Re: [PATCH] keys: Move permissions checking decisions into the checking code

From: David Howells
Date: Fri May 15 2020 - 18:27:31 EST

Stephen Smalley <stephen.smalley.work@xxxxxxxxx> wrote:

> > I can go back to the enum patch for the moment if you and Casey can put up
> > with that for the moment?
>
> Yes, let's do that.

Okay. I'll use the attached. I've added a note into the commit message to
indicate what should be done in future. I won't put the other patch into my
keys-next branch yet as it will conflict with this.

David
---
commit e9c98329b2201e6df4edb34dc386e6ef1eeeb78b
Author: David Howells <dhowells@xxxxxxxxxx>
Date: Tue May 12 15:16:29 2020 +0100

keys: Make the KEY_NEED_* perms an enum rather than a mask

Since the meaning of combining the KEY_NEED_* constants is undefined, make
it so that you can't do that by turning them into an enum.

The enum is also given some extra values to represent special
circumstances, such as:

(1) The '0' value is reserved and causes a warning to trap the parameter
being unset.

(2) The key is to be unlinked and we require no permissions on it, only
the keyring, (this replaces the KEY_LOOKUP_FOR_UNLINK flag).

(3) An override due to CAP_SYS_ADMIN.

(4) An override due to an instantiation token being present.

(5) The permissions check is being deferred to later key_permission()
calls.

The extra values give the opportunity for LSMs to audit these situations.

[Note: This really needs overhauling so that lookup_user_key() tells
key_task_permission() and the LSM what operation is being done and leaves
it to those functions to decide how to map that onto the available
permits. However, I don't really want to make these change in the middle
of the notifications patchset.]

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
cc: Paul Moore <paul@xxxxxxxxxxxxxx>
cc: Stephen Smalley <stephen.smalley.work@xxxxxxxxx>
cc: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
cc: keyrings@xxxxxxxxxxxxxxx
cc: selinux@xxxxxxxxxxxxxxx

diff --git a/include/linux/key.h b/include/linux/key.h
index b99b40db08fc..0f2e24f13c2b 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -71,6 +71,23 @@ struct net;

#define KEY_PERM_UNDEF 0xffffffff

+/*
+ * The permissions required on a key that we're looking up.
+ */
+enum key_need_perm {
+ KEY_NEED_UNSPECIFIED, /* Needed permission unspecified */
+ KEY_NEED_VIEW, /* Require permission to view attributes */
+ KEY_NEED_READ, /* Require permission to read content */
+ KEY_NEED_WRITE, /* Require permission to update / modify */
+ KEY_NEED_SEARCH, /* Require permission to search (keyring) or find (key) */
+ KEY_NEED_LINK, /* Require permission to link */
+ KEY_NEED_SETATTR, /* Require permission to change attributes */
+ KEY_NEED_UNLINK, /* Require permission to unlink key */
+ KEY_SYSADMIN_OVERRIDE, /* Special: override by CAP_SYS_ADMIN */
+ KEY_AUTHTOKEN_OVERRIDE, /* Special: override by possession of auth token */
+ KEY_DEFER_PERM_CHECK, /* Special: permission check is deferred */
+};
+
struct seq_file;
struct user_struct;
struct signal_struct;
@@ -420,20 +437,9 @@ static inline key_serial_t key_serial(const struct key *key)
extern void key_set_timeout(struct key *, unsigned);

extern key_ref_t lookup_user_key(key_serial_t id, unsigned long flags,
- key_perm_t perm);
+ enum key_need_perm need_perm);
extern void key_free_user_ns(struct user_namespace *);

-/*
- * The permissions required on a key that we're looking up.
- */
-#define KEY_NEED_VIEW 0x01 /* Require permission to view attributes */
-#define KEY_NEED_READ 0x02 /* Require permission to read content */
-#define KEY_NEED_WRITE 0x04 /* Require permission to update / modify */
-#define KEY_NEED_SEARCH 0x08 /* Require permission to search (keyring) or find (key) */
-#define KEY_NEED_LINK 0x10 /* Require permission to link */
-#define KEY_NEED_SETATTR 0x20 /* Require permission to change attributes */
-#define KEY_NEED_ALL 0x3f /* All the above permissions */
-
static inline short key_read_state(const struct key *key)
{
/* Barrier versus mark_key_instantiated(). */
diff --git a/include/linux/security.h b/include/linux/security.h
index e7914e4e0b02..57aac14e3418 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -1767,8 +1767,8 @@ static inline int security_path_chroot(const struct path *path)

int security_key_alloc(struct key *key, const struct cred *cred, unsigned long flags);
void security_key_free(struct key *key);
-int security_key_permission(key_ref_t key_ref,
- const struct cred *cred, unsigned perm);
+int security_key_permission(key_ref_t key_ref, const struct cred *cred,
+ enum key_need_perm need_perm);
int security_key_getsecurity(struct key *key, char **_buffer);

