Re: [PATCH] malloc: Implement heap protector

[Florian Weimer <fweimer at redhat dot com>]

On 10/28/2016 07:55 PM, Paul Eggert wrote:
> Thanks for working on this. The idea looks good. I second Carlos's
> comments on comment and NEWS. Some other (minor) issues:
>
> > +/* The heap cookie.  The lowest three bits (corresponding to
> > +   SIZE_BITS) in __malloc_guard_header must be clear. Initialized
>
> It's __malloc_header_guard, not __malloc_guard_header.

Fixed.

> > +   during libc startup, and computed by elf/dl-keysetup.c.  */
> > +INTERNAL_SIZE_T __malloc_header_guard; /* For size.  */
> > +INTERNAL_SIZE_T __malloc_footer_guard; /* For prev_size.  */
>
> Why two cookies? Doesn't one suffice, and mightn't that be a tad more
> efficient? The comment should explain.

I think we need two cookies because on is more likely to leak through 
uninitialized heap chunks because it protects data which overlaps with 
application data once the chunk is in use.

The question is whether the prev_size field needs protecting at all. 
It's easy just to mark the previous chunk as allocated in an injected 
chunk header, so that the attacker does not have to spoof it.  It is 
probably more beneficial to obfuscate the fastbin next pointer, and the 
forward/backward pointers of binned chunks.

I hope to receive feedback on this point later this week.

> > +/* Decrypt a heap header chunk.  */
>
> These macros both encrypt and decrypt, right?  At least, they're used
> that way.  The comment should say so.

> Come to think of it, suppose we want to microimprove the implementation
> to use val+guard to encrypt and val-guard to decrypt; this might be bit
> faster on some platforms, and would be safe in unsigned arithmetic.
> Would we then need two sets of macros, one for encryption and one for
> decryption?  Then perhaps we should have two sets of macros now, even
> though the implementations are the same now, to make invoking code
> clearer and more-portable to a +/- approach.

I'm not aware of any architectures where add/sub is faster than xor.  I 
assumed the uniform instruction encoding between encryption and 
decryption would be beneficial for other reasons.

Anyway, I want to experiment with byte-swapping as well.  (Big endian is 
much more difficult to exploit with single-byte overflows than little 
endian.)  I have introduced the additional macros.

> > +#define HEAP_CRYPT_SIZE(val) (__malloc_header_guard ^
> ((INTERNAL_SIZE_T) val))
> > +#define HEAP_CRYPT_PREVSIZE(val) \
> > +  (__malloc_footer_guard ^ ((INTERNAL_SIZE_T) val))
>
> 'val' needs to be parenthesized in these two macro definientia.

Right.

> > +#define set_foot(p, s) \
> > +  (((mchunkptr) ((char *) (p) + (s)))->mchunk_prev_size \
> > +    = HEAP_CRYPT_PREVSIZE (s))
>
> Last line is indented one space too much.

I don't have firm opinion on this one.  In general, operators on 
continued lines are indented by two spaces.

> > +      else if (usable > size + 4096)
>
> Rephrase as 'usable - size > 4096' so that we don't have to worry about
> SIZE being ridiculously large.  The subtraction cannot possibly overflow
> here.

Okay.

> > +     are treated as fake mmapped chunks.  We cannot use the regular
> > +     accessors because the chunks we read are not yet encrypted.  */
>
> Avoid the royal "we", and use imperative voice instead: "We cannot use"
> -> "Don't use", "the chunks we read" -> "the chunks read".

This writing style is used in comments throughout glibc.  I see nothing 
wrong with it.

I'm attaching an updated version.  As I said above, it's still not final 
because we'll likely want to apply the secondary/prev_size chunk 
hardening to other malloc_chunk members instead.

