	/*
> +	 * No truncation possible: size is computed over the same enabled
> +	 * xattrs under xattr_list_mutex, so offset never exceeds size.
> +	 */
>  	list_for_each_entry(xattr, &evm_config_xattrnames, list) {
>  		if (!xattr->enabled)
>  			continue;
>  
> -		sprintf(temp + offset, "%s\n", xattr->name);
> -		offset += strlen(xattr->name) + 1;
> +		offset += snprintf(temp + offset, size + 1 - offset, "%s\n",
> +				   xattr->name);
>  	}
>  
>  	mutex_unlock(&xattr_list_mutex);
> -	rc = simple_read_from_buffer(buf, count, ppos, temp, strlen(temp));
> +	rc = simple_read_from_buffer(buf, count, ppos, temp, offset);
>  
>  	kfree(temp);
>