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[libvirt] [RFC PATCH] Add proxy FS (9p) support to libvirt



From: "M. Mohan Kumar" <mohan in ibm com>

This patch
*) adds new sub element <virtfs-proxy-helper> under <device> element.
This sub-element is used to specify the virtfs-proxy-helper binary
(part of QEMU) to be executed when proxy FS driver is used.

*) invokes proxy_helper binary specified by the <virtfs-proxy-helper>
sub-element with passing one of the created socket pair descriptor and
adds "-sock_fd" paramter to qemu.

Is it okay to add the sub-element "virtfs-proxy-helper" under "devices"
element? Proxy helper binary is common for all 9p proxy FS devices, so it
can not be placed under "filesystems" element.

About Proxy FS driver
======================
Proxy FS driver is added to qemu 9p server to overcome two issues with
pass-through security model (needs root privilege) security model.

1) TOCTTOU vulnerability: Following symbolic links in the server could
provide access to files beyond 9p export path.

2) Running QEMU with root privilege could be a security issue.

Proxy FS uses chroot + socket combination for securing the vulnerability
known with following symbolic links. Intention of adding a new filesystem
type is to allow qemu to run in non-root mode, but doing privileged
operations using socket IO.

Proxy helper(a stand alone binary part of qemu) is invoked with
root privileges. Proxy helper chroots into 9p export path and creates
a socket pair or a named socket based on the command line parameter.
Qemu and proxy helper communicate using this socket. QEMU proxy fs
driver sends filesystem request to proxy helper and receives the
response from it.

Proxy helper is designed such a way that it needs only few capabilities
related to filesystem operations (such as CAP_DAC_OVERRIDE, CAP_FOWNER,
etc) and all other capabilities are dropped (CAP_SYS_CHROOT, etc)

Proxy patches
    http://permalink.gmane.org/gmane.comp.emulators.qemu/128735

Signed-off-by: M. Mohan Kumar <mohan in ibm com>
---
 docs/formatdomain.html.in    |    3 +-
 src/conf/domain_conf.c       |    7 ++++-
 src/conf/domain_conf.h       |    4 +-
 src/qemu/qemu_capabilities.c |    3 ++
 src/qemu/qemu_capabilities.h |    1 +
 src/qemu/qemu_command.c      |   64 ++++++++++++++++++++++++++++++++++++++++--
 src/qemu/qemu_command.h      |    3 +-
 tests/qemuhelptest.c         |    3 +-
 8 files changed, 79 insertions(+), 9 deletions(-)

diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 18b7e22..e398779 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -1336,7 +1336,8 @@
         This mode also has an optional
         sub-element <code>driver</code>, with an
         attribute <code>type='path'</code>
-        or <code>type='handle'</code> <span class="since">(since
+        or <code>type='handle'</code>
+        or <code>type='proxy'</code> <span class="since">(since
         0.9.7)</span>.
         </dd>
         <dt><code>type='template'</code></dt>
diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 0190a81..694c166 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -250,7 +250,8 @@ VIR_ENUM_IMPL(virDomainFS, VIR_DOMAIN_FS_TYPE_LAST,
 VIR_ENUM_IMPL(virDomainFSDriverType, VIR_DOMAIN_FS_DRIVER_TYPE_LAST,
               "default",
               "path",
-              "handle")
+              "handle",
+              "proxy")
 
 VIR_ENUM_IMPL(virDomainFSAccessMode, VIR_DOMAIN_FS_ACCESSMODE_LAST,
               "passthrough",
@@ -1465,6 +1466,7 @@ void virDomainDefFree(virDomainDefPtr def)
     VIR_FREE(def->name);
     VIR_FREE(def->cpumask);
     VIR_FREE(def->emulator);
+    VIR_FREE(def->virtfs_proxy_helper);
     VIR_FREE(def->description);
 
     virBlkioDeviceWeightArrayClear(def->blkio.devices,
@@ -7490,6 +7492,9 @@ static virDomainDefPtr virDomainDefParseXML(virCapsPtr caps,
             goto error;
     }
 
+    def->virtfs_proxy_helper =
+        virXPathString("string(./devices/virtfs-proxy-helper[1])", ctxt);
+
     /* analysis of the disk devices */
     if ((n = virXPathNodeSet("./devices/disk", ctxt, &nodes)) < 0) {
         goto error;
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 03aa5b6..a7d248a 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -450,7 +450,6 @@ struct _virDomainControllerDef {
     virDomainDeviceInfo info;
 };
 
