[syslog-ng] [PATCH] Use CAP_SYSLOG instead of CAP_SYS_ADMIN, if available.

[Gergely Nagy]

If cap_syslog exists, the kernel will complain (once) that we only
have cap_sys_admin.  Additionally, using cap_syslog instead of
cap_sys_admin significantly lowers the unneeded privs we are
using.

Upon startup, syslog-ng will detect whether CAP_SYSLOG is available,
and use capabilities based on that finding. This detection will also
have a side-effect, which will make it so that
g_process_cap_modify(CAP_SYSLOG) will fall back to CAP_SYS_ADMIN, if
CAP_SYSLOG support was not detected.

Thanks to Andrew Morgan for pointing out a nice way to detect whether
the kernel has CAP_SYSLOG. Original code by Serge Hallyn, with minor
changes based on Balazs Scheidler's review by Gergely Nagy.

Signed-off-by: Serge Hallyn <serge.hallyn at ubuntu.com>
Signed-off-by: Gergely Nagy <algernon at balabit.hu>
---
 lib/gprocess.c          |   24 ++++++++++++++++++++++++
 lib/gprocess.h          |    5 +++++
 modules/affile/affile.c |    2 +-
 syslog-ng/main.c        |   31 +++++++++++++++++++++++--------
 4 files changed, 53 insertions(+), 9 deletions(-)

diff --git a/lib/gprocess.c b/lib/gprocess.c
index 38bcb12..8876d51 100644
--- a/lib/gprocess.c
+++ b/lib/gprocess.c
@@ -98,6 +98,7 @@ static gint startup_result_pipe[2] = { -1, -1 };
 static gint init_result_pipe[2] = { -1, -1 };
 static GProcessKind process_kind = G_PK_STARTUP;
 static gboolean stderr_present = TRUE;
+static int have_capsyslog = FALSE;
 
 /* global variables */
 static struct
@@ -216,6 +217,13 @@ g_process_cap_modify(int capability, int onoff)
   if (!process_opts.caps)
     return TRUE;
 
+  /*
+   * if libcap or kernel doesn't support cap_syslog, then resort to
+   * cap_sys_admin
+   */
+  if (capability == CAP_SYSLOG && (!have_capsyslog || CAP_SYSLOG == -1))
+    capability = CAP_SYS_ADMIN;
+
   caps = cap_get_proc();
   if (!caps)
     return FALSE;
@@ -885,6 +893,22 @@ g_process_change_dir(void)
   
 }
 
+gboolean
+g_process_check_cap_syslog(void)
+{
+  int ret;
+
+  if (CAP_SYSLOG == -1)
+    return FALSE;
+
+  ret = prctl(PR_CAPBSET_READ, CAP_SYSLOG);
+  if (ret == -1)
+    return FALSE;
+
+  have_capsyslog = TRUE;
+  return TRUE;
+}
+
 /**
  * g_process_send_result:
  * @ret_num: exit code of the process
diff --git a/lib/gprocess.h b/lib/gprocess.h
index a6dd7c4..19e1d34 100644
--- a/lib/gprocess.h
+++ b/lib/gprocess.h
@@ -46,6 +46,10 @@ gboolean g_process_cap_modify(int capability, int onoff);
 cap_t g_process_cap_save(void);
 void g_process_cap_restore(cap_t r);
 
+#ifndef CAP_SYSLOG
+#define CAP_SYSLOG -1
+#endif
+
 #else
 
 typedef gpointer cap_t;
@@ -66,6 +70,7 @@ void g_process_set_chroot(const gchar *chroot);
 void g_process_set_pidfile(const gchar *pidfile);
 void g_process_set_pidfile_dir(const gchar *pidfile_dir);
 void g_process_set_working_dir(const gchar *cwd);
+gboolean g_process_check_cap_syslog(void);
 void g_process_set_caps(const gchar *caps);
 void g_process_set_argv_space(gint argc, gchar **argv);
 void g_process_set_use_fdlimit(gboolean use);
diff --git a/modules/affile/affile.c b/modules/affile/affile.c
index 67a4632..6e4f5f0 100644
--- a/modules/affile/affile.c
+++ b/modules/affile/affile.c
@@ -59,7 +59,7 @@ affile_open_file(gchar *name, gint flags,
   if (privileged)
     {
       g_process_cap_modify(CAP_DAC_READ_SEARCH, TRUE);
-      g_process_cap_modify(CAP_SYS_ADMIN, TRUE);
+      g_process_cap_modify(CAP_SYSLOG, TRUE);
     }
   else
     {
diff --git a/syslog-ng/main.c b/syslog-ng/main.c
index 7e81f2f..fa06492 100644
--- a/syslog-ng/main.c
+++ b/syslog-ng/main.c
@@ -155,6 +155,26 @@ version(void)
 
 }
 
+#define BASE_CAPS "cap_net_bind_service,cap_net_broadcast,cap_net_raw," \
+  "cap_dac_read_search,cap_dac_override,cap_chown,cap_fowner=p "
+
+static void
+setup_caps (gboolean with_cap_syslog)
+{
+  static gchar *capsstr_syslog = BASE_CAPS "cap_syslog=ep";
+  static gchar *capsstr_sys_admin = BASE_CAPS "cap_sys_admin=ep";
+
+  /* Set up the minimal privilege we'll need
+   *
+   * NOTE: polling /proc/kmsg requires cap_sys_admin, otherwise it'll always
+   * indicate readability. Enabling/disabling cap_sys_admin on every poll
+   * invocation seems to be too expensive. So I enable it for now.
+   */
+  if (with_cap_syslog)
+    g_process_set_caps(capsstr_syslog);
+  else
+    g_process_set_caps(capsstr_sys_admin);
+}
 
 int 
 main(int argc, char *argv[])
@@ -166,14 +186,9 @@ main(int argc, char *argv[])
   z_mem_trace_init("syslog-ng.trace");
 
   g_process_set_argv_space(argc, (gchar **) argv);
-  
-  /* NOTE: polling /proc/kmsg requires cap_sys_admin, otherwise it'll always
-   * indicate readability. Enabling/disabling cap_sys_admin on every poll
-   * invocation seems to be too expensive. So I enable it for now. */
-  
-  g_process_set_caps("cap_net_bind_service,cap_net_broadcast,cap_net_raw,"
-                     "cap_dac_read_search,cap_dac_override,cap_chown,cap_fowner=p "
-                     "cap_sys_admin=ep");
+
+  setup_caps(g_process_check_cap_syslog ());
+
   ctx = g_option_context_new("syslog-ng");
   g_process_add_option_group(ctx);
   msg_add_option_group(ctx);
-- 
1.7.2.5