Move __cleanup into mprotect'ed page to prevent unintentional modifications

similar to the atexit handlers. Idea and help deraadt@, ok deraadt@

src/lib/libc/stdlib/abort.c

@@ -32,19 +32,19 @@
  */
 
 #if defined(LIBC_SCCS) && !defined(lint)
-static char *rcsid = "$OpenBSD: abort.c,v 1.7 2001/08/12 12:03:01 heko Exp $";
+static char *rcsid = "$OpenBSD: abort.c,v 1.8 2002/09/14 22:03:14 dhartmei Exp $";
 #endif /* LIBC_SCCS and not lint */
 
 #include <signal.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include "thread_private.h"
-
-void (*__cleanup)();
+#include "atexit.h"
 
 void
 abort()
 {
+	struct atexit *p = __atexit;
 	static int cleanup_called = 0;
 	sigset_t mask;
 
@@ -64,9 +64,13 @@ abort()
 	/*
 	 * POSIX requires we flush stdio buffers on abort
 	 */
-	if (cleanup_called == 0 && __cleanup != NULL) {
-		cleanup_called = 1;
-		(*__cleanup)();
+	if (cleanup_called == 0) {
+		while (p != NULL && p->next != NULL)
+			p = p->next;
+		if (p != NULL && p->fns[0] != NULL) {
+			cleanup_called = 1;
+			(*p->fns[0])();
+		}
 	}
 
 	(void)kill(getpid(), SIGABRT);

src/lib/libc/stdlib/atexit.c

@@ -29,7 +29,7 @@
  */
 
 #if defined(LIBC_SCCS) && !defined(lint)
-static char *rcsid = "$OpenBSD: atexit.c,v 1.6 2002/09/06 22:48:34 henning Exp $";
+static char *rcsid = "$OpenBSD: atexit.c,v 1.7 2002/09/14 22:03:14 dhartmei Exp $";
 #endif /* LIBC_SCCS and not lint */
 
 #include <sys/types.h>
@@ -41,6 +41,20 @@ static char *rcsid = "$OpenBSD: atexit.c,v 1.6 2002/09/06 22:48:34 henning Exp $
 int __atexit_invalid = 1;
 struct atexit *__atexit;
 
+/*
+ * Function pointers are stored in a linked list of pages. The list
+ * is initially empty, and pages are allocated on demand. The first
+ * function pointer in the first allocated page (the last one in
+ * the linked list) is reserved for the cleanup function.
+ *
+ * Outside the following two functions, all pages are mprotect()'ed
+ * to prevent unintentional/malicious corruption.
+ *
+ * The free(malloc(1)) is a workaround causing malloc_init() to
+ * ensure that malloc.c gets the first mmap() call for its sbrk()
+ * games.
+ */
+
 /*
  * Register a function to be performed at exit.
  */
@@ -61,9 +75,6 @@ atexit(fn)
 	}
 	if (p == NULL) {
 		if (__atexit_invalid) {
-			/* malloc.c wants the first mmap() for sbrk()
-			   games ('nice hack'), so enforce
-			   malloc_init() with a dummy call.  */
 			free(malloc(1));
 			__atexit_invalid = 0;
 		}
@@ -71,7 +82,11 @@ atexit(fn)
 		    MAP_ANON | MAP_PRIVATE, -1, 0);
 		if (p == MAP_FAILED)
 			return (-1);
-		p->ind = 0;
+		if (__atexit == NULL) {
+			p->fns[0] = NULL;
+			p->ind = 1;
+		} else
+			p->ind = 0;
 		p->max = (pgsize - ((char *)&p->fns[0] - (char *)p)) /
 		    sizeof(p->fns[0]);
 		p->next = __atexit;
@@ -82,3 +97,39 @@ atexit(fn)
 		return (-1);
 	return (0);
 }
+
+/*
+ * Register the cleanup function
+ */
+void
+__atexit_register_cleanup(fn)
+	void (*fn)();
+{
+	register struct atexit *p = __atexit;
+	register int pgsize = getpagesize();
+
+	if (pgsize < sizeof(*p))
+		return;
+	while (p != NULL && p->next != NULL)
+		p = p->next;
+	if (p == NULL) {
+		if (__atexit_invalid) {
+			free(malloc(1));
+			__atexit_invalid = 0;
+		}
+		p = mmap(NULL, pgsize, PROT_READ | PROT_WRITE,
+		    MAP_ANON | MAP_PRIVATE, -1, 0);
+		if (p == MAP_FAILED)
+			return;
+		p->ind = 1;
+		p->max = (pgsize - ((char *)&p->fns[0] - (char *)p)) /
+		    sizeof(p->fns[0]);
+		p->next = NULL;
+		__atexit = p;
+	} else {
+		if (mprotect(p, pgsize, PROT_READ | PROT_WRITE))
+		    return;
+	}
+	p->fns[0] = fn;
+	mprotect(p, pgsize, PROT_READ);
+}

src/lib/libc/stdlib/exit.c

@@ -32,7 +32,7 @@
  */
 
 #if defined(LIBC_SCCS) && !defined(lint)
-static char *rcsid = "$OpenBSD: exit.c,v 1.7 2002/08/30 07:58:07 dhartmei Exp $";
+static char *rcsid = "$OpenBSD: exit.c,v 1.8 2002/09/14 22:03:14 dhartmei Exp $";
 #endif /* LIBC_SCCS and not lint */
 
 #include <sys/types.h>
@@ -42,8 +42,6 @@ static char *rcsid = "$OpenBSD: exit.c,v 1.7 2002/08/30 07:58:07 dhartmei Exp $"
 #include "atexit.h"
 #include "thread_private.h"
 
-void (*__cleanup)();
-
 /*
  * This variable is zero until a process has created a thread.
  * It is used to avoid calling locking functions in libc when they
@@ -67,13 +65,13 @@ exit(status)
 		p = __atexit;
 		while (p != NULL) {
 			for (n = p->ind; --n >= 0;)
-				(*p->fns[n])();
+				if (p->fns[n] != NULL)
+					(*p->fns[n])();
 			q = p;
 			p = p->next;
 			munmap(q, pgsize);
 		}
 	}
-	if (__cleanup)
-		(*__cleanup)();
+	/* cleanup, if registered, was called through fns[0] in the last page */
 	_exit(status);
 }