security hardening: ensure suid programs have valid stdin/out/err

this behavior (opening fds 0-2 for a suid program) is explicitly
allowed (but not required) by POSIX to protect badly-written suid
programs from clobbering files they later open.

this commit does add some cost in startup code, but the availability
of auxv and the security flag will be useful elsewhere in the future.
in particular auxv is needed for static-linked vdso support, which is
still waiting to be committed (sorry nik!)

3 files changed, 38 insertions, 13 deletions

diff --git a/src/env/__environ.c b/src/env/__environ.c
index d7bd5e50..0a2786fd 100644
--- a/src/env/__environ.c
+++ b/src/env/__environ.c
@@ -1,7 +1,6 @@
 #include "libc.h"
 
 #undef environ
-char **___environ = 0;
-weak_alias(___environ, __environ);
-weak_alias(___environ, _environ);
-weak_alias(___environ, environ);
+char **__environ = 0;
+weak_alias(__environ, _environ);
+weak_alias(__environ, environ);
diff --git a/src/env/__init_security.c b/src/env/__init_security.c
new file mode 100644
index 00000000..5fd12ecb
--- /dev/null
+++ b/src/env/__init_security.c
@@ -0,0 +1,26 @@
+#include <stddef.h>
+#include <elf.h>
+#include <poll.h>
+#include <fcntl.h>
+#include "syscall.h"
+#include "libc.h"
+#include "atomic.h"
+
+#define AUX_CNT 24
+
+void __init_security(size_t *auxv)
+{
+	size_t i, aux[AUX_CNT] = { 0 };
+	struct pollfd pfd[3] = { {.fd=0}, {.fd=1}, {.fd=2} };
+
+	for (; auxv[0]; auxv+=2) if (auxv[0]<AUX_CNT) aux[auxv[0]] = auxv[1];
+	if (aux[AT_UID]==aux[AT_EUID] && aux[AT_GID]==aux[AT_EGID]
+		&& !aux[AT_SECURE]) return;
+
+	__syscall(SYS_poll, pfd, 3, 0);
+	for (i=0; i<3; i++)
+		if (pfd[i].revents&POLLNVAL)
+			if (__syscall(SYS_open, "/dev/null", O_RDWR)<0)
+				a_crash();
+	libc.secure = 1;
+}
diff --git a/src/env/__libc_start_main.c b/src/env/__libc_start_main.c
index 70af77b5..f31222b2 100644
--- a/src/env/__libc_start_main.c
+++ b/src/env/__libc_start_main.c
@@ -1,21 +1,21 @@
 #include "libc.h"
 
-/* Any use of __environ/environ will override this symbol. */
-char **__dummy_environ = (void *)-1;
-weak_alias(__dummy_environ, ___environ);
+void __init_security(size_t *);
 
 int __libc_start_main(
 	int (*main)(int, char **, char **), int argc, char **argv,
 	int (*init)(int, char **, char **), void (*fini)(void),
 	void (*ldso_fini)(void))
 {
-	/* Save the environment if it may be used by libc/application */
-	char **envp = argv+argc+1;
-	if (___environ != (void *)-1) ___environ = envp;
+	char **envp = argv+argc+1, **auxv = envp;
 
-	/* Avoid writing 0 and triggering unnecessary COW */
-	if (ldso_fini) libc.ldso_fini = ldso_fini;
-	if (fini) libc.fini = fini;
+	__environ = envp;
+	do auxv++; while (*auxv);
+	libc.auxv = (void *)++auxv;
+	libc.ldso_fini = ldso_fini;
+	libc.fini = fini;
+
+	__init_security((void *)auxv);
 
 	/* Execute constructors (static) linked into the application */
 	if (init) init(argc, argv, envp);