[lttng-ust,v3] Add setuid wrappers for per-UIDbuffers

  In case of a per-UID buffer, events following a change of user ID should be
forwarded to buffers of the new UID. In order to do so, we add a wrapper around
setuid(3), setreuid(3) and setresuid(2) that unregister and re-register the
application from the session daemon.

While there exist some other functions such as seteuid(3) or setfsuid(2), we
don't need to add a wrapper for them since they do not edit the real user ID,
the actual ID returned by getuid(3) and used for creating the buffers.

Signed-off-by: Gabriel-Andrew Pollo-Guilbert <gabriel.pollo-guilbert at efficios.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
---
 Makefile.am                     |   1 +
 configure.ac                    |   1 +
 doc/man/lttng-ust.3.txt         |  19 ++++++
 include/lttng/ust.h             |   1 +
 liblttng-ust-setuid/Makefile.am |  10 +++
 liblttng-ust-setuid/ustsetuid.c | 105 ++++++++++++++++++++++++++++++++
 liblttng-ust/lttng-ust-comm.c   |  29 ++++++++-
 7 files changed, 164 insertions(+), 2 deletions(-)
 create mode 100644 liblttng-ust-setuid/Makefile.am
 create mode 100644 liblttng-ust-setuid/ustsetuid.c
  

Hello,

That's an interesting feature.


On 2019-06-11 11:57 a.m., Gabriel-Andrew Pollo-Guilbert wrote:
> In case of a per-UID buffer, events following a change of user ID should be
> forwarded to buffers of the new UID. In order to do so, we add a wrapper around
> setuid(3), setreuid(3) and setresuid(2) that unregister and re-register the
> application from the session daemon.
> 
> While there exist some other functions such as seteuid(3) or setfsuid(2), we

