Introducing freezero(3) a version of free that guarantees the process

no longer has access to the content of a memmory object. It does
this by either clearing (if the object memory remains cached) or
by calling munmap(2). ok millert@, deraadt@, guenther@

2 files changed, 130 insertions, 36 deletions

diff --git a/src/lib/libc/stdlib/malloc.3 b/src/lib/libc/stdlib/malloc.3
index c65c08ef98..c7a79b5e3d 100644
--- a/src/lib/libc/stdlib/malloc.3
+++ b/src/lib/libc/stdlib/malloc.3
@@ -30,18 +30,19 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"     $OpenBSD: malloc.3,v 1.109 2017/04/06 17:00:52 otto Exp $
+.\"     $OpenBSD: malloc.3,v 1.110 2017/04/10 05:45:02 otto Exp $
 .\"
-.Dd $Mdocdate: April 6 2017 $
+.Dd $Mdocdate: April 10 2017 $
 .Dt MALLOC 3
 .Os
 .Sh NAME
 .Nm malloc ,
 .Nm calloc ,
-.Nm reallocarray ,
-.Nm recallocarray ,
 .Nm realloc ,
 .Nm free
+.Nm reallocarray ,
+.Nm recallocarray ,
+.Nm freezero ,
 .Nd memory allocation and deallocation
 .Sh SYNOPSIS
 .In stdlib.h
@@ -50,15 +51,23 @@
 .Ft void *
 .Fn calloc "size_t nmemb" "size_t size"
 .Ft void *
+.Fn realloc "void *ptr" "size_t size"
+.Ft void
+.Fn free "void *ptr"
+.Ft void *
 .Fn reallocarray "void *ptr" "size_t nmemb" "size_t size"
 .Ft void *
 .Fn recallocarray "void *ptr" "size_t oldnmemb" "size_t nmemb" "size_t size"
-.Ft void *
-.Fn realloc "void *ptr" "size_t size"
 .Ft void
-.Fn free "void *ptr"
+.Fn freezero "void *ptr" "size_t size"
 .Vt char *malloc_options ;
 .Sh DESCRIPTION
+The standard functions
+.Fn malloc ,
+.Fn calloc ,
+and
+.Fn realloc
+allocate memory space.
 The
 .Fn malloc
 function allocates uninitialized space for an object of
@@ -103,6 +112,26 @@ behaves like
 and allocates a new object.
 .Pp
 The
+.Fn free
+function causes the space pointed to by
+.Fa ptr
+to be either placed on a list of free blocks to make it available for future
+allocation or, when appropiate, to be returned to the kernel using
+.Xr munmap 2 .
+If
+.Fa ptr
+is a
+.Dv NULL
+pointer, no action occurs.
+If
+.Fa ptr
+was previously freed by
+.Fn free
+or a reallocation function,
+the behavior is undefined and the double free is a security concern.
+.Pp
+Designed for safe allocation of arrays,
+the
 .Fn reallocarray
 function is similar to
 .Fn realloc
@@ -115,7 +144,8 @@ and checks for integer overflow in the calculation
 *
 .Fa size .
 .Pp
-The
+Used for the allocation of memory holding sensitive data,
+the
 .Fn recallocarray
 function is similar to
 .Fn reallocarray
@@ -150,23 +180,25 @@ is the size of the earlier allocation that returned
 otherwise the behaviour is undefined.
 .Pp
 The
+.Fn freezero
+function is similar to the
 .Fn free
-function causes the space pointed to by
-.Fa ptr
-to be either placed on a list of free pages to make it available for future
-allocation or, if required, to be returned to the kernel using
-.Xr munmap 2 .
+function except it ensures the memory being deallocated is explicitly
+discarded.
 If
 .Fa ptr
-is a
-.Dv NULL
-pointer, no action occurs.
+is
+.Dv NULL ,
+no action occurs.
 If
 .Fa ptr
-was previously freed by
-.Fn free
-or a reallocation function,
-the behavior is undefined and the double free is a security concern.
+is not
+.Dv NULL ,
+the
+.Fa size
+argument must be the size of the earlier allocation that returned
+.Fa ptr ,
+otherwise the behaviour is undefined.
 .Sh RETURN VALUES
 Upon successful completion, the allocation functions
 return a pointer to the allocated space; otherwise, a
@@ -319,10 +351,8 @@ function should be used for resizing objects containing sensitive data like
 keys.
 To avoid leaking information,
 it guarantees memory is cleared before placing it on the internal free list.
-A
-.Fn free
-call for such an object should still be preceded by a call to
-.Xr explicit_bzero 3 .
+Deallocation of such an object should be done by calling
+.Fn freezero .
 .Sh ENVIRONMENT
 .Bl -tag -width "/etc/malloc.conf"
 .It Ev MALLOC_OPTIONS
@@ -539,6 +569,10 @@ The
 .Fn recallocarray
 function appeared in
 .Ox 6.1 .
+The
+.Fn freezero
+function appeared in
+.Ox 6.2 .
 .Sh CAVEATS
 When using
 .Fn malloc ,
diff --git a/src/lib/libc/stdlib/malloc.c b/src/lib/libc/stdlib/malloc.c
index f2b8b1549b..07c73ca774 100644
--- a/src/lib/libc/stdlib/malloc.c
+++ b/src/lib/libc/stdlib/malloc.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: malloc.c,v 1.219 2017/04/06 08:39:47 otto Exp $       */
+/*      $OpenBSD: malloc.c,v 1.220 2017/04/10 05:45:02 otto Exp $       */
 /*
  * Copyright (c) 2008, 2010, 2011, 2016 Otto Moerbeek <otto@drijf.net>
  * Copyright (c) 2012 Matthew Dempsky <matthew@openbsd.org>
@@ -185,7 +185,7 @@ struct malloc_readonly {
         int     malloc_realloc;         /* always realloc? */
         int     malloc_xmalloc;         /* xmalloc behaviour? */
         int     chunk_canaries;         /* use canaries after chunks? */
-        int     internal_recallocarray; /* use better recallocarray? */
+        int     internal_funcs;         /* use better recallocarray/freezero? */
         u_int   malloc_cache;           /* free pages we cache */
         size_t  malloc_guard;           /* use guard pages after allocations? */
 #ifdef MALLOC_STATS
@@ -343,7 +343,14 @@ unmap(struct dir_info *d, void *p, size_t sz, int clear)
         if (sz != PAGEROUND(sz))
                 wrterror(d, "munmap round");
 
