security: Don't increase XATTRs refcounter on failure

If user has two domains, each have the same disk (configured for
RW) but each runs with different seclabel then we deny start of
the second domain because in order to do that we would need to
relabel the disk but that would cut the first domain off. Even if
we did not do that, qemu would fail to start because it would be
unable to lock the disk image for the second time. So far, this
behaviour is expected. But what is not expected is that we
increase the refcounter in XATTRs and leave it like that.

What happens is that when the second domain starts,
virSecuritySetRememberedLabel() is called, and since there are
XATTRs from the first domain it increments the refcounter and
returns it (refcounter == 2 at this point). Then callers
(virSecurityDACSetOwnership() and
virSecuritySELinuxSetFileconHelper()) realize that refcounter is
greater than 1 and desired seclabel doesn't match the one the
disk image already has and an error is produced. But the
refcounter is never decremented.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1740024Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NMartin Kletzander <mkletzan@redhat.com>

src/security/security_dac.c

@@ -751,6 +751,7 @@ virSecurityDACSetOwnership(virSecurityManagerPtr mgr,
                            bool remember)
 {
     virSecurityDACDataPtr priv = virSecurityManagerGetPrivateData(mgr);
+    virErrorPtr origerr;
     struct stat sb;
     int refcount;
     int rc;
@@ -784,12 +785,13 @@ virSecurityDACSetOwnership(virSecurityManagerPtr mgr,
              * is @refcount domains using the @path. Do not
              * change the label (as it would almost certainly
              * cause the other domains to lose access to the
-             * @path). */
+             * @path). However, the refcounter was incremented in
+             * XATTRs so decrease it. */
             if (sb.st_uid != uid || sb.st_gid != gid) {
                 virReportError(VIR_ERR_OPERATION_INVALID,
                                _("Setting different DAC user or group on %s "
                                  "which is already in use"), path);
-                return -1;
+                goto error;
             }
         }
     }
@@ -797,25 +799,26 @@ virSecurityDACSetOwnership(virSecurityManagerPtr mgr,
     VIR_INFO("Setting DAC user and group on '%s' to '%ld:%ld'",
              NULLSTR(src ? src->path : path), (long)uid, (long)gid);
 
-    if (virSecurityDACSetOwnershipInternal(priv, src, path, uid, gid) < 0) {
-        virErrorPtr origerr;
-
-        virErrorPreserveLast(&origerr);
-        /* Try to restore the label. This is done so that XATTRs
-         * are left in the same state as when the control entered
-         * this function. However, if our attempt fails, there's
-         * not much we can do. XATTRs refcounting is fubar'ed and
-         * the only option we have is warn users. */
-        if (virSecurityDACRestoreFileLabelInternal(mgr, src, path, remember) < 0)
-            VIR_WARN("Unable to restore label on '%s'. "
-                     "XATTRs might have been left in inconsistent state.",
-                     NULLSTR(src ? src->path : path));
-
-        virErrorRestore(&origerr);
-        return -1;
-    }
+    if (virSecurityDACSetOwnershipInternal(priv, src, path, uid, gid) < 0)
+        goto error;
 
     return 0;
+
+ error:
+    virErrorPreserveLast(&origerr);
+    /* Try to restore the label. This is done so that XATTRs
+     * are left in the same state as when the control entered
+     * this function. However, if our attempt fails, there's
+     * not much we can do. XATTRs refcounting is fubar'ed and
+     * the only option we have is warn users. */
+    if (virSecurityDACRestoreFileLabelInternal(mgr, src, path, remember) < 0)
+        VIR_WARN("Unable to restore label on '%s'. "
+                 "XATTRs might have been left in inconsistent state.",
+                 NULLSTR(src ? src->path : path));
+
+    virErrorRestore(&origerr);
+
+    return -1;
 }
 
 

src/security/security_selinux.c

@@ -1334,6 +1334,7 @@ virSecuritySELinuxSetFileconHelper(virSecurityManagerPtr mgr,
     security_context_t econ = NULL;
     int refcount;
     int rc;
+    bool rollback = false;
     int ret = -1;
 
     if ((rc = virSecuritySELinuxTransactionAppend(path, tcon,
@@ -1353,6 +1354,8 @@ virSecuritySELinuxSetFileconHelper(virSecurityManagerPtr mgr,
 
         if (econ) {
             refcount = virSecuritySELinuxRememberLabel(path, econ);
+            if (refcount > 0)
+                rollback = true;
             if (refcount == -2) {
                 /* Not supported. Don't error though. */
             } else if (refcount < 0) {
@@ -1362,7 +1365,8 @@ virSecuritySELinuxSetFileconHelper(virSecurityManagerPtr mgr,
                  * is @refcount domains using the @path. Do not
                  * change the label (as it would almost certainly
                  * cause the other domains to lose access to the
-                 * @path). */
+                 * @path). However, the refcounter was
+                 * incremented in XATTRs so decrease it. */
                 if (STRNEQ(econ, tcon)) {
                     virReportError(VIR_ERR_OPERATION_INVALID,
                                    _("Setting different SELinux label on %s "
@@ -1373,7 +1377,12 @@ virSecuritySELinuxSetFileconHelper(virSecurityManagerPtr mgr,
         }
     }
 
-    if (virSecuritySELinuxSetFileconImpl(path, tcon, optional, privileged) < 0) {
+    if (virSecuritySELinuxSetFileconImpl(path, tcon, optional, privileged) < 0)
+        goto cleanup;
+
+    ret = 0;
+ cleanup:
+    if (ret < 0 && rollback) {
         virErrorPtr origerr;
 
         virErrorPreserveLast(&origerr);
@@ -1388,11 +1397,8 @@ virSecuritySELinuxSetFileconHelper(virSecurityManagerPtr mgr,
                      path);
 
         virErrorRestore(&origerr);
-        goto cleanup;
-    }
 
-    ret = 0;
- cleanup:
+    }
     freecon(econ);
     return ret;
 }