-
 /* Two types of disk backends */
 enum virDomainFSType {
     VIR_DOMAIN_FS_TYPE_MOUNT,   /* Better named 'bind' */
@@ -466,6 +465,7 @@ enum virDomainFSDriverType {
     VIR_DOMAIN_FS_DRIVER_TYPE_DEFAULT = 0,
     VIR_DOMAIN_FS_DRIVER_TYPE_PATH,
     VIR_DOMAIN_FS_DRIVER_TYPE_HANDLE,
+    VIR_DOMAIN_FS_DRIVER_TYPE_PROXY,
 
     VIR_DOMAIN_FS_DRIVER_TYPE_LAST
 };
@@ -491,7 +491,6 @@ struct _virDomainFSDef {
     virDomainDeviceInfo info;
 };
 
-
 /* 5 different types of networking config */
 enum virDomainNetType {
     VIR_DOMAIN_NET_TYPE_USER,
@@ -1503,6 +1502,7 @@ struct _virDomainDef {
 
     void *namespaceData;
     virDomainXMLNamespace ns;
+    char *virtfs_proxy_helper;
 };
 
 enum virDomainTaintFlags {
diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index 43c7578..9c01a3b 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -144,6 +144,7 @@ VIR_ENUM_IMPL(qemuCaps, QEMU_CAPS_LAST,
               "ich9-ahci",
               "no-acpi",
               "fsdev-readonly",
+              "fsdev-proxy",
     );
 
 struct qemu_feature_flags {
@@ -1083,6 +1084,8 @@ qemuCapsComputeCmdFlags(const char *help,
         qemuCapsSet(flags, QEMU_CAPS_FSDEV);
         if (strstr(fsdev, "readonly"))
             qemuCapsSet(flags, QEMU_CAPS_FSDEV_READONLY);
+        if (strstr(fsdev, "sock_fd"))
+            qemuCapsSet(flags, QEMU_CAPS_FSDEV_PROXY);
     }
     if (strstr(help, "-smbios type"))
         qemuCapsSet(flags, QEMU_CAPS_SMBIOS_TYPE);
diff --git a/src/qemu/qemu_capabilities.h b/src/qemu/qemu_capabilities.h
index c759baf..e129e35 100644
--- a/src/qemu/qemu_capabilities.h
+++ b/src/qemu/qemu_capabilities.h
@@ -117,6 +117,7 @@ enum qemuCapsFlags {
     QEMU_CAPS_ICH9_AHCI         = 77, /* -device ich9-ahci */
     QEMU_CAPS_NO_ACPI		= 78, /* -no-acpi */
     QEMU_CAPS_FSDEV_READONLY    =79, /* -fsdev readonly supported */
+    QEMU_CAPS_FSDEV_PROXY      = 80, /* -fsdev proxy supported */
 
     QEMU_CAPS_LAST,                   /* this must always be the last item */
 };
diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index f2e9cfa..9ac5a4c 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -106,7 +106,8 @@ VIR_ENUM_DECL(qemuDomainFSDriver)
 VIR_ENUM_IMPL(qemuDomainFSDriver, VIR_DOMAIN_FS_DRIVER_TYPE_LAST,
               "local",
               "local",
-              "handle");
+              "handle",
+              "proxy");
 
 
 static void
@@ -2080,9 +2081,32 @@ error:
     return NULL;
 }
 
+/*
+ * Invokes the Proxy Helper with one of the socketpair as its parameter
+ *
+ */
+static int qemuInvokeProxyHelper(const char *binary, int sock, const char *path)
+{
+    int ret_val, status;
+    virCommandPtr cmd;
+
+    cmd = virCommandNewArgList(binary, NULL);
+    virCommandAddArg(cmd, "-f");
+    virCommandAddArgFormat(cmd, "%d", sock);
+    virCommandAddArg(cmd, "-p");
+    virCommandAddArgFormat(cmd, "%s", path);
+    virCommandTransferFD(cmd, sock);
+    virCommandDaemonize(cmd);
+    ret_val = virCommandRun(cmd, &status);
+    if (ret_val < 0)
+        qemuReportError(VIR_ERR_INTERNAL_ERROR,
+                            _("%s can't execute"), binary);
+    virCommandFree(cmd);
+    return ret_val;
+}
 