While there exists
                 ^

> don't need to add a wrapper for them since they do not edit the real user ID,
> the actual ID returned by getuid(3) and used for creating the buffers.
> 
> Signed-off-by: Gabriel-Andrew Pollo-Guilbert <gabriel.pollo-guilbert at efficios.com>
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> ---
>  Makefile.am                     |   1 +
>  configure.ac                    |   1 +
>  doc/man/lttng-ust.3.txt         |  19 ++++++
>  include/lttng/ust.h             |   1 +
>  liblttng-ust-setuid/Makefile.am |  10 +++
>  liblttng-ust-setuid/ustsetuid.c | 105 ++++++++++++++++++++++++++++++++
>  liblttng-ust/lttng-ust-comm.c   |  29 ++++++++-
>  7 files changed, 164 insertions(+), 2 deletions(-)
>  create mode 100644 liblttng-ust-setuid/Makefile.am
>  create mode 100644 liblttng-ust-setuid/ustsetuid.c
> 
> diff --git a/Makefile.am b/Makefile.am
> index 810761ca..e8812e59 100644
> --- a/Makefile.am
> +++ b/Makefile.am
> @@ -5,6 +5,7 @@ SUBDIRS = . include snprintf libringbuffer liblttng-ust-comm \
>  		liblttng-ust-ctl \
>  		liblttng-ust-fd \
>  		liblttng-ust-fork \
> +		liblttng-ust-setuid \
>  		liblttng-ust-libc-wrapper \
>  		liblttng-ust-cyg-profile \
>  		tools
> diff --git a/configure.ac b/configure.ac
> index 52fc3f68..95780dba 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -522,6 +522,7 @@ AC_CONFIG_FILES([
>  	liblttng-ust/Makefile
>  	liblttng-ust-ctl/Makefile
>  	liblttng-ust-fork/Makefile
> +	liblttng-ust-setuid/Makefile
>  	liblttng-ust-dl/Makefile
>  	liblttng-ust-fd/Makefile
>  	liblttng-ust-java/Makefile
> diff --git a/doc/man/lttng-ust.3.txt b/doc/man/lttng-ust.3.txt
> index 5c9c9f5d..511ff210 100644
> --- a/doc/man/lttng-ust.3.txt
> +++ b/doc/man/lttng-ust.3.txt
> @@ -744,6 +744,25 @@ library before you start the application. Typical use cases include
>  daemons closing all file descriptors after man:fork(2), and buggy
>  applications doing ``double-closes''.
>  
> ---
> v3:
> 	* fix some typos
> 	* added documentation concerning CAP_SETUID/SUID and LD_PRELOAD
> ---
> 
> +Using LTTng-UST with applications that change effective user ID
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +If LTTng traces your application using per-UID buffers (see the
> +nloption:--buffers-uid option of man:lttng-enable-channel(1)) and changes
> +the process's effective user ID with man:setuid(2), the library
> +`liblttng-ust-setuid.so` needs to be preloaded before starting the
> +application with the `LD_PRELOAD` environment variable (see man:ld.so(8)).
> +This way, event records generated after the user ID change are forwarded to
> +the correct buffer.
> +
> +This workaround requires that the tracing session be global (created by a
> +root session daemon; see man:lttng-sessiond(1)) to avoid permission problems
> +of a session daemon started by another user.
> +
> +That said, if your application has the `CAP_SETUID` capability or the SUID bit,
> +the dynamic linker will ignore libraries in `LD_PRELOAD` if they aren't in the
> +standard search directories and don't have the SUID bit too (see man:ld.so(8)).
> +The install script of LTTng-UST will not set the SUID bit to
> +`liblttng-ust-setuid.so` for security reason.
>  
>  Context information
>  ~~~~~~~~~~~~~~~~~~~
> diff --git a/include/lttng/ust.h b/include/lttng/ust.h
> index 2779d7a7..5495e564 100644
> --- a/include/lttng/ust.h
> +++ b/include/lttng/ust.h
> @@ -32,6 +32,7 @@ extern "C" {
>  extern void ust_before_fork(sigset_t *save_sigset);
>  extern void ust_after_fork_parent(sigset_t *restore_sigset);
>  extern void ust_after_fork_child(sigset_t *restore_sigset);
> +extern void ust_after_setuid(void);
>  
>  #ifdef __cplusplus 
>  }
> diff --git a/liblttng-ust-setuid/Makefile.am b/liblttng-ust-setuid/Makefile.am
> new file mode 100644
> index 00000000..df3cd622
> --- /dev/null
> +++ b/liblttng-ust-setuid/Makefile.am
> @@ -0,0 +1,10 @@
> +AM_CPPFLAGS = -I$(top_srcdir)/include
> +AM_CFLAGS += -fno-strict-aliasing
> +
> +lib_LTLIBRARIES = liblttng-ust-setuid.la
> +liblttng_ust_setuid_la_SOURCES = ustsetuid.c
> +liblttng_ust_setuid_la_LIBADD = \
> +	$(top_builddir)/liblttng-ust/liblttng-ust.la \
> +	$(DL_LIBS)
> +