-        if (psz > mopts.malloc_cache) {
+        rsz = mopts.malloc_cache - d->free_regions_size;
+
+        /*
+         * normally the cache holds recently freed regions, but if the region
+         * to unmap is larger than the cache size or we're clearing and the
+         * cache is full, just munmap
+         */
+        if (psz > mopts.malloc_cache || (clear && rsz == 0)) {
                 i = munmap(p, sz);
                 if (i)
                         wrterror(d, "munmap %p", p);
@@ -351,7 +358,6 @@ unmap(struct dir_info *d, void *p, size_t sz, int clear)
                 return;
         }
         tounmap = 0;
-        rsz = mopts.malloc_cache - d->free_regions_size;
         if (psz > rsz)
                 tounmap = psz - rsz;
         offset = getrbyte(d);
@@ -1234,7 +1240,7 @@ _malloc_init(int from_rthreads)
         if (from_rthreads)
                 mopts.malloc_mt = 1;
         else
-                mopts.internal_recallocarray = 1;
+                mopts.internal_funcs = 1;
 
         /*
          * Options have been set and will never be reset.
@@ -1297,7 +1303,7 @@ validate_junk(struct dir_info *pool, void *p)
 }
 
 static void
-ofree(struct dir_info *argpool, void *p, int clear)
+ofree(struct dir_info *argpool, void *p, int clear, int check, size_t argsz)
 {
         struct dir_info *pool;
         struct region_info *r;
@@ -1326,6 +1332,25 @@ ofree(struct dir_info *argpool, void *p, int clear)
         }
 
