Move __cleanup into mprotect'ed page to prevent unintentional modifications

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

1 files changed, 56 insertions, 5 deletions

diff --git a/src/lib/libc/stdlib/atexit.c b/src/lib/libc/stdlib/atexit.c
index da5a0ddda0..98564d0dd3 100644
--- a/src/lib/libc/stdlib/atexit.c
+++ b/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>
@@ -42,6 +42,20 @@ 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.
  */
 int
@@ -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);
+}