[Xen-devel] [PATCH v5] run QEMU as non-root

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

Try to use "xen-qemudepriv-domid$domid" first, then
"xen-qemudepriv-shared" and root if everything else fails.

The uids need to be manually created by the user or, more likely, by the
xen package maintainer.

To actually secure QEMU when running in Dom0, we need at least to
deprivilege the privcmd and xenstore interfaces, this is just the first
step in that direction.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>

---
Changes in v5:
- improve wording in doc
- fix wording in warning message
- fix example in doc
- drop xen-qemudepriv-$domname

Changes in v4:
- rename qemu-deprivilege to qemu-deprivilege.txt
- add a note about qemu-deprivilege.txt to INSTALL
- instead of xen-qemudepriv-base + $domid, try xen-qemudepriv-domid$domid
- introduce libxl__dm_runas_helper to make the code nicer

Changes in v3:
- clarify doc
- handle errno == ERANGE
---
 INSTALL                        |    7 ++++++
 docs/misc/qemu-deprivilege.txt |   26 +++++++++++++++++++++
 tools/libxl/libxl_dm.c         |   50 ++++++++++++++++++++++++++++++++++++++++
 tools/libxl/libxl_internal.h   |    4 ++++
 4 files changed, 87 insertions(+)
 create mode 100644 docs/misc/qemu-deprivilege.txt

diff --git a/INSTALL b/INSTALL
index a0f2e7b..fe83c20 100644
--- a/INSTALL
+++ b/INSTALL
@@ -297,6 +297,13 @@ systemctl enable xendomains.service
 systemctl enable xen-watchdog.service
 
 
+QEMU Deprivilege
+================
+It is recommended to run QEMU as non-root.
+See docs/misc/qemu-deprivilege.txt for an explanation on what you need
+to do at installation time to run QEMU as a dedicated user.
+
+
 History of options
 ==================
 
diff --git a/docs/misc/qemu-deprivilege.txt b/docs/misc/qemu-deprivilege.txt
new file mode 100644
index 0000000..783874b
--- /dev/null
+++ b/docs/misc/qemu-deprivilege.txt
@@ -0,0 +1,26 @@
+For security reasons, libxl tries to create QEMU as non-root.
+Libxl looks for the following users in this order:
+
+1) a user named "xen-qemuuser-domid$domid", 
+Where $domid is the domid of the domain being created.
+This requires the reservation of 65535 uids from xen-qemuuser-domid1
+to xen-qemuuser-domid65535. To use this mechanism, you might want to
+create a large number of users at installation time. For example:
+
+for ((i=1; i<65536; i++))
+do
+    adduser --system xen-qemuuser-domid$i
+done
+
+
+2) a user named "xen-qemuuser-shared"
+As a fall back if both 1) and 2) fail, libxl will use a single user for
+all QEMU instances. The user is named xen-qemuuser-shared. This is
+less secure but still better than running QEMU as root. Using this is as
+simple as creating just one more user on your host:
+
+adduser --system xen-qemuuser-shared
+
+
+3) root
+As a last resort, libxl will start QEMU as root.
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index 0c6408d..66d638d 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -19,6 +19,8 @@
 
 #include "libxl_internal.h"
 #include <xen/hvm/e820.h>
+#include <sys/types.h>
+#include <pwd.h>
 
 static const char *libxl_tapif_script(libxl__gc *gc)
 {
@@ -418,6 +420,33 @@ static char *dm_spice_options(libxl__gc *gc,
     return opt;
 }
 
+/* return 1 if the user was found, 0 if it was not, -1 on error */
+static int libxl__dm_runas_helper(libxl__gc *gc, char *username)
+{
+    struct passwd pwd, *user = NULL;
+    char *buf = NULL;
+    long buf_size;
+
+    buf_size = sysconf(_SC_GETPW_R_SIZE_MAX);
+    if (buf_size < 0) {
+        LOGE(ERROR, "sysconf(_SC_GETPW_R_SIZE_MAX) returned error %ld",
+                buf_size);
+        return -1;
+    }
+
+retry:
+    buf = libxl__realloc(gc, buf, buf_size);
+    errno = 0;
+    getpwnam_r(username, &pwd, buf, buf_size, &user);
+    if (user != NULL)
+        return 1;
+    if (errno == ERANGE) {
+        buf_size += 128;
+        goto retry;
+    }
+    return 0;
+}
+
 static char ** libxl__build_device_model_args_new(libxl__gc *gc,
                                         const char *dm, int guest_domid,
                                         const libxl_domain_config 
*guest_config,
@@ -439,6 +468,7 @@ static char ** libxl__build_device_model_args_new(libxl__gc 
*gc,
     int i, connection, devid;
     uint64_t ram_size;
     const char *path, *chardev;
+    char *user;
 
     dm_args = flexarray_make(gc, 16, 1);
 
@@ -878,6 +908,26 @@ static char ** 
libxl__build_device_model_args_new(libxl__gc *gc,
         default:
             break;
         }
+
+        user = libxl__sprintf(gc, "%s%d", LIBXL_QEMU_USER_BASE, guest_domid);
+        if (libxl__dm_runas_helper(gc, user) > 0)
+            goto end_search;
+
+        user = LIBXL_QEMU_USER_SHARED;
+        if (libxl__dm_runas_helper(gc, user) > 0) {
+            LOG(WARN, "Could not find user %s%d, falling back to %s",
+                    LIBXL_QEMU_USER_BASE, guest_domid, LIBXL_QEMU_USER_SHARED);
+            goto end_search;
+        }
+
+        user = NULL;
+        LOG(WARN, "Could not find user %s, starting QEMU as root", 
LIBXL_QEMU_USER_SHARED);
+
+end_search:
+        if (user) {
+            flexarray_append(dm_args, "-runas");
+            flexarray_append(dm_args, user);
+        }
     }
     flexarray_append(dm_args, NULL);
     return (char **) flexarray_contents(dm_args);
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 8eb38aa..7d0af40 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -3692,6 +3692,10 @@ static inline void libxl__update_config_vtpm(libxl__gc 
*gc,
  */
 void libxl__bitmap_copy_best_effort(libxl__gc *gc, libxl_bitmap *dptr,
                                     const libxl_bitmap *sptr);
+
+#define LIBXL_QEMU_USER_PREFIX "xen-qemuuser"
+#define LIBXL_QEMU_USER_BASE   LIBXL_QEMU_USER_PREFIX"-domid"
+#define LIBXL_QEMU_USER_SHARED LIBXL_QEMU_USER_PREFIX"-shared"
 #endif
 
 /*
-- 
1.7.10.4


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