 char *qemuBuildFSStr(virDomainFSDefPtr fs,
-                     virBitmapPtr qemuCaps ATTRIBUTE_UNUSED)
+                     virBitmapPtr qemuCaps ATTRIBUTE_UNUSED, int qemuSocket)
 {
     virBuffer opt = VIR_BUFFER_INITIALIZER;
     const char *driver = qemuDomainFSDriverTypeToString(fs->fsdriver);
@@ -2108,6 +2132,10 @@ char *qemuBuildFSStr(virDomainFSDefPtr fs,
         virBufferAddLit(&opt, ",security_model=none");
     }
     virBufferAsprintf(&opt, ",id=%s%s", QEMU_FSDEV_HOST_PREFIX, fs->info.alias);
+
+    if (fs->fsdriver == VIR_DOMAIN_FS_DRIVER_TYPE_PROXY)
+        virBufferAsprintf(&opt, ",sock_fd=%d", qemuSocket);
+
     virBufferAsprintf(&opt, ",path=%s", fs->src);
 
     if (fs->readonly) {
@@ -4426,10 +4454,40 @@ qemuBuildCommandLine(virConnectPtr conn,
     if (qemuCapsGet(qemuCaps, QEMU_CAPS_FSDEV)) {
         for (i = 0 ; i < def->nfss ; i++) {
             char *optstr;
+            int sockets[2] = {-1, -1};
             virDomainFSDefPtr fs = def->fss[i];
 
+            /*
+             * If its a proxy FS, we need to create a socket pair
+             * and invoke proxy_helper
+             */
+            if (fs->fsdriver == VIR_DOMAIN_FS_DRIVER_TYPE_PROXY) {
+                if (qemuCapsGet(qemuCaps, QEMU_CAPS_FSDEV_PROXY) < 0) {
+                    qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                        _("proxy helper not supported"));
+                    goto error;
+                }
+                if (!def->virtfs_proxy_helper) {
+                    qemuReportError(VIR_ERR_XML_ERROR, "%s",
+                        _("proxy helper not specified"));
+                    goto error;
+                }
+                /* create a socket pair */
+                if (socketpair(PF_UNIX, SOCK_STREAM, 0, sockets) < 0) {
+                    qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                        _("socketpair failed"));
+                    goto error;
+                }
+                virCommandTransferFD(cmd, sockets[1]);
+                if (qemuInvokeProxyHelper(def->virtfs_proxy_helper, sockets[0],
+                                    fs->src) < 0) {
+                    VIR_FORCE_CLOSE(sockets[0]);
+                    VIR_FORCE_CLOSE(sockets[1]);
+                    goto error;
+                }
+            }
             virCommandAddArg(cmd, "-fsdev");
-            if (!(optstr = qemuBuildFSStr(fs, qemuCaps)))
+            if (!(optstr = qemuBuildFSStr(fs, qemuCaps, sockets[1])))
                 goto error;
             virCommandAddArg(cmd, optstr);
             VIR_FREE(optstr);
diff --git a/src/qemu/qemu_command.h b/src/qemu/qemu_command.h
index de61cf3..8e4f335 100644
--- a/src/qemu/qemu_command.h
+++ b/src/qemu/qemu_command.h
@@ -87,7 +87,8 @@ char *qemuBuildDriveStr(virConnectPtr conn,
                         bool bootable,
                         virBitmapPtr qemuCaps);
 char *qemuBuildFSStr(virDomainFSDefPtr fs,
-                     virBitmapPtr qemuCaps);
+                     virBitmapPtr qemuCaps,
+                     int qemuSocket);
 
 /* Current, best practice */
 char * qemuBuildDriveDevStr(virDomainDiskDefPtr disk,
diff --git a/tests/qemuhelptest.c b/tests/qemuhelptest.c
index 60155e7..455a7e1 100644
--- a/tests/qemuhelptest.c
+++ b/tests/qemuhelptest.c
@@ -648,7 +648,8 @@ mymain(void)
             QEMU_CAPS_PCI_ROMBAR,
             QEMU_CAPS_ICH9_AHCI,
             QEMU_CAPS_NO_ACPI,
-            QEMU_CAPS_FSDEV_READONLY);
+            QEMU_CAPS_FSDEV_READONLY,
+            QEMU_CAPS_FSDEV_PROXY);
 
     return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
 }
-- 
1.7.6


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