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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH v10] run QEMU as non-root
Try to use "xen-qemuuser-domid$domid" first, then
"xen-qemuuser-shared" and root if everything else fails.
The uids need to be manually created by the user or, more likely, by the
xen package maintainer.
Expose a device_model_user setting in libxl_domain_build_info, so that
opinionated callers, such as libvirt, can set any user they like. Do not
fall back to root if device_model_user is set. Users can also set
device_model_user by hand in the xl domain config file.
QEMU is going to setuid and setgid to the user ID and the group ID of
the specified user, soon after initialization, before starting to deal
with any guest IO.
To actually secure QEMU when running in Dom0, we need at least to
deprivilege the privcmd and xenstore interfaces, this is just the first
step in that direction.
Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
---
Changes in v10:
- rebase
- git add docs/misc/qemu-deprivilege.txt
- fix commit message to reflect the names chosen (xen-qemudepriv ->
xen-qemuuser)
Changes in v9:
- add a device_model_user option to the xl domain config file
Changes in v8:
- no need to pass the -runas option if the user requested for root
- return ERROR_FAIL from libxl__dm_runas_helper in case of errors
- return NULL from libxl__build_device_model_args_new if libxl__dm_runas_helper
failed
- fix line too long
- remove setting errno
- replace retry goto loop, with a while loop
- const char * as argument to libxl__dm_runas_helper
- fix comment
Changes in v7:
- do not fall back to root if the user explicitly set
b_info->device_model_user.
Changes in v6:
- add device_model_user to libxl_domain_build_info
- improve doc
- improve wording in commit message
Changes in v5:
- improve wording in doc
- fix wording in warning message
- fix example in doc
- drop xen-qemudepriv-$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/man/xl.cfg.pod.5 | 5 +++
docs/misc/qemu-deprivilege.txt | 31 +++++++++++++++++++
tools/libxl/libxl.h | 5 +++
tools/libxl/libxl_dm.c | 67 +++++++++++++++++++++++++++++++++++++++-
tools/libxl/libxl_internal.h | 5 +++
tools/libxl/libxl_types.idl | 1 +
tools/libxl/xl_cmdimpl.c | 3 ++
8 files changed, 123 insertions(+), 1 deletion(-)
create mode 100644 docs/misc/qemu-deprivilege.txt
diff --git a/INSTALL b/INSTALL
index 56e2950..b7e426c 100644
--- a/INSTALL
+++ b/INSTALL
@@ -304,6 +304,13 @@ 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/man/xl.cfg.pod.5 b/docs/man/xl.cfg.pod.5
index b63846a..2aca8dd 100644
--- a/docs/man/xl.cfg.pod.5
+++ b/docs/man/xl.cfg.pod.5
@@ -1825,6 +1825,11 @@ Pass additional arbitrary options on the device-model
command line for
an HVM device model only. Each element in the list is passed as an
option to the device-model.
+=item B<device_model_user="username">
+
+Run the device model as user "username", instead of
+xen-qemudepriv-domid$domid or xen-qemudepriv-shared or root.
+
=back
=head2 Keymaps
diff --git a/docs/misc/qemu-deprivilege.txt b/docs/misc/qemu-deprivilege.txt
new file mode 100644
index 0000000..dde74ab
--- /dev/null
+++ b/docs/misc/qemu-deprivilege.txt
@@ -0,0 +1,31 @@
+For security reasons, libxl tries to pass a non-root username to QEMU as
+argument. During initialization QEMU calls setuid and setgid with the
+user ID and the group ID of the user passed as argument.
+Libxl looks for the following users in this order:
+
+1) a user named "xen-qemuuser-domid$domid",
+Where $domid is the domid of the domain being created.
+This requires the reservation of 65535 uids from xen-qemuuser-domid1
+to xen-qemuuser-domid65535. To use this mechanism, you might want to
+create a large number of users at installation time. For example:
+
+for ((i=1; i<65536; i++))
+do
+ adduser --no-create-home --system xen-qemuuser-domid$i
+done
+
+You might want to consider passing --group to adduser to create a new
+group for each new user.
+
+
+2) a user named "xen-qemuuser-shared"
+As a fall back if both 1) fails, 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 --no-create-home --system xen-qemuuser-shared
+
+
+3) root
+As a last resort, libxl will start QEMU as root.
diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
index 168fedd..5edeb30 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -199,6 +199,11 @@
*/
#define LIBXL_HAVE_DEVICETREE_PASSTHROUGH 1
+/* libxl_domain_build_info has device_model_user to specify the user to
+ * run the device model with. See docs/misc/qemu-deprivilege.txt.
