util: eliminate "use after free" in callers of virNetDevLinkDump

virNetDevLinkDump() gets a message from netlink into "resp", then
calls nlmsg_parse() to fill the table "tb" with pointers into resp. It
then returns tb to its caller, but not before freeing the buffer at
resp. That means that all the callers of virNetDevLinkDump() are
examining memory that has already been freed. This can be verified by
filling the buffer at resp with garbage prior to freeing it (or, I
suppose, just running libvirtd under valgrind) then performing some
operation that calls virNetDevLinkDump().

The upstream commit log incorrectly states that the code has been like
this ever since virNetDevLinkDump() was written. In reality, the
problem was introduced with commit e95de74d, first in libvirt-1.0.5,
which was attempting to eliminate a typecast that caused compiler
warnings. It has only been pure luck (or maybe a lack of heavy load,
and/or maybe an allocation algorithm in malloc() that delays re-use of
just-freed memory) that has kept this from causing errors, for example
when configuring a PCI passthrough or macvtap passthrough network
interface.

The solution taken in this patch is the simplest - just return resp to
the caller along with tb, then have the caller free it after they are
finished using the data (pointers) in tb. I alternately could have
made a cleaner interface by creating a new struct that put tb and resp
together along with a vir*Free() function for it, but this function is
only used in a couple places, and I'm not sure there will be
additional new uses of virNetDevLinkDump(), so the value of adding a
new type, extra APIs, etc. is dubious.

(cherry picked from commit f9f9699f)

src/util/virnetdev.c

@@ -1354,23 +1354,25 @@ static struct nla_policy ifla_vf_policy[IFLA_VF_MAX+1] = {
 /**
  * virNetDevLinkDump:
  *
- * @ifname: The name of the interface; only use if ifindex < 0
- * @ifindex: The interface index; may be < 0 if ifname is given
- * @nlattr: pointer to a pointer of netlink attributes that will contain
- *          the results
+ * @ifname:  The name of the interface; only use if ifindex <= 0
+ * @ifindex: The interface index; may be <= 0 if ifname is given
+ * @data:    Gets a pointer to the raw data from netlink.
+             MUST BE FREED BY CALLER!
+ * @nlattr:  Pointer to a pointer of netlink attributes that will contain
+ *           the results
  * @src_pid: pid used for nl_pid of the local end of the netlink message
  *           (0 == "use getpid()")
  * @dst_pid: pid of destination nl_pid if the kernel
  *           is not the target of the netlink message but it is to be
  *           sent to another process (0 if sending to the kernel)
  *
- * Get information about an interface given its name or index.
+ * Get information from netlink about an interface given its name or index.
  *
  * Returns 0 on success, -1 on fatal error.
  */
 int
 virNetDevLinkDump(const char *ifname, int ifindex,
-                  struct nlattr **tb,
+                  void **nlData, struct nlattr **tb,
                   uint32_t src_pid, uint32_t dst_pid)
 {
     int rc = -1;
@@ -1452,7 +1454,9 @@ virNetDevLinkDump(const char *ifname, int ifindex,
     rc = 0;
  cleanup:
     nlmsg_free(nl_msg);
-    VIR_FREE(resp);
+    if (rc < 0)
+       VIR_FREE(resp);
+    *nlData = resp;
     return rc;
 
  malformed_resp:
@@ -1648,15 +1652,18 @@ virNetDevGetVfConfig(const char *ifname, int vf, virMacAddrPtr mac,
                      int *vlanid)
 {
     int rc = -1;
+    void *nlData = NULL;
     struct nlattr *tb[IFLA_MAX + 1] = {NULL, };
     int ifindex = -1;
 
-    rc = virNetDevLinkDump(ifname, ifindex, tb, 0, 0);
+    rc = virNetDevLinkDump(ifname, ifindex, &nlData, tb, 0, 0);
     if (rc < 0)
-        return rc;
+        goto cleanup;
 
     rc = virNetDevParseVfConfig(tb, vf, mac, vlanid);
 