Thanks,
Florian

malloc: Implement heap protector

2016-11-08  Florian Weimer  <fweimer@redhat.com>

	* malloc/mallc-internal.h (__malloc_header_guard)
	(__malloc_footer_guard): Declare.
	* malloc/malloc-guard.c: New file.
	* malloc/Makefile (routines): Add it.
	* malloc/malloc.c: Update malloc_chunk comment.
	(HEAP_MANGLE_SIZE, HEAP_DEMANGLE_SIZE)
	(HEAP_MANGLE_PREVSIZE, HEAP_DEMANGLE_PREVSIZE): Define.
	(chunksize_nomask, prev_size, set_prev_size, set_head_size)
	(set_head, set_foot): Add encryption.
	* malloc/arena.c (ptmalloc_init): For shared builds, initialize
	the heap guard variables.  Initialize the top chunk.
	* malloc/hooks.c (malloc_set_state): Apply the heap guard to the
	dumped heap.
	* malloc/tst-mallocstate.c (malloc_usable_size_valid): New
	variable.
	(check_allocation): Check malloc_usable_size result if
	malloc_usable_size_valid.
	(init_heap): Set malloc_usable_size_valid.
	* csu/libc-start.c (LIBC_START_MAIN): Initialize heap guard
	variables.
	* sysdeps/generic/ldsodefs.h (struct rtld_global_ro): Add members
	_dl_malloc_header_guard, _dl_malloc_footer_guard.
	* elf/rtld.c (security_init): Initialize temporary copy of the
	heap guard variables.

diff --git a/NEWS b/NEWS
index 65184b1..432000b 100644
--- a/NEWS
+++ b/NEWS
@@ -71,6 +71,10 @@ Version 2.25
   for the Linux quota interface which predates kernel version 2.4.22 has
   been removed.
 
+* The malloc implementation attempts to stop certain exploitation techniques
+  targeted at malloc heap metadata.  This is not intended as a security
+  feature, merely as a post-exploitation hardening measure.
+
 * The malloc_get_state and malloc_set_state functions have been removed.
   Already-existing binaries that dynamically link to these functions will
   get a hidden implementation in which malloc_get_state is a stub.  As far
diff --git a/csu/libc-start.c b/csu/libc-start.c
index 333a4cc..ae6abde 100644
--- a/csu/libc-start.c
+++ b/csu/libc-start.c
@@ -22,6 +22,7 @@
 #include <ldsodefs.h>
 #include <exit-thread.h>
 #include <elf/dl-keysetup.h>
+#include <malloc/malloc-internal.h>
 
 extern void __libc_init_first (int argc, char **argv, char **envp);
 
@@ -210,6 +211,11 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
   __pointer_chk_guard_local = keys.pointer;
 # endif
 
+  /* In the non-shared case, we initialize the heap guard
+     directly.  */
+  __malloc_header_guard = keys.heap_header;
+  __malloc_footer_guard = keys.heap_footer;
+
 #endif
 
   /* Register the destructor of the dynamic linker if there is any.  */
diff --git a/elf/rtld.c b/elf/rtld.c
index de965da..7ea06e4 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -42,6 +42,7 @@
 #include <stap-probe.h>
 #include <stackinfo.h>
 #include <dl-keysetup.h>
+#include <malloc/malloc-internal.h>
 
 #include <assert.h>
 
@@ -716,6 +717,11 @@ security_init (void)
 #endif
   __pointer_chk_guard_local = keys.pointer;
 
+  /* Keep a copy of the computed keys, so that they can be obtained
+     during malloc initialization in libc.so.  */
+  GLRO (dl_malloc_header_guard) = keys.heap_header;
+  GLRO (dl_malloc_footer_guard) = keys.heap_footer;
+
   /* We do not need the _dl_random value anymore.  The less
      information we leave behind, the better, so clear the
      variable.  */
diff --git a/malloc/Makefile b/malloc/Makefile
index b8efcd6..cd289f8 100644
--- a/malloc/Makefile
+++ b/malloc/Makefile
@@ -41,7 +41,7 @@ tests-static := \
 tests += $(tests-static)
 test-srcs = tst-mtrace
 
