Allows run with restricted privileges.

Adds option -u and -g to specify user and group.
When different user (than root) is specified,
linux capabilities CAP_NET_* are kept.

2 files changed, 63 insertions, 0 deletions

diff --git a/sysdep/linux/Modules b/sysdep/linux/Modules
index 1b867d8..09f4a47 100644
--- a/sysdep/linux/Modules
+++ b/sysdep/linux/Modules
@@ -3,3 +3,4 @@ krt-scan.c
 krt-scan.h
 #endif
 sysio.h
+syspriv.h
diff --git a/sysdep/linux/syspriv.h b/sysdep/linux/syspriv.h
new file mode 100644
index 0000000..bfe19ac
--- /dev/null
+++ b/sysdep/linux/syspriv.h
@@ -0,0 +1,62 @@
+
+#include <sys/prctl.h>
+#include <linux/capability.h>
+
+#ifndef _LINUX_CAPABILITY_VERSION_3
+#define _LINUX_CAPABILITY_VERSION_3  0x20080522
+#define _LINUX_CAPABILITY_U32S_3     2
+#endif
+
+/* capset() prototype is missing ... */
+int capset(cap_user_header_t hdrp, const cap_user_data_t datap);
+
+static inline int
+set_capabilities(u32 caps)
+{
+  struct __user_cap_header_struct cap_hdr;
+  struct __user_cap_data_struct cap_dat[_LINUX_CAPABILITY_U32S_3];
+  int err;
+
+  cap_hdr.version = _LINUX_CAPABILITY_VERSION_3;
+  cap_hdr.pid = 0;
+
+  memset(cap_dat, 0, sizeof(cap_dat));
+  cap_dat[0].effective = cap_dat[0].permitted = caps;
+
+  err = capset(&cap_hdr, cap_dat);
+  if (!err)
+    return 0;
+
+  /* Kernel may support do not support our version of capability interface.
+       The last call returned supported version so we just retry it. */
+  if (errno == EINVAL)
+  {
+    err = capset(&cap_hdr, cap_dat);
+    if (!err)
+      return 0;
+  }
+
+  return -1;
+}
+
+static void
+drop_uid(uid_t uid)
+{
+  u32 caps =
+    CAP_TO_MASK(CAP_NET_BIND_SERVICE) |
+    CAP_TO_MASK(CAP_NET_BROADCAST) |
+    CAP_TO_MASK(CAP_NET_ADMIN) |
+    CAP_TO_MASK(CAP_NET_RAW);
+
+  if (seteuid(uid) < 0)
+    die("seteuid: %m");
+
+  if (set_capabilities(caps) < 0)
+    die("capset: %m");
+
+  if (prctl(PR_SET_KEEPCAPS, 1) < 0)
+    die("prctl: %m");
+
+  if (setresuid(uid, uid, uid) < 0)
+    die("setresuid: %m");
+}