> +liblttng_ust_setuid_la_CFLAGS = -DUST_COMPONENT=liblttng-ust-setuid $(AM_CFLAGS)
> diff --git a/liblttng-ust-setuid/ustsetuid.c b/liblttng-ust-setuid/ustsetuid.c
> new file mode 100644
> index 00000000..d221e8fb
> --- /dev/null
> +++ b/liblttng-ust-setuid/ustsetuid.c
> @@ -0,0 +1,105 @@
> +/*
> + * Copyright (C) 2019 Gabriel-Andrew Pollo-Guilbert
> + *
> + * This library is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; version 2.1 of
> + * the License.
> + *
> + * This library is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with this library; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA
> + */
> +
> +#define _GNU_SOURCE
> +#include <unistd.h>
> +#include <errno.h>
> +#include <stdio.h>
> +#include <lttng/ust-dlfcn.h>
> +#include <lttng/ust.h>
> +
> +int setuid(uid_t uid)
> +{
> +	static int (*plibc_func)(uid_t) = NULL;
> +	int retval;
> +	uid_t before_uid;
> +
> +	if (plibc_func == NULL) {
> +		plibc_func = dlsym(RTLD_NEXT, __func__);
> +		if (plibc_func == NULL) {
> +			fprintf(stderr, "libustsetuid: unable to find \"%s\" symbol\n", __func__);
> +			errno = ENOSYS;
> +			return -1;
> +		}
> +	}
> +
> +	before_uid = getuid();
> +
> +	retval = plibc_func(uid);
> +	if (retval < 0)
> +		return retval;
> +
> +	if (before_uid != getuid())
> +		ust_after_setuid();
> +
> +	return retval;
> +}
> +
> +int setreuid(uid_t ruid, uid_t euid)
> +{
> +	static int (*plibc_func)(uid_t, uid_t) = NULL;
> +	int retval;
> +	uid_t before_uid;
> +
> +	if (plibc_func == NULL) {
> +		plibc_func = dlsym(RTLD_NEXT, __func__);
> +		if (plibc_func == NULL) {
> +			fprintf(stderr, "libustsetuid: unable to find \"%s\" symbol\n", __func__);
> +			errno = ENOSYS;
> +			return -1;
> +		}
> +	}
> +
> +	before_uid = getuid();
> +
> +	retval = plibc_func(ruid, euid);
> +	if (retval < 0)
> +		return retval;
> +
> +	if (before_uid != getuid())
> +		ust_after_setuid();
> +
> +	return retval;
> +}
> +
> +int setresuid(uid_t ruid, uid_t euid, uid_t suid)
> +{
> +	static int (*plibc_func)(uid_t, uid_t, uid_t) = NULL;
> +	int retval;
> +	uid_t before_uid;
> +
> +	if (plibc_func == NULL) {
> +		plibc_func = dlsym(RTLD_NEXT, __func__);
> +		if (plibc_func == NULL) {
> +			fprintf(stderr, "libustsetuid: unable to find \"%s\" symbol\n", __func__);
> +			errno = ENOSYS;
> +			return -1;
> +		}
> +	}
> +
> +	before_uid = getuid();
> +
> +	retval = plibc_func(ruid, euid, suid);
> +	if (retval < 0)
> +		return retval;
> +
> +	if (before_uid != getuid())
> +		ust_after_setuid();
> +
> +	return retval;
> +}
> diff --git a/liblttng-ust/lttng-ust-comm.c b/liblttng-ust/lttng-ust-comm.c
> index 61dbb41b..36c876a4 100644
> --- a/liblttng-ust/lttng-ust-comm.c
> +++ b/liblttng-ust/lttng-ust-comm.c
> @@ -225,10 +225,12 @@ void ust_unlock(void)
>   *   daemon problems).
>   */
>  static sem_t constructor_wait;
> +
>  /*
>   * Doing this for both the global and local sessiond.
>   */
> -static int sem_count = { 2 };
> +enum { LTTNG_UST_INIT_SEM_COUNT = 2 };
> +static int sem_count = LTTNG_UST_INIT_SEM_COUNT;
>  
>  /*
>   * Counting nesting within lttng-ust. Used to ensure that calling fork()
> @@ -1922,7 +1924,7 @@ void lttng_ust_cleanup(int exiting)
>  	exit_tracepoint();
>  	if (!exiting) {
>  		/* Reinitialize values for fork */
> -		sem_count = 2;
> +		sem_count = LTTNG_UST_INIT_SEM_COUNT;
>  		lttng_ust_comm_should_quit = 0;
>  		initialized = 0;
>  	}
> @@ -2072,3 +2074,26 @@ void lttng_ust_sockinfo_session_enabled(void *owner)
>  	struct sock_info *sock_info = owner;
>  	sock_info->statedump_pending = 1;
>  }
> +
> +/*
> + * Re-register the application when changing user ID. This is especially
> + * important for per-UID buffers. It is not strictly needed for per-PID
> + * buffers, but a slight extra overhead when changing user ID is considered
> + * harmless for a relatively infrequent operation.
> + */
> +void ust_after_setuid(void)
> +{
> +	DBG("Unregistering the process");
> +	lttng_ust_fixup_tls();
> +	lttng_ust_exit();
> +
> +	sem_count = LTTNG_UST_INIT_SEM_COUNT;
> +	lttng_ust_comm_should_quit = 0;
> +	initialized = 0;
> +
> +	global_apps.wait_shm_mmap = NULL;
> +	local_apps.wait_shm_mmap = NULL;
> +
> +	DBG("Registering the process under new UID=%u", getuid());
> +	lttng_ust_init();
> +}
>