Re: [RFC PATCH] tee: add support for application-based session login methods

[Elvira Khabirova]

On Mon, 28 Sep 2020 15:43:47 +0200
Jens Wiklander <jens.wiklander@xxxxxxxxxx> wrote:

> Hi Elvira,
> 
> On Thu, Sep 17, 2020 at 06:38:03PM +0300, Elvira Khabirova wrote:
> > GP TEE Client API in addition to login methods already supported
> > in the kernel also defines several application-based methods:
> > TEEC_LOGIN_APPLICATION, TEEC_LOGIN_USER_APPLICATION, and
> > TEEC_LOGIN_GROUP_APPLICATION.
> > 
> > It specifies credentials generated for TEEC_LOGIN_APPLICATION as only
> > depending on the identity of the program, being persistent within one
> > implementation, across multiple invocations of the application
> > and across power cycles, enabling them to be used to disambiguate
> > persistent storage. The exact nature is REE-specific.
> > 
> > As the exact method of generating application identifier strings may
> > vary between vendors, setups and installations, add two suggested
> > methods and an exact framework for vendors to extend upon.
> > 
> > Signed-off-by: Elvira Khabirova <e.khabirova@xxxxxxxxxxxx>
> > ---
> >  drivers/tee/Kconfig    |  29 +++++++++
> >  drivers/tee/tee_core.c | 136 ++++++++++++++++++++++++++++++++++++++++-
> >  2 files changed, 164 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/tee/Kconfig b/drivers/tee/Kconfig
> > index e99d840c2511..4cd6e0d2aad5 100644
> > --- a/drivers/tee/Kconfig
> > +++ b/drivers/tee/Kconfig
> > @@ -11,6 +11,35 @@ config TEE
> >  	  This implements a generic interface towards a Trusted Execution
> >  	  Environment (TEE).
> >  
> > +choice
> > +	prompt "Application ID for client UUID"
> > +	depends on TEE
> > +	default TEE_APPID_PATH
> > +	help
> > +	  This option allows to choose which method will be used to generate
> > +	  application identifiers for client UUID generation when login methods
> > +	  TEE_LOGIN_APPLICATION, TEE_LOGIN_USER_APPLICATION
> > +	  and TEE_LOGIN_GROUP_APPLICATION are used.
> > +	  Please be mindful of the security of each method in your particular
> > +	  installation.
> > +
> > +	config TEE_APPID_PATH
> > +		bool "Path-based application ID"
> > +		help
> > +		  Use the executable's path as an application ID.
> > +
> > +	config TEE_APPID_SECURITY
> > +		bool "Security extended attribute based application ID"
> > +		help
> > +		  Use the executable's security extended attribute as an application ID.
> > +endchoice
> > +
> > +config TEE_APPID_SECURITY_XATTR
> > +	string "Security extended attribute to use for application ID"
> > +	depends on TEE_APPID_SECURITY
> > +	help
> > +	  Attribute to be used as an application ID (with the security prefix removed).
> > +
> >  if TEE
> >  
> >  menu "TEE drivers"
> > diff --git a/drivers/tee/tee_core.c b/drivers/tee/tee_core.c
> > index 64637e09a095..19c965dd212b 100644
> > --- a/drivers/tee/tee_core.c
> > +++ b/drivers/tee/tee_core.c
> > @@ -7,8 +7,10 @@
> >  
> >  #include <linux/cdev.h>
> >  #include <linux/cred.h>
> > +#include <linux/file.h>
> >  #include <linux/fs.h>
> >  #include <linux/idr.h>
> > +#include <linux/mm.h>
> >  #include <linux/module.h>
> >  #include <linux/slab.h>
> >  #include <linux/tee_drv.h>
> > @@ -17,11 +19,15 @@
> >  #include <crypto/sha.h>
> >  #include "tee_private.h"
> >  
> > +#ifdef CONFIG_TEE_APPID_SECURITY
> No need for the ifdef, just inlude the file unconditionally above.