+ cleanup:
+    VIR_FREE(nlData);
     return rc;
 }
 
@@ -1799,6 +1806,7 @@ virNetDevRestoreNetConfig(const char *linkdev, int vf, const char *stateDir)
 int
 virNetDevLinkDump(const char *ifname ATTRIBUTE_UNUSED,
                   int ifindex ATTRIBUTE_UNUSED,
+                  void **nlData ATTRIBUTE_UNUSED,
                   struct nlattr **tb ATTRIBUTE_UNUSED,
                   uint32_t src_pid ATTRIBUTE_UNUSED,
                   uint32_t dst_pid ATTRIBUTE_UNUSED)

src/util/virnetdev.h

@@ -130,7 +130,7 @@ int virNetDevGetVirtualFunctions(const char *pfname,
     ATTRIBUTE_NONNULL(4) ATTRIBUTE_RETURN_CHECK;
 
 int virNetDevLinkDump(const char *ifname, int ifindex,
-                      struct nlattr **tb,
+                      void **nlData, struct nlattr **tb,
                       uint32_t src_pid, uint32_t dst_pid)
     ATTRIBUTE_RETURN_CHECK;
 

src/util/virnetdevvportprofile.c

@@ -787,7 +787,8 @@ virNetDevVPortProfileGetNthParent(const char *ifname, int ifindex, unsigned int
                                   int *parent_ifindex, char *parent_ifname,
                                   unsigned int *nth)
 {
-    int rc;
+    int rc = -1;
+    void *nlData = NULL;
     struct nlattr *tb[IFLA_MAX + 1] = { NULL, };
     bool end = false;
     size_t i = 0;
@@ -798,7 +799,8 @@ virNetDevVPortProfileGetNthParent(const char *ifname, int ifindex, unsigned int
         return -1;
 
     while (!end && i <= nthParent) {
-        rc = virNetDevLinkDump(ifname, ifindex, tb, 0, 0);
+        VIR_FREE(nlData);
+        rc = virNetDevLinkDump(ifname, ifindex, &nlData, tb, 0, 0);
         if (rc < 0)
             break;
 
@@ -807,7 +809,8 @@ virNetDevVPortProfileGetNthParent(const char *ifname, int ifindex, unsigned int
                            IFNAMSIZ)) {
                 virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                                _("buffer for root interface name is too small"));
-                return -1;
+                rc = -1;
+                goto cleanup;
             }
             *parent_ifindex = ifindex;
         }
@@ -823,6 +826,8 @@ virNetDevVPortProfileGetNthParent(const char *ifname, int ifindex, unsigned int
 
     *nth = i - 1;
 
+ cleanup:
+    VIR_FREE(nlData);
     return rc;
 }
 
@@ -844,6 +849,7 @@ virNetDevVPortProfileOpCommon(const char *ifname, int ifindex,
     int rc;
     int src_pid = 0;
     uint32_t dst_pid = 0;
+    void *nlData = NULL;
     struct nlattr *tb[IFLA_MAX + 1] = { NULL, };
     int repeats = STATUS_POLL_TIMEOUT_USEC / STATUS_POLL_INTERVL_USEC;
     uint16_t status = 0;
@@ -876,7 +882,8 @@ virNetDevVPortProfileOpCommon(const char *ifname, int ifindex,
     }
 
     while (--repeats >= 0) {
-        rc = virNetDevLinkDump(NULL, ifindex, tb, src_pid, dst_pid);
+        VIR_FREE(nlData);
+        rc = virNetDevLinkDump(NULL, ifindex, &nlData, tb, src_pid, dst_pid);
         if (rc < 0)
             goto cleanup;
 
@@ -908,7 +915,7 @@ virNetDevVPortProfileOpCommon(const char *ifname, int ifindex,
     }
 
  cleanup:
-
+    VIR_FREE(nlData);
     return rc;
 }