#else
@@ -1786,7 +1786,7 @@ static inline void security_key_free(struct key *key)

static inline int security_key_permission(key_ref_t key_ref,
const struct cred *cred,
- unsigned perm)
+ enum key_need_perm need_perm)
{
return 0;
}
diff --git a/security/keys/internal.h b/security/keys/internal.h
index 28e17f4f3328..1fc17cb317a9 100644
--- a/security/keys/internal.h
+++ b/security/keys/internal.h
@@ -167,7 +167,6 @@ extern bool lookup_user_key_possessed(const struct key *key,
const struct key_match_data *match_data);
#define KEY_LOOKUP_CREATE 0x01
#define KEY_LOOKUP_PARTIAL 0x02
-#define KEY_LOOKUP_FOR_UNLINK 0x04

extern long join_session_keyring(const char *name);
extern void key_change_session_keyring(struct callback_head *twork);
@@ -183,7 +182,7 @@ extern void key_gc_keytype(struct key_type *ktype);

extern int key_task_permission(const key_ref_t key_ref,
const struct cred *cred,
- key_perm_t perm);
+ enum key_need_perm need_perm);

static inline void notify_key(struct key *key,
enum key_notification_subtype subtype, u32 aux)
@@ -205,9 +204,10 @@ static inline void notify_key(struct key *key,
/*
* Check to see whether permission is granted to use a key in the desired way.
*/
-static inline int key_permission(const key_ref_t key_ref, unsigned perm)
+static inline int key_permission(const key_ref_t key_ref,
+ enum key_need_perm need_perm)
{
- return key_task_permission(key_ref, current_cred(), perm);
+ return key_task_permission(key_ref, current_cred(), need_perm);
}

extern struct key_type key_type_request_key_auth;
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 7d8de1c9a478..6763ee45e04d 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -434,7 +434,7 @@ long keyctl_invalidate_key(key_serial_t id)

/* Root is permitted to invalidate certain special keys */
if (capable(CAP_SYS_ADMIN)) {
- key_ref = lookup_user_key(id, 0, 0);
+ key_ref = lookup_user_key(id, 0, KEY_SYSADMIN_OVERRIDE);
if (IS_ERR(key_ref))
goto error;
if (test_bit(KEY_FLAG_ROOT_CAN_INVAL,
@@ -479,7 +479,8 @@ long keyctl_keyring_clear(key_serial_t ringid)

/* Root is permitted to invalidate certain special keyrings */
if (capable(CAP_SYS_ADMIN)) {
- keyring_ref = lookup_user_key(ringid, 0, 0);
+ keyring_ref = lookup_user_key(ringid, 0,
+ KEY_SYSADMIN_OVERRIDE);
if (IS_ERR(keyring_ref))
goto error;
if (test_bit(KEY_FLAG_ROOT_CAN_CLEAR,
@@ -563,7 +564,7 @@ long keyctl_keyring_unlink(key_serial_t id, key_serial_t ringid)
goto error;
}

- key_ref = lookup_user_key(id, KEY_LOOKUP_FOR_UNLINK, 0);
+ key_ref = lookup_user_key(id, KEY_LOOKUP_PARTIAL, KEY_NEED_UNLINK);
if (IS_ERR(key_ref)) {
ret = PTR_ERR(key_ref);
goto error2;
@@ -663,7 +664,7 @@ long keyctl_describe_key(key_serial_t keyid,
key_put(instkey);
key_ref = lookup_user_key(keyid,
KEY_LOOKUP_PARTIAL,
- 0);
+ KEY_AUTHTOKEN_OVERRIDE);
if (!IS_ERR(key_ref))
goto okay;
}
@@ -833,7 +834,7 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
size_t key_data_len;

/* find the key first */
- key_ref = lookup_user_key(keyid, 0, 0);
+ key_ref = lookup_user_key(keyid, 0, KEY_DEFER_PERM_CHECK);
if (IS_ERR(key_ref)) {
ret = -ENOKEY;
goto out;
@@ -1471,7 +1472,7 @@ long keyctl_set_timeout(key_serial_t id, unsigned timeout)
key_put(instkey);
key_ref = lookup_user_key(id,
KEY_LOOKUP_PARTIAL,
- 0);
+ KEY_AUTHTOKEN_OVERRIDE);
if (!IS_ERR(key_ref))
goto okay;
}
@@ -1579,7 +1580,8 @@ long keyctl_get_security(key_serial_t keyid,
return PTR_ERR(instkey);
key_put(instkey);