+ */
+#define LIBXL_HAVE_DEVICE_MODEL_USER 1
+
/*
* libxl_domain_build_info has the arm.gic_version field.
*/
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index 9c9eaa3..151b6ed 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -21,6 +21,8 @@
#include <xc_dom.h>
#include <xen/hvm/e820.h>
+#include <sys/types.h>
+#include <pwd.h>
static const char *libxl_tapif_script(libxl__gc *gc)
{
@@ -722,6 +724,36 @@ libxl__detect_gfx_passthru_kind(libxl__gc *gc,
return LIBXL_GFX_PASSTHRU_KIND_DEFAULT;
}
+/* return 1 if the user was found, 0 if it was not, -1 on error */
+static int libxl__dm_runas_helper(libxl__gc *gc, const char *username)
+{
+ struct passwd pwd, *user = NULL;
+ char *buf = NULL;
+ long buf_size;
+ int ret;
+
+ 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 ERROR_FAIL;
+ }
+
+ while (1) {
+ buf = libxl__realloc(gc, buf, buf_size);
+ ret = getpwnam_r(username, &pwd, buf, buf_size, &user);
+ if (ret == ERANGE) {
+ buf_size += 128;
+ continue;
+ }
+ if (ret != 0)
+ return ERROR_FAIL;
+ if (user != NULL)
+ return 1;
+ return 0;
+ }
+}
+
static int libxl__build_device_model_args_new(libxl__gc *gc,
const char *dm, int guest_domid,
const libxl_domain_config
*guest_config,
@@ -740,9 +772,10 @@ static int libxl__build_device_model_args_new(libxl__gc
*gc,
const char *keymap = dm_keymap(guest_config);
char *machinearg;
flexarray_t *dm_args, *dm_envs;
- int i, connection, devid;
+ int i, connection, devid, ret;
uint64_t ram_size;
const char *path, *chardev;
+ char *user = NULL;
dm_args = flexarray_make(gc, 16, 1);
dm_envs = flexarray_make(gc, 16, 1);
@@ -1222,6 +1255,38 @@ static int libxl__build_device_model_args_new(libxl__gc
*gc,
default:
break;
}
+
+ if (b_info->device_model_user) {
+ user = b_info->device_model_user;
+ goto end_search;
+ }
+
+ user = libxl__sprintf(gc, "%s%d", LIBXL_QEMU_USER_BASE, guest_domid);
+ ret = libxl__dm_runas_helper(gc, user);
+ if (ret < 0)
+ return ret;
+ if (ret > 0)
+ goto end_search;
+
+ user = LIBXL_QEMU_USER_SHARED;
+ ret = libxl__dm_runas_helper(gc, user);
+ if (ret < 0)
+ return ret;
+ if (ret > 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 != NULL && strcmp(user, "root")) {
+ flexarray_append(dm_args, "-runas");
+ flexarray_append(dm_args, user);
+ }
}
flexarray_append(dm_args, NULL);
*args = (char **) flexarray_contents(dm_args);
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index b00add0..952b668 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -4018,6 +4018,11 @@ void libxl__bitmap_copy_best_effort(libxl__gc *gc,
libxl_bitmap *dptr,
const libxl_bitmap *sptr);
int libxl__count_physical_sockets(libxl__gc *gc, int *sockets);
+
+
+#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
/*
diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
index 4d78f86..6808f2b 100644
--- a/tools/libxl/libxl_types.idl
+++ b/tools/libxl/libxl_types.idl
@@ -440,6 +440,7 @@ libxl_domain_build_info = Struct("domain_build_info",[
("device_model", string),
("device_model_ssidref", uint32),
("device_model_ssid_label", string),
+ ("device_model_user", string),
# extra parameters pass directly to qemu, NULL terminated
("extra", libxl_string_list),
diff --git a/tools/libxl/xl_cmdimpl.c b/tools/libxl/xl_cmdimpl.c
index 03442e1..2b6371d 100644
--- a/tools/libxl/xl_cmdimpl.c
+++ b/tools/libxl/xl_cmdimpl.c
@@ -2201,6 +2201,9 @@ skip_vfb:
xlu_cfg_replace_string(config, "device_model_stubdomain_seclabel",
&b_info->device_model_ssid_label, 0);
+ xlu_cfg_replace_string(config, "device_model_user",
+ &b_info->device_model_user, 0);
+
#define parse_extra_args(type) \
e = xlu_cfg_get_list_as_string_list(config, "device_model_args"#type, \
&b_info->extra##type, 0); \
--
1.7.10.4
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