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authorAristeu Rozanski <aris@redhat.com>2014-04-21 12:13:03 -0400
committerTejun Heo <tj@kernel.org>2014-04-21 18:21:42 -0400
commit79d719749d23234e9b725098aa49133f3ef7299d (patch)
tree454b591af0e26506982cf92a9712b67bcf77c96c /security
parente37a06f10994c2ba86f54d8f96734f2483a869b8 (diff)
device_cgroup: rework device access check and exception checking
Whenever a device file is opened and checked against current device cgroup rules, it uses the same function (may_access()) as when a new exception rule is added by writing devices.{allow,deny}. And in both cases, the algorithm is the same, doesn't matter the behavior. First problem is having device access to be considered the same as rule checking. Consider the following structure: A (default behavior: allow, exceptions disallow access) \ B (default behavior: allow, exceptions disallow access) A new exception is added to B by writing devices.deny: c 12:34 rw When checking if that exception is allowed in may_access(): if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) { if (behavior == DEVCG_DEFAULT_ALLOW) { /* the exception will deny access to certain devices */ return true; Which is ok, since B is not getting more privileges than A, it doesn't matter and the rule is accepted Now, consider it's a device file open check and the process belongs to cgroup B. The access will be generated as: behavior: allow exception: c 12:34 rw The very same chunk of code will allow it, even if there's an explicit exception telling to do otherwise. A simple test case: # mkdir new_group # cd new_group # echo $$ >tasks # echo "c 1:3 w" >devices.deny # echo >/dev/null # echo $? 0 This is a serious bug and was introduced on c39a2a3018f8 devcg: prepare may_access() for hierarchy support To solve this problem, the device file open function was split from the new exception check. Second problem is how exceptions are processed by may_access(). The first part of the said function tries to match fully with an existing exception: list_for_each_entry_rcu(ex, &dev_cgroup->exceptions, list) { if ((refex->type & DEV_BLOCK) && !(ex->type & DEV_BLOCK)) continue; if ((refex->type & DEV_CHAR) && !(ex->type & DEV_CHAR)) continue; if (ex->major != ~0 && ex->major != refex->major) continue; if (ex->minor != ~0 && ex->minor != refex->minor) continue; if (refex->access & (~ex->access)) continue; match = true; break; } That means the new exception should be contained into an existing one to be considered a match: New exception Existing match? notes b 12:34 rwm b 12:34 rwm yes b 12:34 r b *:34 rw yes b 12:34 rw b 12:34 w no extra "r" b *:34 rw b 12:34 rw no too broad "*" b *:34 rw b *:34 rwm yes Which is fine in some cases. Consider: A (default behavior: deny, exceptions allow access) \ B (default behavior: deny, exceptions allow access) In this case the full match makes sense, the new exception cannot add more access than the parent allows But this doesn't always work, consider: A (default behavior: allow, exceptions disallow access) \ B (default behavior: deny, exceptions allow access) In this case, a new exception in B shouldn't match any of the exceptions in A, after all you can't allow something that was forbidden by A. But consider this scenario: New exception Existing in A match? outcome b 12:34 rw b 12:34 r no exception is accepted Because the new exception has "w" as extra, it doesn't match, so it'll be added to B's exception list. The same problem can happen during a file access check. Consider a cgroup with allow as default behavior: Access Exception match? b 12:34 rw b 12:34 r no In this case, the access didn't match any of the exceptions in the cgroup, which is required since exceptions will disallow access. To solve this problem, two new functions were created to match an exception either fully or partially. In the example above, a partial check will be performed and it'll produce a match since at least "b 12:34 r" from "b 12:34 rw" access matches. Cc: cgroups@vger.kernel.org Cc: Tejun Heo <tj@kernel.org> Cc: Serge Hallyn <serge.hallyn@canonical.com> Cc: Li Zefan <lizefan@huawei.com> Cc: stable@vger.kernel.org Signed-off-by: Aristeu Rozanski <arozansk@redhat.com> Signed-off-by: Tejun Heo <tj@kernel.org>
Diffstat (limited to 'security')
-rw-r--r--security/device_cgroup.c162
1 files changed, 122 insertions, 40 deletions
diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 8365909f5f8c..b9048dc46b1a 100644
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -306,57 +306,139 @@ static int devcgroup_seq_show(struct seq_file *m, void *v)
}
/**
- * may_access - verifies if a new exception is part of what is allowed
- * by a dev cgroup based on the default policy +
- * exceptions. This is used to make sure a child cgroup
- * won't have more privileges than its parent or to
- * verify if a certain access is allowed.
- * @dev_cgroup: dev cgroup to be tested against
- * @refex: new exception
- * @behavior: behavior of the exception
+ * match_exception - iterates the exception list trying to match a rule
+ * based on type, major, minor and access type. It is
+ * considered a match if an exception is found that
+ * will contain the entire range of provided parameters.
