log: fix deadlock obtaining hostname (related CVE-2018-6764)

The fix for CVE-2018-6764 introduced a potential deadlock scenario
that gets triggered by the NSS module when virGetHostname() calls
getaddrinfo to resolve the hostname:

 #0  0x00007f6e714b57e7 in futex_wait
 #1  futex_wait_simple
 #2  __pthread_once_slow
 #3  0x00007f6e71d16e7d in virOnce
 #4  0x00007f6e71d0997c in virLogInitialize
 #5  0x00007f6e71d0a09a in virLogVMessage
 #6  0x00007f6e71d09ffd in virLogMessage
 #7  0x00007f6e71d0db22 in virObjectNew
 #8  0x00007f6e71d0dbf1 in virObjectLockableNew
 #9  0x00007f6e71d0d3e5 in virMacMapNew
 #10 0x00007f6e71cdc50a in findLease
 #11 0x00007f6e71cdcc56 in _nss_libvirt_gethostbyname4_r
 #12 0x00007f6e724631fc in gaih_inet
 #13 0x00007f6e72464697 in __GI_getaddrinfo
 #14 0x00007f6e71d19e81 in virGetHostnameImpl
 #15 0x00007f6e71d1a057 in virGetHostnameQuiet
 #16 0x00007f6e71d09936 in virLogOnceInit
 #17 0x00007f6e71d09952 in virLogOnce
 #18 0x00007f6e714b5829 in __pthread_once_slow
 #19 0x00007f6e71d16e7d in virOnce
 #20 0x00007f6e71d0997c in virLogInitialize
 #21 0x00007f6e71d0a09a in virLogVMessage
 #22 0x00007f6e71d09ffd in virLogMessage
 #23 0x00007f6e71d0db22 in virObjectNew
 #24 0x00007f6e71d0dbf1 in virObjectLockableNew
 #25 0x00007f6e71d0d3e5 in virMacMapNew
 #26 0x00007f6e71cdc50a in findLease
 #27 0x00007f6e71cdc839 in _nss_libvirt_gethostbyname3_r
 #28 0x00007f6e71cdc724 in _nss_libvirt_gethostbyname2_r
 #29 0x00007f6e7248f72f in __gethostbyname2_r
 #30 0x00007f6e7248f494 in gethostbyname2
 #31 0x000056348c30c36d in hosts_keys
 #32 0x000056348c30b7d2 in main

Fortunately the extra stuff virGetHostname does is totally irrelevant to
the needs of the logging code, so we can just inline a call to the
native hostname() syscall directly.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

cfg.mk

@@ -1183,7 +1183,7 @@ _src2=src/(util/vircommand|libvirt|lxc/lxc_controller|locking/lock_daemon|loggin
 exclude_file_name_regexp--sc_prohibit_fork_wrappers = \
   (^($(_src2)|tests/testutils|daemon/libvirtd)\.c$$)
 
-exclude_file_name_regexp--sc_prohibit_gethostname = ^src/util/virutil\.c$$
+exclude_file_name_regexp--sc_prohibit_gethostname = ^src/util/vir(util|log)\.c$$
 
 exclude_file_name_regexp--sc_prohibit_internal_functions = \
   ^src/(util/(viralloc|virutil|virfile)\.[hc]|esx/esx_vi\.c)$$

src/util/virlog.c

@@ -64,7 +64,7 @@
 VIR_LOG_INIT("util.log");
 
 static regex_t *virLogRegex;
-static char *virLogHostname;
+static char virLogHostname[HOST_NAME_MAX+1];
 
 
 #define VIR_LOG_DATE_REGEX "[0-9]{4}-[0-9]{2}-[0-9]{2}"
@@ -261,6 +261,8 @@ virLogPriorityString(virLogPriority lvl)
 static int
 virLogOnceInit(void)
 {
+    int r;
+
     if (virMutexInit(&virLogMutex) < 0)
         return -1;
 
@@ -275,8 +277,17 @@ virLogOnceInit(void)
     /* We get and remember the hostname early, because at later time
      * it might not be possible to load NSS modules via getaddrinfo()
      * (e.g. at container startup the host filesystem will not be
-     * accessible anymore. */
-    virLogHostname = virGetHostnameQuiet();
+     * accessible anymore.
+     * Must not use virGetHostname though as that causes re-entrancy
+     * problems if it triggers logging codepaths
+     */
+    r = gethostname(virLogHostname, sizeof(virLogHostname));
+    if (r == -1) {
+        ignore_value(virStrcpy(virLogHostname,
+                               "(unknown)", sizeof(virLogHostname)));
+    } else {
+        NUL_TERMINATE(virLogHostname);
+    }
 
     virLogUnlock();
     return 0;
@@ -475,9 +486,6 @@ virLogHostnameString(char **rawmsg,
 {
     char *hoststr;
 
-    if (!virLogHostname)
-        return -1;
-
     if (virAsprintfQuiet(&hoststr, "hostname: %s", virLogHostname) < 0)
         return -1;