audit: allow interfield comparison in audit rules

We wish to be able to audit when a uid=500 task accesses a file which is
uid=0.  Or vice versa.  This patch introduces a new audit filter type
AUDIT_FIELD_COMPARE which takes as an 'enum' which indicates which fields
should be compared.  At this point we only define the task->uid vs
inode->uid, but other comparisons can be added.

Signed-off-by: Eric Paris <eparis@redhat.com>

2 files changed, 33 insertions, 2 deletions

diff --git a/kernel/auditfilter.c b/kernel/auditfilter.c
index f10605c787e6..a6c3f1abd206 100644
--- a/kernel/auditfilter.c
+++ b/kernel/auditfilter.c
@@ -526,7 +526,6 @@ static struct audit_entry *audit_data_to_entry(struct audit_rule_data *data,
                                 goto exit_free;
                         break;
                 case AUDIT_FILTERKEY:
-                        err = -EINVAL;
                         if (entry->rule.filterkey || f->val > AUDIT_MAX_KEY_LEN)
                                 goto exit_free;
                         str = audit_unpack_string(&bufp, &remain, f->val);
@@ -543,6 +542,10 @@ static struct audit_entry *audit_data_to_entry(struct audit_rule_data *data,
                         if (f->val & ~S_IFMT)
                                 goto exit_free;
                         break;
+                case AUDIT_FIELD_COMPARE:
+                        if (f->val > AUDIT_MAX_FIELD_COMPARE)
+                                goto exit_free;
+                        break;
                 default:
                         goto exit_free;
                 }
diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index 9161e70a4379..8fb2c8e6d624 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -463,6 +463,32 @@ static int match_tree_refs(struct audit_context *ctx, struct audit_tree *tree)
         return 0;
 }
 
+static int audit_field_compare(struct task_struct *tsk,
+                               const struct cred *cred,
+                               struct audit_field *f,
+                               struct audit_context *ctx,
+                               struct audit_names *name)
+{
+        struct audit_names *n;
+
+        switch (f->val) {
+        case AUDIT_COMPARE_UID_TO_OBJ_UID:
+                if (name) {
+                        return audit_comparator(cred->uid, f->op, name->uid);
+                } else if (ctx) {
+                        list_for_each_entry(n, &ctx->names_list, list) {
+                                if (audit_comparator(cred->uid, f->op, n->uid))
+                                        return 1;
+                        }
+                }
+                break;
+        default:
+                WARN(1, "Missing AUDIT_COMPARE define.  Report as a bug\n");
+                return 0;
+        }
+        return 0;
+}
+
 /* Determine if any context name data matches a rule's watch data */
 /* Compare a task_struct with an audit_rule.  Return 1 on match, 0
  * otherwise.
@@ -693,8 +719,10 @@ static int audit_filter_rules(struct task_struct *tsk,
                 case AUDIT_FILETYPE:
                         result = audit_match_filetype(ctx, f->val);
                         break;
+                case AUDIT_FIELD_COMPARE:
+                        result = audit_field_compare(tsk, cred, f, ctx, name);
+                        break;
                 }
-
                 if (!result)
                         return 0;
         }