- key_ref = lookup_user_key(keyid, KEY_LOOKUP_PARTIAL, 0);
+ key_ref = lookup_user_key(keyid, KEY_LOOKUP_PARTIAL,
+ KEY_AUTHTOKEN_OVERRIDE);
if (IS_ERR(key_ref))
return PTR_ERR(key_ref);
}
diff --git a/security/keys/permission.c b/security/keys/permission.c
index 085f907b64ac..4a61f804e80f 100644
--- a/security/keys/permission.c
+++ b/security/keys/permission.c
@@ -13,7 +13,7 @@
* key_task_permission - Check a key can be used
* @key_ref: The key to check.
* @cred: The credentials to use.
- * @perm: The permissions to check for.
+ * @need_perm: The permission required.
*
* Check to see whether permission is granted to use a key in the desired way,
* but permit the security modules to override.
@@ -24,12 +24,30 @@
* permissions bits or the LSM check.
*/
int key_task_permission(const key_ref_t key_ref, const struct cred *cred,
- unsigned perm)
+ enum key_need_perm need_perm)
{
struct key *key;
- key_perm_t kperm;
+ key_perm_t kperm, mask;
int ret;

+ switch (need_perm) {
+ default:
+ WARN_ON(1);
+ return -EACCES;
+ case KEY_NEED_UNLINK:
+ case KEY_SYSADMIN_OVERRIDE:
+ case KEY_AUTHTOKEN_OVERRIDE:
+ case KEY_DEFER_PERM_CHECK:
+ goto lsm;
+
+ case KEY_NEED_VIEW: mask = KEY_OTH_VIEW; break;
+ case KEY_NEED_READ: mask = KEY_OTH_READ; break;
+ case KEY_NEED_WRITE: mask = KEY_OTH_WRITE; break;
+ case KEY_NEED_SEARCH: mask = KEY_OTH_SEARCH; break;
+ case KEY_NEED_LINK: mask = KEY_OTH_LINK; break;
+ case KEY_NEED_SETATTR: mask = KEY_OTH_SETATTR; break;
+ }
+
key = key_ref_to_ptr(key_ref);

/* use the second 8-bits of permissions for keys the caller owns */
@@ -64,13 +82,12 @@ int key_task_permission(const key_ref_t key_ref, const struct cred *cred,
if (is_key_possessed(key_ref))
kperm |= key->perm >> 24;

- kperm = kperm & perm & KEY_NEED_ALL;
-
- if (kperm != perm)
+ if ((kperm & mask) != mask)
return -EACCES;

/* let LSM be the final arbiter */
- return security_key_permission(key_ref, cred, perm);
+lsm:
+ return security_key_permission(key_ref, cred, need_perm);
}
EXPORT_SYMBOL(key_task_permission);

diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 09541de31f2f..7e0232db1707 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -609,7 +609,7 @@ bool lookup_user_key_possessed(const struct key *key,
* returned key reference.
*/
key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
- key_perm_t perm)
+ enum key_need_perm need_perm)
{
struct keyring_search_context ctx = {
.match_data.cmp = lookup_user_key_possessed,
@@ -773,35 +773,33 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,

/* unlink does not use the nominated key in any way, so can skip all
* the permission checks as it is only concerned with the keyring */
- if (lflags & KEY_LOOKUP_FOR_UNLINK) {
- ret = 0;
- goto error;
- }
-
- if (!(lflags & KEY_LOOKUP_PARTIAL)) {
- ret = wait_for_key_construction(key, true);
- switch (ret) {
- case -ERESTARTSYS:
- goto invalid_key;
- default:
- if (perm)
+ if (need_perm != KEY_NEED_UNLINK) {
+ if (!(lflags & KEY_LOOKUP_PARTIAL)) {
+ ret = wait_for_key_construction(key, true);
+ switch (ret) {
+ case -ERESTARTSYS:
+ goto invalid_key;
+ default:
+ if (need_perm != KEY_AUTHTOKEN_OVERRIDE &&
+ need_perm != KEY_DEFER_PERM_CHECK)
+ goto invalid_key;
+ case 0:
+ break;
+ }
+ } else if (need_perm != KEY_DEFER_PERM_CHECK) {
+ ret = key_validate(key);
+ if (ret < 0)
goto invalid_key;
- case 0:
- break;
}
- } else if (perm) {
- ret = key_validate(key);
- if (ret < 0)
+
+ ret = -EIO;
+ if (!(lflags & KEY_LOOKUP_PARTIAL) &&
+ key_read_state(key) == KEY_IS_UNINSTANTIATED)
goto invalid_key;
}