> 
> > +#include <linux/security.h>
> > +#endif
> > +
> >  #define TEE_NUM_DEVICES	32
> >  
> >  #define TEE_IOCTL_PARAM_SIZE(x) (sizeof(struct tee_param) * (x))
> >  
> > -#define TEE_UUID_NS_NAME_SIZE	128
> > +#define TEE_UUID_NS_NAME_SIZE	PATH_MAX
> >  
> >  /*
> >   * TEE Client UUID name space identifier (UUIDv4)
> > @@ -125,6 +131,67 @@ static int tee_release(struct inode *inode, struct file *filp)
> >  	return 0;
> >  }
> >  
> > +#ifdef CONFIG_TEE_APPID_SECURITY
> > +static const char *tee_session_get_application_id(void **data)
> static char *get_app_id(void) should be enough.
> 
> > +{
> > +	struct file *exe_file;
> > +	const char *name = CONFIG_TEE_APPID_SECURITY_XATTR;
> > +	int len;
> > +
> > +	exe_file = get_mm_exe_file(current->mm);
> > +	if (!exe_file)
> > +		return ERR_PTR(-ENOENT);
> > +
> > +	if (!exe_file->f_inode) {
> > +		fput(exe_file);
> > +		return ERR_PTR(-ENOENT);
> > +	}
> > +
> 
> You could perhaps add a comment here on the expected properties of this
> data. Something along (don't know if I'm even close) "string
> representation of the the hash of the binary and time stamp".
> 
> > +	len = security_inode_getsecurity(exe_file->f_inode, name, data, true);
> > +	if (len < 0)
> > +		return ERR_PTR(len);
> > +
> > +	fput(exe_file);
> > +
> > +	return *data;
> > +}
> > +#endif /* CONFIG_TEE_APPID_SECURITY */
> > +
> > +#ifdef CONFIG_TEE_APPID_PATH
> > +static const char *tee_session_get_application_id(void **data)
> > +{
> > +	struct file *exe_file;
> > +	char *path;
> > +
> > +	*data = kzalloc(TEE_UUID_NS_NAME_SIZE, GFP_KERNEL);
> > +	if (!*data)
> > +		return ERR_PTR(-ENOMEM);
> > +
> > +	exe_file = get_mm_exe_file(current->mm);
> > +	if (!exe_file) {
> > +		kfree(*data);
> > +		return ERR_PTR(-ENOENT);
> > +	}
> > +
> > +	path = file_path(exe_file, *data, TEE_UUID_NS_NAME_SIZE);
> > +	if (IS_ERR(path)) {
> > +		kfree(*data);
> > +		return path;
> > +	}
> > +
> > +	fput(exe_file);
> > +
> > +	return path;
> > +}
> > +#endif /* CONFIG_TEE_APPID_PATH */
> > +
> > +#if defined(CONFIG_TEE_APPID_PATH) || defined(CONFIG_TEE_APPID_SECURITY)
> > +static void tee_session_free_application_id(void *data)
> > +{
> > +	kfree(data);
> > +}
> This function isn't needed, we can call kfree() directly.
> 
> > +#endif /* CONFIG_TEE_APPID_PATH || CONFIG_TEE_APPID_SECURITY */
> > +
> >  /**
> >   * uuid_v5() - Calculate UUIDv5
> >   * @uuid: Resulting UUID
> > @@ -197,6 +264,8 @@ int tee_session_calc_client_uuid(uuid_t *uuid, u32 connection_method,
> >  	gid_t ns_grp = (gid_t)-1;
> >  	kgid_t grp = INVALID_GID;
> >  	char *name = NULL;
> > +	void *application_id_data = NULL;
> This isn't needed
The only reason application_id_data exists (and get_application_id has a parameter)
is because when defined(CONFIG_TEE_APPID_PATH), file_path() returns a pointer
that often starts at an offset into the buffer (see the comment above d_path()).
Therefore we end up with a buffer that's not equal to application_id which we
want to free later. The other way (other than having a void **data parameter)
would be to allocate a new buffer, copy the resulting path there, and return
the new buffer instead of the one used by file_path().
Let me know which solution is preferred.

