selinux: revert "stop passing MAY_NOT_BLOCK to the AVC upon follow_link"

This reverts commit e46e01ee ("selinux: stop passing MAY_NOT_BLOCK
to the AVC upon follow_link"). The correct fix is to instead fall
back to ref-walk if audit is required irrespective of the specific
audit data type.  This is done in the next commit.

Fixes: e46e01ee ("selinux: stop passing MAY_NOT_BLOCK to the AVC upon follow_link")
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/avc.c

@@ -862,8 +862,9 @@ static int avc_update_node(struct selinux_avc *avc,
 	 * permissive mode that only appear when in enforcing mode.
 	 *
 	 * See the corresponding handling in slow_avc_audit(), and the
-	 * logic in selinux_inode_permission for the MAY_NOT_BLOCK flag,
-	 * which is transliterated into AVC_NONBLOCKING.
+	 * logic in selinux_inode_follow_link and selinux_inode_permission
+	 * for the VFS MAY_NOT_BLOCK flag, which is transliterated into
+	 * AVC_NONBLOCKING for avc_has_perm_noaudit().
 	 */
 	if (flags & AVC_NONBLOCKING)
 		return 0;
@@ -1205,6 +1206,25 @@ int avc_has_perm(struct selinux_state *state, u32 ssid, u32 tsid, u16 tclass,
 	return rc;
 }
 
+int avc_has_perm_flags(struct selinux_state *state,
+		       u32 ssid, u32 tsid, u16 tclass, u32 requested,
+		       struct common_audit_data *auditdata,
+		       int flags)
+{
+	struct av_decision avd;
+	int rc, rc2;
+
+	rc = avc_has_perm_noaudit(state, ssid, tsid, tclass, requested,
+				  (flags & MAY_NOT_BLOCK) ? AVC_NONBLOCKING : 0,
+				  &avd);
+
+	rc2 = avc_audit(state, ssid, tsid, tclass, requested, &avd, rc,
+			auditdata, flags);
+	if (rc2)
+		return rc2;
+	return rc;
+}
+
 u32 avc_policy_seqno(struct selinux_state *state)
 {
 	return state->avc->avc_cache.latest_notif;

security/selinux/hooks.c

@@ -3004,8 +3004,9 @@ static int selinux_inode_follow_link(struct dentry *dentry, struct inode *inode,
 	if (IS_ERR(isec))
 		return PTR_ERR(isec);
 
-	return avc_has_perm(&selinux_state,
-			    sid, isec->sid, isec->sclass, FILE__READ, &ad);
+	return avc_has_perm_flags(&selinux_state,
+				  sid, isec->sid, isec->sclass, FILE__READ, &ad,
+				  rcu ? MAY_NOT_BLOCK : 0);
 }
 
 static noinline int audit_inode_permission(struct inode *inode,

security/selinux/include/avc.h

@@ -153,6 +153,11 @@ int avc_has_perm(struct selinux_state *state,
 		 u32 ssid, u32 tsid,
 		 u16 tclass, u32 requested,
 		 struct common_audit_data *auditdata);
+int avc_has_perm_flags(struct selinux_state *state,
+		       u32 ssid, u32 tsid,
+		       u16 tclass, u32 requested,
+		       struct common_audit_data *auditdata,
+		       int flags);
 
 int avc_has_extended_perms(struct selinux_state *state,
 			   u32 ssid, u32 tsid, u16 tclass, u32 requested,