-routines = malloc morecore mcheck mtrace obstack \
+routines = malloc morecore mcheck mtrace obstack malloc-guard \
   scratch_buffer_grow scratch_buffer_grow_preserve \
   scratch_buffer_set_array_size
 
diff --git a/malloc/arena.c b/malloc/arena.c
index eed4247..ada79f6 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -340,6 +340,19 @@ ptmalloc_init (void)
       if (check_action != 0)
         __malloc_check_init ();
     }
+
+#ifdef SHARED
+  /* For a shared library, elf/rtld.c performed key setup in
+     security_init, and we copy the keys.  In static builds, the guard
+     cookies have already been initialized in csu/libc-start.c.  */
+  __malloc_header_guard = GLRO (dl_malloc_header_guard);
+  __malloc_footer_guard = GLRO (dl_malloc_footer_guard);
+#endif
+
+  /* Initialize the top chunk, based on the heap protector guards.  */
+  malloc_init_state (&main_arena);
+  set_head (main_arena.top, 0);
+
 #if HAVE_MALLOC_INIT_HOOK
   void (*hook) (void) = atomic_forced_read (__malloc_initialize_hook);
   if (hook != NULL)
diff --git a/malloc/hooks.c b/malloc/hooks.c
index 12995d3..17a2752 100644
--- a/malloc/hooks.c
+++ b/malloc/hooks.c
@@ -537,35 +537,38 @@ malloc_set_state (void *msptr)
      dumped_main_arena_end, realloc and free will recognize these
      chunks as dumped fake mmapped chunks and never free them.  */
 
-  /* Find the chunk with the lowest address with the heap.  */
-  mchunkptr chunk = NULL;
+  /* Find the chunk with the lowest address with the heap.  If
+     successful, size_header will point to the mchunk_size member (not
+     the chunk start, i.e. the mchunck_prev_size member).  */
+  size_t *size_header = NULL;
   {
     size_t *candidate = (size_t *) ms->sbrk_base;
     size_t *end = (size_t *) (ms->sbrk_base + ms->sbrked_mem_bytes);
     while (candidate < end)
       if (*candidate != 0)
 	{
-	  chunk = mem2chunk ((void *) (candidate + 1));
+	  size_header = candidate;
 	  break;
 	}
       else
 	++candidate;
   }
-  if (chunk == NULL)
+  if (size_header == NULL)
     return 0;
 
   /* Iterate over the dumped heap and patch the chunks so that they
-     are treated as fake mmapped chunks.  */
+     are treated as fake mmapped chunks.  We cannot use the regular
+     accessors because the chunks we read are not yet encrypted.  */
   mchunkptr top = ms->av[2];
-  while (chunk < top)
+  size_t *top_size_header = ((size_t *) top) + 1;
+  while (size_header < top_size_header)
     {
-      if (inuse (chunk))
-	{
-	  /* Mark chunk as mmapped, to trigger the fallback path.  */
-	  size_t size = chunksize (chunk);
-	  set_head (chunk, size | IS_MMAPPED);
-	}
-      chunk = next_chunk (chunk);
+      size_t size = *size_header & ~SIZE_BITS;
+      /* We treat all chunks as allocated.  The heap consistency
+	 checks do not trigger because they are not active for the
+	 dumped heap.  */
+      *size_header = HEAP_MANGLE_SIZE (size) | IS_MMAPPED;
+      size_header += size / sizeof (*size_header);
     }
 
   /* The dumped fake mmapped chunks all lie in this address range.  */
diff --git a/malloc/malloc-guard.c b/malloc/malloc-guard.c
new file mode 100644
index 0000000..8eb43c4
--- /dev/null
+++ b/malloc/malloc-guard.c
@@ -0,0 +1,31 @@
+/* Heap protector variables.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; see the file COPYING.LIB.  If
+   not, see <http://www.gnu.org/licenses/>.  */
+
+/* These variables are defined in a separate file because the static
+   startup code initializes them, but this should not pull the rest of
+   the libc malloc implementation into the link.  */
+
+#include <malloc-internal.h>
+
+/* The heap cookies.  The lowest three bits (corresponding to
+   SIZE_BITS) in __malloc_header_guard must be clear.  Computed by
+   elf/dl-keysetup.c, and initialized from rtld_global_ro in
+   ptmalloc_init.  See "malloc_chunk details" in malloc.c for
+   information on how these values are used.  */
+INTERNAL_SIZE_T __malloc_header_guard; /* For size.  */
+INTERNAL_SIZE_T __malloc_footer_guard; /* For prev_size.  */
diff --git a/malloc/malloc-internal.h b/malloc/malloc-internal.h
index a3df8c3..e723539 100644
--- a/malloc/malloc-internal.h
+++ b/malloc/malloc-internal.h
@@ -81,5 +81,8 @@ void __malloc_fork_unlock_parent (void) internal_function attribute_hidden;
 /* Called in the child process after a fork.  */
 void __malloc_fork_unlock_child (void) internal_function attribute_hidden;
 