+ * @exceptions: list of exceptions
+ * @type: device type (DEV_BLOCK or DEV_CHAR)
+ * @major: device file major number, ~0 to match all
+ * @minor: device file minor number, ~0 to match all
+ * @access: permission mask (ACC_READ, ACC_WRITE, ACC_MKNOD)
+ *
+ * returns: true in case it matches an exception completely
*/
-static bool may_access(struct dev_cgroup *dev_cgroup,
- struct dev_exception_item *refex,
- enum devcg_behavior behavior)
+static bool match_exception(struct list_head *exceptions, short type,
+ u32 major, u32 minor, short access)
{
struct dev_exception_item *ex;
- bool match = false;
- rcu_lockdep_assert(rcu_read_lock_held() ||
- lockdep_is_held(&devcgroup_mutex),
- "device_cgroup::may_access() called without proper synchronization");
+ list_for_each_entry_rcu(ex, exceptions, list) {
+ if ((type & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
+ continue;
+ if ((type & DEV_CHAR) && !(ex->type & DEV_CHAR))
+ continue;
+ if (ex->major != ~0 && ex->major != major)
+ continue;
+ if (ex->minor != ~0 && ex->minor != minor)
+ continue;
+ /* provided access cannot have more than the exception rule */
+ if (access & (~ex->access))
+ continue;
+ return true;
+ }
+ return false;
+}
+
+/**
+ * match_exception_partial - iterates the exception list trying to match a rule
+ * based on type, major, minor and access type. It is
+ * considered a match if an exception's range is
+ * found to contain *any* of the devices specified by
+ * provided parameters. This is used to make sure no
+ * extra access is being granted that is forbidden by
+ * any of the exception list.
+ * @exceptions: list of exceptions
+ * @type: device type (DEV_BLOCK or DEV_CHAR)
+ * @major: device file major number, ~0 to match all
+ * @minor: device file minor number, ~0 to match all
+ * @access: permission mask (ACC_READ, ACC_WRITE, ACC_MKNOD)
+ *
+ * returns: true in case the provided range mat matches an exception completely
+ */
+static bool match_exception_partial(struct list_head *exceptions, short type,
+ u32 major, u32 minor, short access)
+{
+ struct dev_exception_item *ex;
- list_for_each_entry_rcu(ex, &dev_cgroup->exceptions, list) {
- if ((refex->type & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
+ list_for_each_entry_rcu(ex, exceptions, list) {
+ if ((type & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
continue;
- if ((refex->type & DEV_CHAR) && !(ex->type & DEV_CHAR))
+ if ((type & DEV_CHAR) && !(ex->type & DEV_CHAR))
continue;
- if (ex->major != ~0 && ex->major != refex->major)
+ /*
+ * We must be sure that both the exception and the provided
+ * range aren't masking all devices
+ */
+ if (ex->major != ~0 && major != ~0 && ex->major != major)
continue;
- if (ex->minor != ~0 && ex->minor != refex->minor)
+ if (ex->minor != ~0 && minor != ~0 && ex->minor != minor)
continue;
- if (refex->access & (~ex->access))
+ /*
+ * In order to make sure the provided range isn't matching
+ * an exception, all its access bits shouldn't match the
+ * exception's access bits
+ */
+ if (!(access & ex->access))
continue;
- match = true;
- break;
+ return true;
}
+ return false;
+}
+
+/**
+ * verify_new_ex - verifies if a new exception is part of what is allowed
+ * by a dev cgroup based on the default policy +
+ * exceptions. This is used to make sure a child cgroup
+ * won't have more privileges than its parent
+ * @dev_cgroup: dev cgroup to be tested against
+ * @refex: new exception
+ * @behavior: behavior of the exception's dev_cgroup
+ */
+static bool verify_new_ex(struct dev_cgroup *dev_cgroup,
+ struct dev_exception_item *refex,
+ enum devcg_behavior behavior)
+{
+ bool match = false;
+
+ rcu_lockdep_assert(rcu_read_lock_held() ||
+ lockdep_is_held(&devcgroup_mutex),
+ "device_cgroup:verify_new_ex called without proper synchronization");
if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) {
if (behavior == DEVCG_DEFAULT_ALLOW) {
- /* the exception will deny access to certain devices */
+ /*
+ * new exception in the child doesn't matter, only
+ * adding extra restrictions
+ */
return true;
} else {
- /* the exception will allow access to certain devices */
+ /*
+ * new exception in the child will add more devices
+ * that can be acessed, so it can't match any of
+ * parent's exceptions, even slightly
+ */
+ match = match_exception_partial(&dev_cgroup->exceptions,
+ refex->type,
+ refex->major,
+ refex->minor,
+ refex->access);
+
if (match)
- /*
- * a new exception allowing access shouldn't
- * match an parent's exception
- */
return false;
return true;
}
} else {
- /* only behavior == DEVCG_DEFAULT_DENY allowed here */
+ /*
+ * Only behavior == DEVCG_DEFAULT_DENY allowed here, therefore
+ * the new exception will add access to more devices and must
+ * be contained completely in an parent's exception to be
+ * allowed
+ */
+ match = match_exception(&dev_cgroup->exceptions, refex->type,
+ refex->major, refex->minor,
+ refex->access);
+
if (match)
/* parent has an exception that matches the proposed */
return true;
@@ -378,7 +460,7 @@ static int parent_has_perm(struct dev_cgroup *childcg,
if (!parent)
return 1;
- return may_access(parent, ex, childcg->behavior);
+ return verify_new_ex(parent, ex, childcg->behavior);
}
/**
@@ -704,18 +786,18 @@ static int __devcgroup_check_permission(short type, u32 major, u32 minor,
short access)
{
struct dev_cgroup *dev_cgroup;
- struct dev_exception_item ex;
- int rc;
-
- memset(&ex, 0, sizeof(ex));
- ex.type = type;
- ex.major = major;
- ex.minor = minor;
- ex.access = access;
+ bool rc;
rcu_read_lock();
dev_cgroup = task_devcgroup(current);
- rc = may_access(dev_cgroup, &ex, dev_cgroup->behavior);
+ if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW)
+ /* Can't match any of the exceptions, even partially */
+ rc = !match_exception_partial(&dev_cgroup->exceptions,
+ type, major, minor, access);
+ else
+ /* Need to match completely one exception to be allowed */
+ rc = match_exception(&dev_cgroup->exceptions, type, major,
+ minor, access);
rcu_read_unlock();
if (!rc)

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