security_dac: compute supplemental groups before fork

https://bugzilla.redhat.com/show_bug.cgi?id=964358

Commit 75c12564 states that virGetGroupList must not be called
between fork and exec, then commit ee777e99 promptly violated
that for lxc's use of virSecurityManagerSetProcessLabel.  Hoist
the supplemental group detection to the time that the security
manager needs to fork.  Qemu is safe, as it uses
virSecurityManagerSetChildProcessLabel which in turn uses
virCommand to determine supplemental groups.

This does not fix the fact that virSecurityManagerSetProcessLabel
calls virSecurityDACParseIds calls parseIds which eventually
calls getpwnam_r, which also violates fork/exec async-signal-safe
safety rules, but so far no one has complained of hitting
deadlock in that case.

* src/security/security_dac.c (_virSecurityDACData): Track groups
in private data.
(virSecurityDACPreFork): New function, to set them.
(virSecurityDACClose): Clean up new fields.
(virSecurityDACGetIds): Alter signature.
(virSecurityDACSetSecurityHostdevLabelHelper)
(virSecurityDACSetChardevLabel, virSecurityDACSetProcessLabel)
(virSecurityDACSetChildProcessLabel): Update callers.
Signed-off-by: NEric Blake <eblake@redhat.com>
(cherry picked from commit 29fe5d74)

src/security/security_dac.c

@@ -43,6 +43,8 @@ typedef virSecurityDACData *virSecurityDACDataPtr;
 struct _virSecurityDACData {
     uid_t user;
     gid_t group;
+    gid_t *groups;
+    int ngroups;
     bool dynamicOwnership;
 };
 
@@ -146,7 +148,8 @@ virSecurityDACParseIds(virDomainDefPtr def, uid_t *uidPtr, gid_t *gidPtr)
 
 static int
 virSecurityDACGetIds(virDomainDefPtr def, virSecurityDACDataPtr priv,
-                     uid_t *uidPtr, gid_t *gidPtr)
+                     uid_t *uidPtr, gid_t *gidPtr,
+                     gid_t **groups, int *ngroups)
 {
     int ret;
 
@@ -157,8 +160,13 @@ virSecurityDACGetIds(virDomainDefPtr def, virSecurityDACDataPtr priv,
         return -1;
     }
 
-    if ((ret = virSecurityDACParseIds(def, uidPtr, gidPtr)) <= 0)
+    if ((ret = virSecurityDACParseIds(def, uidPtr, gidPtr)) <= 0) {
+        if (groups)
+            *groups = NULL;
+        if (ngroups)
+            *ngroups = 0;
         return ret;
+    }
 
     if (!priv) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
@@ -171,6 +179,10 @@ virSecurityDACGetIds(virDomainDefPtr def, virSecurityDACDataPtr priv,
         *uidPtr = priv->user;
     if (gidPtr)
         *gidPtr = priv->group;
+    if (groups)
+        *groups = priv->groups;
+    if (ngroups)
+        *ngroups = priv->ngroups;
 
     return 0;
 }
@@ -250,8 +262,10 @@ virSecurityDACOpen(virSecurityManagerPtr mgr ATTRIBUTE_UNUSED)
 }
 
 static int
-virSecurityDACClose(virSecurityManagerPtr mgr ATTRIBUTE_UNUSED)
+virSecurityDACClose(virSecurityManagerPtr mgr)
 {
+    virSecurityDACDataPtr priv = virSecurityManagerGetPrivateData(mgr);
+    VIR_FREE(priv->groups);
     return 0;
 }
 
@@ -268,6 +282,21 @@ virSecurityDACGetDOI(virSecurityManagerPtr mgr ATTRIBUTE_UNUSED)
     return "0";
 }
 
+static int
+virSecurityDACPreFork(virSecurityManagerPtr mgr)
+{
+    virSecurityDACDataPtr priv = virSecurityManagerGetPrivateData(mgr);
+    int ngroups;
+
+    VIR_FREE(priv->groups);
+    priv->ngroups = 0;
+    if ((ngroups = virGetGroupList(priv->user, priv->group,
+                                   &priv->groups)) < 0)
+        return -1;
+    priv->ngroups = ngroups;
+    return 0;
+}
+
 static int
 virSecurityDACSetOwnership(const char *path, uid_t uid, gid_t gid)
 {
@@ -448,7 +477,7 @@ virSecurityDACSetSecurityHostdevLabelHelper(const char *file,
     uid_t user;
     gid_t group;
 
-    if (virSecurityDACGetIds(def, priv, &user, &group))
+    if (virSecurityDACGetIds(def, priv, &user, &group, NULL, NULL))
         return -1;
 
     return virSecurityDACSetOwnership(file, user, group);
@@ -702,7 +731,7 @@ virSecurityDACSetChardevLabel(virSecurityManagerPtr mgr,
     uid_t user;
     gid_t group;
 
-    if (virSecurityDACGetIds(def, priv, &user, &group))
+    if (virSecurityDACGetIds(def, priv, &user, &group, NULL, NULL))
         return -1;
 
     switch (dev->type) {
@@ -1000,26 +1029,20 @@ virSecurityDACSetProcessLabel(virSecurityManagerPtr mgr,
 {
     uid_t user;
     gid_t group;
-    virSecurityDACDataPtr priv = virSecurityManagerGetPrivateData(mgr);
     gid_t *groups;
     int ngroups;
-    int ret = -1;
+    virSecurityDACDataPtr priv = virSecurityManagerGetPrivateData(mgr);
 
-    if (virSecurityDACGetIds(def, priv, &user, &group))
-        return -1;
-    ngroups = virGetGroupList(user, group, &groups);
-    if (ngroups < 0)
+    if (virSecurityDACGetIds(def, priv, &user, &group, &groups, &ngroups))
         return -1;
 
-    VIR_DEBUG("Dropping privileges of DEF to %u:%u",
-              (unsigned int) user, (unsigned int) group);
+    VIR_DEBUG("Dropping privileges of DEF to %u:%u, %d supplemental groups",
+              (unsigned int) user, (unsigned int) group, ngroups);
 
     if (virSetUIDGID(user, group, groups, ngroups) < 0)
-        goto cleanup;
-    ret = 0;
-cleanup:
-    VIR_FREE(groups);
-    return ret;
+        return -1;
+
+    return 0;
 }
 
 
@@ -1032,7 +1055,7 @@ virSecurityDACSetChildProcessLabel(virSecurityManagerPtr mgr,
     gid_t group;
     virSecurityDACDataPtr priv = virSecurityManagerGetPrivateData(mgr);
 
-    if (virSecurityDACGetIds(def, priv, &user, &group))
+    if (virSecurityDACGetIds(def, priv, &user, &group, NULL, NULL))
         return -1;
 
     VIR_DEBUG("Setting child to drop privileges of DEF to %u:%u",
@@ -1212,6 +1235,8 @@ virSecurityDriver virSecurityDriverDAC = {
     .getModel                           = virSecurityDACGetModel,
     .getDOI                             = virSecurityDACGetDOI,
 
+    .preFork                            = virSecurityDACPreFork,
+
     .domainSecurityVerify               = virSecurityDACVerify,
 
     .domainSetSecurityImageLabel        = virSecurityDACSetSecurityImageLabel,