[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.

Changelog:
	v2: At startup, detect whether libcap knows about CAP_SYSLOG.
	    (Thanks to Gergely Nagy for pointing out that case)
	v3: Andrew Morgan pointed out a nice way to detect whether
	    the kernel has CAP_SYSLOG.  Thanks, Andrew!

Signed-off-by: Serge Hallyn <serge.hallyn at ubuntu.com>
Signed-off-by: Gergely Nagy <algernon at balabit.hu>
---
 lib/gprocess.c          |   48 ++++++++++++++++++++++++++++++++++++++++++++++-
 lib/gprocess.h          |    2 +-
 modules/affile/affile.c |    4 +++
 syslog-ng/main.c        |   10 ++------
 4 files changed, 55 insertions(+), 9 deletions(-)

diff --git a/lib/gprocess.c b/lib/gprocess.c
index d9a2dfb..4a4ce52 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
@@ -210,6 +211,14 @@ g_process_cap_modify(int capability, int onoff)
   if (!process_opts.caps)
     return TRUE;
 
+#ifdef CAP_SYSLOG
+  /*
+   * if libcap or kernel doesn't support cap_syslog, then resort to
+   * cap_sys_admin
+   */
+  if (capability == CAP_SYSLOG && !have_capsyslog)
+    capability = CAP_SYS_ADMIN;
+#endif
   caps = cap_get_proc();
   if (!caps)
     return FALSE;
@@ -416,7 +425,7 @@ g_process_set_working_dir(const gchar *cwd)
  * capability set. The process will change its capabilities to this value
  * during startup, provided it has enough permissions to do so.
  **/
-void 
+static void
 g_process_set_caps(const gchar *caps)
 {
   if (!process_opts.caps)
@@ -879,6 +888,43 @@ g_process_change_dir(void)
 }
 
 /**
+ * g_process_setup_caps(void)
+ *
+ * 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.
+ */
+void
+g_process_setup_caps(void)
+{
+  int ret;
+  gchar * capsstr = "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";
+#ifdef CAP_SYSLOG
+  cap_t caps;
+
+  /* Check whether cap_syslog exists.  If not, get cap_sys_admin, else get
+   * cap_syslog */
+  caps = cap_from_text("cap_syslog=p");
+  if (caps) {
+    cap_free(caps);
+    /* libcap knows it, does the kernel? */
+    ret = prctl(PR_CAPBSET_READ, CAP_SYSLOG);
+    if (ret != -1) {
+	    /* kernel knows cap_syslog!  Use it */
+	    capsstr = "cap_net_bind_service,cap_net_broadcast,cap_net_raw,"
+		      "cap_dac_read_search,cap_dac_override,cap_chown,cap_fowner=p "
+		      "cap_syslog=ep";
+	    have_capsyslog = TRUE;
+    }
+  }
+#endif
+
+  g_process_set_caps(capsstr);
+}
+
+/**
  * g_process_send_result:
  * @ret_num: exit code of the process
  *
diff --git a/lib/gprocess.h b/lib/gprocess.h
index cda35b0..4ad0866 100644
--- a/lib/gprocess.h
+++ b/lib/gprocess.h
@@ -66,7 +66,7 @@ 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);
-void g_process_set_caps(const gchar *caps);
+void g_process_setup_caps(void);
 void g_process_set_argv_space(gint argc, gchar **argv);
 void g_process_set_use_fdlimit(gboolean use);
 void g_process_set_check(gint check_period, gboolean (*check_fn)(void));
diff --git a/modules/affile/affile.c b/modules/affile/affile.c
index 67a4632..5c56bc5 100644
--- a/modules/affile/affile.c
+++ b/modules/affile/affile.c
@@ -59,7 +59,11 @@ affile_open_file(gchar *name, gint flags,
   if (privileged)
     {
       g_process_cap_modify(CAP_DAC_READ_SEARCH, TRUE);
+#ifdef CAP_SYSLOG
+      g_process_cap_modify(CAP_SYSLOG, TRUE);
+#else
       g_process_cap_modify(CAP_SYS_ADMIN, TRUE);
+#endif
     }
   else
     {
diff --git a/syslog-ng/main.c b/syslog-ng/main.c
index 7e81f2f..5e62c1f 100644
--- a/syslog-ng/main.c
+++ b/syslog-ng/main.c
@@ -167,13 +167,9 @@ main(int argc, char *argv[])
 
   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");
+  /* Set up the minimal privilege we'll need */
+  g_process_setup_caps();
+
   ctx = g_option_context_new("syslog-ng");
   g_process_add_option_group(ctx);
   msg_add_option_group(ctx);
-- 
1.7.2.5