> > +	const char *application_id = NULL;
> char *app_id = NULL;
> 
> >  	int name_len;
> >  	int rc;
> >  
> > @@ -217,6 +286,14 @@ int tee_session_calc_client_uuid(uuid_t *uuid, u32 connection_method,
> >  	 * For TEEC_LOGIN_GROUP:
> >  	 * gid=<gid>
> >  	 *
> > +	 * For TEEC_LOGIN_APPLICATION:
> > +	 * app=<application id>
> > +	 *
> > +	 * For TEEC_LOGIN_USER_APPLICATION:
> > +	 * uid=<uid>:app=<application id>
> > +	 *
> > +	 * For TEEC_LOGIN_GROUP_APPLICATION:
> > +	 * gid=<gid>:app=<application id>
> >  	 */
> >  
> >  	name = kzalloc(TEE_UUID_NS_NAME_SIZE, GFP_KERNEL);
> > @@ -249,6 +326,63 @@ int tee_session_calc_client_uuid(uuid_t *uuid, u32 connection_method,
> >  		}
> >  		break;
> >  
> > +	case TEE_IOCTL_LOGIN_APPLICATION:
> > +		application_id = tee_session_get_application_id(&application_id_data);
> > +		if (IS_ERR(application_id)) {
> > +			rc = PTR_ERR(application_id);
> > +			goto out_free_name;
> > +		}
> > +
> > +		name_len = snprintf(name, TEE_UUID_NS_NAME_SIZE, "app=%s",
> > +				    application_id);
> > +		tee_session_free_application_id(application_id_data);
> > +
> > +		if (name_len >= TEE_UUID_NS_NAME_SIZE) {
> > +			rc = -E2BIG;
> > +			goto out_free_name;
> > +		}
> It looks like we remove these tests and replace them with
> if (name_len < TEE_UUID_NS_NAME_SIZE)
>         rc = uuid_v5(uuid, &tee_client_uuid_ns, name, name_len);
> else
>         rc = -E2BIG
> 
> below, just above the "out_free_name:" label.
> 
> This function is small and simple enough that it should be easy to see
> what's going on any way.
> 
> Thanks,
> Jens
> 
> > +		break;
> > +
> > +	case TEE_IOCTL_LOGIN_USER_APPLICATION:
> > +		application_id = tee_session_get_application_id(&application_id_data);
> > +		if (IS_ERR(application_id)) {
> > +			rc = PTR_ERR(application_id);
> > +			goto out_free_name;
> > +		}
> > +
> > +		name_len = snprintf(name, TEE_UUID_NS_NAME_SIZE, "uid=%x:app=%s",
> > +				    current_euid().val, application_id);
> > +		tee_session_free_application_id(application_id_data);
> > +
> > +		if (name_len >= TEE_UUID_NS_NAME_SIZE) {
> > +			rc = -E2BIG;
> > +			goto out_free_name;
> > +		}
> > +		break;
> > +
> > +	case TEE_IOCTL_LOGIN_GROUP_APPLICATION:
> > +		memcpy(&ns_grp, connection_data, sizeof(gid_t));
> > +		grp = make_kgid(current_user_ns(), ns_grp);
> > +		if (!gid_valid(grp) || !in_egroup_p(grp)) {
> > +			rc = -EPERM;
> > +			goto out_free_name;
> > +		}
> > +
> > +		application_id = tee_session_get_application_id(&application_id_data);
> > +		if (IS_ERR(application_id)) {
> > +			rc = PTR_ERR(application_id);
> > +			goto out_free_name;
> > +		}
> > +		name_len = snprintf(name, TEE_UUID_NS_NAME_SIZE, "gid=%x:app=%s",
> > +				    grp.val, application_id);
> > +		tee_session_free_application_id(application_id_data);
> > +
> > +		if (name_len >= TEE_UUID_NS_NAME_SIZE) {
> > +			rc = -E2BIG;
> > +			goto out_free_name;
> > +		}
> > +		break;
> > +
> >  	default:
> >  		rc = -EINVAL;
> >  		goto out_free_name;
> > -- 
> > 2.28.0
> >