+/* Random values for the heap protector.  */
+extern INTERNAL_SIZE_T __malloc_header_guard attribute_hidden;
+extern INTERNAL_SIZE_T __malloc_footer_guard attribute_hidden;
 
 #endif /* _MALLOC_INTERNAL_H */
diff --git a/malloc/malloc.c b/malloc/malloc.c
index 584edbf..18a69c4 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1070,9 +1070,9 @@ struct malloc_chunk {
 
 
     chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-	    |             Size of previous chunk, if unallocated (P clear)  |
+	    | (X1)        Size of previous chunk, if unallocated (P clear)  |
 	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-	    |             Size of chunk, in bytes                     |A|M|P|
+	    | (X0)        Size of chunk, in bytes                     |A|M|P|
       mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 	    |             User data starts here...                          .
 	    .                                                               .
@@ -1081,7 +1081,7 @@ struct malloc_chunk {
 nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 	    |             (size of chunk, but used for application data)    |
 	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-	    |             Size of next chunk, in bytes                |A|0|1|
+	    | (X0)        Size of next chunk, in bytes                |A|0|1|
 	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
     Where "chunk" is the front of the chunk for the purpose of most of
@@ -1095,9 +1095,9 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Free chunks are stored in circular doubly-linked lists, and look like this:
 
     chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-	    |             Size of previous chunk, if unallocated (P clear)  |
+	    | (X1)        Size of previous chunk, if unallocated (P clear)  |
 	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-    `head:' |             Size of chunk, in bytes                     |A|0|P|
+    `head:' | (X0)        Size of chunk, in bytes                     |A|0|P|
       mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 	    |             Forward pointer to next chunk in list             |
 	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
@@ -1107,9 +1107,9 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 	    .                                                               .
 	    .                                                               |
 nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-    `foot:' |             Size of chunk, in bytes                           |
+    `foot:' | (X1)        Size of chunk, in bytes                           |
 	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-	    |             Size of next chunk, in bytes                |A|0|0|
+	    | (X0)        Size of next chunk, in bytes                |A|0|0|
 	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
     The P (PREV_INUSE) bit, stored in the unused low-order bit of the
@@ -1137,6 +1137,13 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     deal with alignments etc but can be very confusing when trying
     to extend or adapt this code.
 
+    The (X0) and (X1) markers in the diagrams above refer to the heap
+    protector.  Fields marked with (X0) are XOR-obfuscated with the
+    __malloc_header_guard cookie.  (X1) chunks are obfuscated with
+    __malloc_footer_guard.  The hope is that this obfuscation makes it
+    more difficult for an attacker to create a valid malloc chunk,
+    after exploiting a heap buffer overflow.
+
     The three exceptions to all this are:
 
      1. The special chunk `top' doesn't bother using the
@@ -1241,6 +1248,17 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 /* Mark a chunk as not being on the main arena.  */
 #define set_non_main_arena(p) ((p)->mchunk_size |= NON_MAIN_ARENA)
 
+/* Encryption and decryption suitable for the mchunk_size member.  */
+#define HEAP_MANGLE_SIZE(val) \
+  (__malloc_header_guard ^ ((INTERNAL_SIZE_T) (val)))
+#define HEAP_DEMANGLE_SIZE(val) \
+  (__malloc_header_guard ^ ((INTERNAL_SIZE_T) (val)))
+
+/* Encryption and decryption suitable for the mchunk_prev_size member.  */
+#define HEAP_MANGLE_PREVSIZE(val)			\
+  (__malloc_footer_guard ^ ((INTERNAL_SIZE_T) (val)))
+#define HEAP_DEMANGLE_PREVSIZE(val) \
+  (__malloc_footer_guard ^ ((INTERNAL_SIZE_T) (val)))
 
 /*
    Bits to mask off when extracting size
@@ -1256,16 +1274,16 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 #define chunksize(p) (chunksize_nomask (p) & ~(SIZE_BITS))
 
