After reading some man pages (mainly credentials(7) and capabilities(7)), I've
come to the conclusion that the definition of a "user" is kind of unclear.
RUID and EUID are pretty much the same, except that when a process wants to
drop its previlieges temporarily, it only needs to change the EUID. Usually,
a process will use the value saved in SUID to regain the original previlieges.
As of right now, the per-UID buffers are created using getuid(3) which returns
the __real__ user ID (RUID).
setfsuid(2): may change FSUID, not RUID, same buffers would be used after
seteuid(2): may change EUID, not RUID, same behavior
setuid(2): may change RUID, EUID and SUID, the should be a wrapper
setreuid(2): may change RUID and EUID, there should be a wrapper
setresuid(2): may change RUID, EUID and SUID, the should be a wrapper
That said, I don't know if `per-RUID` buffers is the actual behavior we want.
Lots of application only changes EUID in order to drop previlieges.
On a side note, I can also add a check in the wrappers so that if the RUID
doesn't change, we don't re-register the application for nothing. Sounds good?
----- Original Message -----
From: "Mathieu Desnoyers" <mathieu.desnoy...@efficios.com>
To: "Gabriel-Andrew Pollo-Guilbert" <gabriel.pollo-guilb...@efficios.com>
Cc: "lttng-dev" <lttng-dev@lists.lttng.org>
Sent: Wednesday, May 22, 2019 12:24:22 PM
Subject: Re: [lttng-dev] [lttng-ust RFC v2] Add setuid wrapper for per-UID
buffers
----- On May 22, 2019, at 11:48 AM, Gabriel-Andrew Pollo-Guilbert
gabriel.pollo-guilb...@efficios.com wrote:
> In case of a per-UID buffer, events following a setuid() call should be
> forwarded to buffers of the new UID. In order to do so, we add a wrapper
> around
> setuid() that unregister and re-register the application from the session
> daemon.
SETUID(2)
SEE ALSO
[...]
seteuid(2), setfsuid(2), setreuid(2)
and:
setresuid(2)
What should we do about those ? What's the policy we want ?
Considering that lttng-ust can be used on BSD systems, we may also want
to check the semantic there.
Thanks,
Mathieu
>
> Signed-off-by: Gabriel-Andrew Pollo-Guilbert
> <gabriel.pollo-guilb...@efficios.com>
> ---
> Makefile.am | 1 +
> configure.ac | 1 +
> doc/man/lttng-ust.3.txt | 11 ++++++++
> include/lttng/ust.h | 1 +
> liblttng-ust-setuid/Makefile.am | 10 +++++++
> liblttng-ust-setuid/ustsetuid.c | 47 +++++++++++++++++++++++++++++++++
> liblttng-ust/lttng-ust-comm.c | 29 ++++++++++++++++++--
> 7 files changed, 98 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..b252c11d 100644
> --- a/doc/man/lttng-ust.3.txt
> +++ b/doc/man/lttng-ust.3.txt
> @@ -744,6 +744,17 @@ 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''.
>
> +Using LTTng-UST with applications that change user
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +If your application is traced using per-UID buffers and changes user ID using
> +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, events generated after the UID change will be
> fowarded
> +to the correct buffer.
> +
> +This workaround requires that the tracing session be global (root
> +lttng-sessiond) in order to avoid permission problems between a session
> +daemon started by another user.
>
> 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..8cae9a43
> --- /dev/null
> +++ b/liblttng-ust-setuid/ustsetuid.c
> @@ -0,0 +1,47 @@
> +/*
> + * 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;
> +
> + if (plibc_func == NULL) {
> + plibc_func = dlsym(RTLD_NEXT, "setuid");
> + if (plibc_func == NULL) {
> + fprintf(stderr, "libustsetuid: unable to find
> \"setuid\" symbol\n");
> + errno = ENOSYS;
> + return -1;
> + }
> + }
> +
> + retval = plibc_func(uid);
> + if (retval < 0)
> + return retval;
> +
> + ust_after_setuid();
> +
> + return retval;
> +}
> diff --git a/liblttng-ust/lttng-ust-comm.c b/liblttng-ust/lttng-ust-comm.c
> index 61dbb41b..0d0b9eb4 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 };
> +#define 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();
> +}
> --
> 2.21.0
>
> _______________________________________________
> lttng-dev mailing list
> lttng-dev@lists.lttng.org
> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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