         REALSIZE(sz, r);
+        if (check) {
+                if (sz <= MALLOC_MAXCHUNK) {
+                        if (mopts.chunk_canaries) {
+                                struct chunk_info *info =
+                                     (struct chunk_info *)r->size;
+                                uint32_t chunknum =
+                                     find_chunknum(pool, r, p, 0);
+
+                                if (info->bits[info->offset + chunknum] !=
+                                    argsz)
+                                        wrterror(pool, "recorded old size %hu"
+                                            " != %zu",
+                                            info->bits[info->offset + chunknum],
+                                            argsz);
+                        }       
+                } else if (argsz != sz - mopts.malloc_guard)
+                        wrterror(pool, "recorded old size %zu != %zu",
+                            sz - mopts.malloc_guard, argsz);
+        }
         if (sz > MALLOC_MAXCHUNK) {
                 if (!MALLOC_MOVE_COND(sz)) {
                         if (r->p != p)
@@ -1411,13 +1436,48 @@ free(void *ptr)
                 malloc_recurse(d);
                 return;
         }
-        ofree(d, ptr, 0);
+        ofree(d, ptr, 0, 0, 0);
         d->active--;
         _MALLOC_UNLOCK(d->mutex);
         errno = saved_errno;
 }
 /*DEF_STRONG(free);*/
 
+static void
+freezero_p(void *ptr, size_t sz)
+{
+        explicit_bzero(ptr, sz);
+        free(ptr);
+}
+
+void
+freezero(void *ptr, size_t sz)
+{
+        struct dir_info *d;
+        int saved_errno = errno;
+
+        /* This is legal. */
+        if (ptr == NULL)
+                return;
+
+        if (!mopts.internal_funcs)
+                return freezero_p(ptr, sz);
+
+        d = getpool();
+        if (d == NULL)
+                wrterror(d, "freezero() called before allocation");
+        _MALLOC_LOCK(d->mutex);
+        d->func = "freezero";
+        if (d->active++) {
+                malloc_recurse(d);
+                return;
+        }
+        ofree(d, ptr, 1, 1, sz);
+        d->active--;
+        _MALLOC_UNLOCK(d->mutex);
+        errno = saved_errno;
+}
+DEF_WEAK(freezero);
 
 static void *
 orealloc(struct dir_info *argpool, void *p, size_t newsz, void *f)
@@ -1591,7 +1651,7 @@ gotit:
                 }
                 if (newsz != 0 && oldsz != 0)
                         memcpy(q, p, oldsz < newsz ? oldsz : newsz);
-                ofree(pool, p, 0);
+                ofree(pool, p, 0, 0, 0);
                 ret = q;
         } else {
                 /* oldsz == newsz */
@@ -1751,7 +1811,7 @@ orecallocarray(struct dir_info *argpool, void *p, size_t oldsize,
         } else
                 memcpy(newptr, p, newsize);
 
-        ofree(pool, p, 1);
+        ofree(pool, p, 1, 0, 0);
 
 done:
         if (argpool != pool) {
@@ -1824,7 +1884,7 @@ recallocarray(void *ptr, size_t oldnmemb, size_t newnmemb, size_t size)
         void *r;
         int saved_errno = errno;
 
-        if (!mopts.internal_recallocarray)
+        if (!mopts.internal_funcs)
                 return recallocarray_p(ptr, oldnmemb, newnmemb, size);
 
         d = getpool();
@@ -2275,8 +2335,8 @@ malloc_exit(void)
                      __progname);
                 write(fd, buf, strlen(buf));
                 snprintf(buf, sizeof(buf),
-                    "MT=%d IRC=%d F=%d U=%d J=%d R=%d X=%d C=%d cache=%u G=%zu\n",
-                    mopts.malloc_mt, mopts.internal_recallocarray,
+                    "MT=%d I=%d F=%d U=%d J=%d R=%d X=%d C=%d cache=%u G=%zu\n",
+                    mopts.malloc_mt, mopts.internal_funcs,
                     mopts.malloc_freenow,
                     mopts.malloc_freeunmap, mopts.malloc_junk,
                     mopts.malloc_realloc, mopts.malloc_xmalloc,