 /* Like chunksize, but do not mask SIZE_BITS.  */
-#define chunksize_nomask(p)         ((p)->mchunk_size)
+#define chunksize_nomask(p) HEAP_DEMANGLE_SIZE ((p)->mchunk_size)
 
 /* Ptr to next physical malloc_chunk. */
 #define next_chunk(p) ((mchunkptr) (((char *) (p)) + chunksize (p)))
 
 /* Size of the chunk below P.  Only valid if prev_inuse (P).  */
-#define prev_size(p) ((p)->mchunk_prev_size)
+#define prev_size(p) HEAP_DEMANGLE_PREVSIZE ((p)->mchunk_prev_size)
 
 /* Set the size of the chunk below P.  Only valid if prev_inuse (P).  */
-#define set_prev_size(p, sz) ((p)->mchunk_prev_size = (sz))
+#define set_prev_size(p, sz) ((p)->mchunk_prev_size = HEAP_MANGLE_PREVSIZE (sz))
 
 /* Ptr to previous physical malloc_chunk.  Only valid if prev_inuse (P).  */
 #define prev_chunk(p) ((mchunkptr) (((char *) (p)) - prev_size (p)))
@@ -1297,13 +1315,16 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
 
 /* Set size at head, without disturbing its use bit */
-#define set_head_size(p, s)  ((p)->mchunk_size = (((p)->mchunk_size & SIZE_BITS) | (s)))
+#define set_head_size(p, s) \
+  ((p)->mchunk_size = ((p)->mchunk_size & SIZE_BITS) | HEAP_MANGLE_SIZE (s))
 
 /* Set size/use field */
-#define set_head(p, s)       ((p)->mchunk_size = (s))
+#define set_head(p, s) ((p)->mchunk_size = HEAP_MANGLE_SIZE (s))
 
 /* Set size at footer (only when chunk is not in use) */
-#define set_foot(p, s)       (((mchunkptr) ((char *) (p) + (s)))->mchunk_prev_size = (s))
+#define set_foot(p, s) \
+  (((mchunkptr) ((char *) (p) + (s)))->mchunk_prev_size \
+   = HEAP_MANGLE_PREVSIZE (s))
 
 
 #pragma GCC poison mchunk_size
diff --git a/malloc/tst-mallocstate.c b/malloc/tst-mallocstate.c
index 7e081c5..ec3bf45 100644
--- a/malloc/tst-mallocstate.c
+++ b/malloc/tst-mallocstate.c
@@ -186,6 +186,10 @@ struct allocation
   unsigned int seed;
 };
 
+/* After heap initialization, we can call malloc_usable_size to check
+   if it gives valid results.  */
+static bool malloc_usable_size_valid;
+
 /* Check that the allocation task allocation has the expected
    contents.  */
 static void
@@ -221,6 +225,23 @@ check_allocation (const struct allocation *alloc, int index)
       putc ('\n', stdout);
       errors = true;
     }
+
+  if (malloc_usable_size_valid)
+    {
+      size_t usable = malloc_usable_size (alloc->data);
+      if (usable < size)
+        {
+          printf ("error: allocation %d has reported size %zu (expected %zu)\n",
+                  index, usable, size);
+          errors = true;
+        }
+      else if (usable - size > 4096)
+        {
+          printf ("error: allocation %d reported as %zu bytes (requested %zu)\n",
+                  index, usable, size);
+          errors = true;
+        }
+    }
 }
 
 /* A heap allocation combined with pending actions on it.  */
@@ -317,6 +338,10 @@ init_heap (void)
       write_message ("error: malloc_set_state failed\n");
       _exit (1);
     }
+
+  /* The heap has been initialized.  We may now call
+     malloc_usable_size.  */
+  malloc_usable_size_valid = true;
 }
 
 /* Interpose the initialization callback.  */
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index f68fdf4..801ded8 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -607,6 +607,10 @@ struct rtld_global_ro
   /* List of auditing interfaces.  */
   struct audit_ifaces *_dl_audit;
   unsigned int _dl_naudit;
+
+  /* malloc protection keys. */
+  uintptr_t _dl_malloc_header_guard;
+  uintptr_t _dl_malloc_footer_guard;
 };
 # define __rtld_global_attribute__
 # if IS_IN (rtld)