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!)

arch/i386/atomic.h

@@ -119,5 +119,10 @@ static inline void a_spin()
 	__asm__ __volatile__( "pause" : : : "memory" );
 }
 
+static inline void a_crash()
+{
+	__asm__ __volatile__( "hlt" : : : "memory" );
+}
+
 
 #endif

arch/x86_64/atomic.h

@@ -118,5 +118,10 @@ static inline void a_spin()
 	__asm__ __volatile__( "pause" : : : "memory" );
 }
 
+static inline void a_crash()
+{
+	__asm__ __volatile__( "hlt" : : : "memory" );
+}
+
 
 #endif

src/env/__environ.c

 #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);

src/env/__init_security.c

+#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;
+}

src/env/__libc_start_main.c

 #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);

src/internal/libc.h

@@ -7,13 +7,15 @@
 struct __libc {
 	void *main_thread;
 	int threaded;
-	int canceldisable;
+	int secure;
+	size_t *auxv;
 	int (*atexit)(void (*)(void));
 	void (*fini)(void);
 	void (*ldso_fini)(void);
 	volatile int threads_minus_1;
-	int ofl_lock;
+	int canceldisable;
 	FILE *ofl_head;
+	int ofl_lock;
 };