- ret = -EIO;
- if (!(lflags & KEY_LOOKUP_PARTIAL) &&
- key_read_state(key) == KEY_IS_UNINSTANTIATED)
- goto invalid_key;
-
/* check the permissions */
- ret = key_task_permission(key_ref, ctx.cred, perm);
+ ret = key_task_permission(key_ref, ctx.cred, need_perm);
if (ret < 0)
goto invalid_key;

diff --git a/security/security.c b/security/security.c
index c73334ab2882..af32d4cd0462 100644
--- a/security/security.c
+++ b/security/security.c
@@ -2398,10 +2398,10 @@ void security_key_free(struct key *key)
call_void_hook(key_free, key);
}

-int security_key_permission(key_ref_t key_ref,
- const struct cred *cred, unsigned perm)
+int security_key_permission(key_ref_t key_ref, const struct cred *cred,
+ enum key_need_perm need_perm)
{
- return call_int_hook(key_permission, 0, key_ref, cred, perm);
+ return call_int_hook(key_permission, 0, key_ref, cred, need_perm);
}

int security_key_getsecurity(struct key *key, char **_buffer)
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 0b4e32161b77..0de048705f0e 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -6541,20 +6541,43 @@ static void selinux_key_free(struct key *k)

static int selinux_key_permission(key_ref_t key_ref,
const struct cred *cred,
- unsigned perm)
+ enum key_need_perm need_perm)
{
struct key *key;
struct key_security_struct *ksec;
- u32 sid;
+ u32 perm, sid;

- /* if no specific permissions are requested, we skip the
- permission check. No serious, additional covert channels
- appear to be created. */
- if (perm == 0)
+ switch (need_perm) {
+ case KEY_NEED_VIEW:
+ perm = KEY__VIEW;
+ break;
+ case KEY_NEED_READ:
+ perm = KEY__READ;
+ break;
+ case KEY_NEED_WRITE:
+ perm = KEY__WRITE;
+ break;
+ case KEY_NEED_SEARCH:
+ perm = KEY__SEARCH;
+ break;
+ case KEY_NEED_LINK:
+ perm = KEY__LINK;
+ break;
+ case KEY_NEED_SETATTR:
+ perm = KEY__SETATTR;
+ break;
+ case KEY_NEED_UNLINK:
+ case KEY_SYSADMIN_OVERRIDE:
+ case KEY_AUTHTOKEN_OVERRIDE:
+ case KEY_DEFER_PERM_CHECK:
return 0;
+ default:
+ WARN_ON(1);
+ return -EPERM;

- sid = cred_sid(cred);
+ }

+ sid = cred_sid(cred);
key = key_ref_to_ptr(key_ref);
ksec = key->security;

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 8c61d175e195..0d6bb53efe74 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -4230,13 +4230,14 @@ static void smack_key_free(struct key *key)
* smack_key_permission - Smack access on a key
* @key_ref: gets to the object
* @cred: the credentials to use
- * @perm: requested key permissions
+ * @need_perm: requested key permission
*
* Return 0 if the task has read and write to the object,
* an error code otherwise
*/
static int smack_key_permission(key_ref_t key_ref,
- const struct cred *cred, unsigned perm)
+ const struct cred *cred,
+ enum key_need_perm need_perm)
{
struct key *keyp;
struct smk_audit_info ad;
@@ -4247,8 +4248,26 @@ static int smack_key_permission(key_ref_t key_ref,
/*
* Validate requested permissions
*/
- if (perm & ~KEY_NEED_ALL)
+ switch (need_perm) {
+ case KEY_NEED_READ:
+ case KEY_NEED_SEARCH:
+ case KEY_NEED_VIEW:
+ request |= MAY_READ;
+ break;
+ case KEY_NEED_WRITE:
+ case KEY_NEED_LINK:
+ case KEY_NEED_SETATTR:
+ request |= MAY_WRITE;
+ break;
+ case KEY_NEED_UNSPECIFIED:
+ case KEY_NEED_UNLINK:
+ case KEY_SYSADMIN_OVERRIDE:
+ case KEY_AUTHTOKEN_OVERRIDE:
+ case KEY_DEFER_PERM_CHECK:
+ return 0;
+ default:
return -EINVAL;
+ }

keyp = key_ref_to_ptr(key_ref);
if (keyp == NULL)
@@ -4273,10 +4292,6 @@ static int smack_key_permission(key_ref_t key_ref,
ad.a.u.key_struct.key = keyp->serial;
ad.a.u.key_struct.key_desc = keyp->description;
#endif
- if (perm & (KEY_NEED_READ | KEY_NEED_SEARCH | KEY_NEED_VIEW))
- request |= MAY_READ;
- if (perm & (KEY_NEED_WRITE | KEY_NEED_LINK | KEY_NEED_SETATTR))
- request |= MAY_WRITE;
rc = smk_access(tkp, keyp->security, request, &ad);
rc = smk_bu_note("key access", tkp, keyp->security, request, rc);
return rc;