cheri: malloc: exact capability check in free/realloc

Capability narrowing uses a lookup table from an address to the
internally used wide capability. Keep the narrow capability in the
table instead of just the address and check it. This allows free
and realloc to check their input and only accept capabilities
exactly matching what was returned by malloc.

When a user adds restrictions on top of malloc returned capabilities
(e.g. narrower bounds), realloc could bypass those restrictions when
it simply looked up the internal wide capability for the address.

malloc/malloc.c

@@ -571,12 +571,12 @@ cap_narrow (void *p, size_t n)
 /* Used in realloc if p is already narrowed or NULL.
    Must match a previous cap_reserve call.  */
 static __always_inline bool
-cap_narrow_check (void *p, void *oldp)
+cap_narrow_check (void *p, void *oldp, void *narrow_oldp)
 {
   if (cap_narrowing_enabled)
     {
       if (p == NULL)
-	(void) __libc_cap_narrow (oldp, 0);
+	__libc_cap_put_back (oldp, narrow_oldp);
       else
 	__libc_cap_unreserve ();
     }
@@ -586,12 +586,12 @@ cap_narrow_check (void *p, void *oldp)
 /* Used in realloc if p is new allocation or NULL but not yet narrowed.
    Must match a previous cap_reserve call.  */
 static __always_inline void *
-cap_narrow_try (void *p, size_t n, void *oldp)
+cap_narrow_try (void *p, size_t n, void *oldp, void *narrow_oldp)
 {
   if (cap_narrowing_enabled)
     {
       if (p == NULL)
-	(void) __libc_cap_narrow (oldp, 0);
+	__libc_cap_put_back (oldp, narrow_oldp);
       else
 	p = __libc_cap_narrow (p, n);
     }
@@ -3588,8 +3588,9 @@ __libc_free (void *mem)
   if (mem == 0)                              /* free(0) has no effect */
     return;
 
+  void *orig_mem = mem;
   mem = cap_widen (mem);
-  cap_drop (mem);
+  cap_drop (orig_mem);
 
   /* Quickly check that the freed pointer matches the tag for the memory.
      This gives a useful double-free detection.  */
@@ -3652,6 +3653,7 @@ __libc_realloc (void *oldmem, size_t bytes)
   if (oldmem == 0)
     return __libc_malloc (bytes);
 
+  void *orig_oldmem = oldmem;
   oldmem = cap_widen (oldmem);
 
   /* Perform a quick check to ensure that the pointer's tag matches the
@@ -3692,7 +3694,7 @@ __libc_realloc (void *oldmem, size_t bytes)
   /* Every return path below should unreserve using the cap_narrow* apis.  */
   if (!cap_reserve ())
     return NULL;
-  cap_drop (oldmem);
+  cap_drop (orig_oldmem);
 
   if (chunk_is_mmapped (oldp))
     {
@@ -3717,7 +3719,7 @@ __libc_realloc (void *oldmem, size_t bytes)
 	     caller for doing this, so we might want to
 	     reconsider.  */
 	  newmem = tag_new_usable (newmem);
-	  newmem = cap_narrow_try (newmem, bytes, oldmem);
+	  newmem = cap_narrow_try (newmem, bytes, oldmem, orig_oldmem);
 	  return newmem;
 	}
 #endif
@@ -3742,7 +3744,7 @@ __libc_realloc (void *oldmem, size_t bytes)
       else
 #endif
       newmem = __libc_malloc (bytes);
-      if (!cap_narrow_check (newmem, oldmem))
+      if (!cap_narrow_check (newmem, oldmem, orig_oldmem))
         return 0;              /* propagate failure */
 
 #ifdef __CHERI_PURE_CAPABILITY__
@@ -3760,7 +3762,7 @@ __libc_realloc (void *oldmem, size_t bytes)
     {
       /* Use memalign, copy, free.  */
       void *newmem = _mid_memalign (align, bytes, 0);
-      if (!cap_narrow_check (newmem, oldmem))
+      if (!cap_narrow_check (newmem, oldmem, orig_oldmem))
 	return newmem;
       size_t sz = memsize (oldp);
       memcpy (newmem, oldmem, sz < bytes ? sz : bytes);
@@ -3774,7 +3776,7 @@ __libc_realloc (void *oldmem, size_t bytes)
       newp = _int_realloc (ar_ptr, oldp, oldsize, nb);
       assert (!newp || chunk_is_mmapped (mem2chunk (newp)) ||
 	      ar_ptr == arena_for_chunk (mem2chunk (newp)));
-      return cap_narrow_try (newp, bytes, oldmem);
+      return cap_narrow_try (newp, bytes, oldmem, orig_oldmem);
     }
 
   __libc_lock_lock (ar_ptr->mutex);
@@ -3790,7 +3792,7 @@ __libc_realloc (void *oldmem, size_t bytes)
       /* Try harder to allocate memory in other arenas.  */
       LIBC_PROBE (memory_realloc_retry, 2, bytes, oldmem);
       newp = __libc_malloc (bytes);
-      if (!cap_narrow_check (newp, oldmem))
+      if (!cap_narrow_check (newp, oldmem, orig_oldmem))
 	return NULL;
       size_t sz = memsize (oldp);
       memcpy (newp, oldmem, sz);

sysdeps/aarch64/morello/libc-cap.h

@@ -30,8 +30,7 @@
 
 struct htentry
 {
-  uint64_t key;
-  uint64_t unused;
+  void *key;
   void *value;
 };
 
@@ -48,19 +47,19 @@ struct ht
 static inline bool
 htentry_isempty (struct htentry *e)
 {
-  return e->key == 0;
+  return (uint64_t) e->key == 0;
 }
 
 static inline bool
 htentry_isdeleted (struct htentry *e)
 {
-  return e->key == -1;
+  return (uint64_t) e->key == -1;
 }
 
 static inline bool
 htentry_isused (struct htentry *e)
 {
-  return e->key != 0 && e->key != -1;
+  return !htentry_isempty (e) && !htentry_isdeleted (e);
 }
 
 static inline uint64_t
@@ -154,9 +153,10 @@ ht_resize (struct ht *ht)
     {
       if (htentry_isused (e))
 	{
-	  uint64_t hash = ht_key_hash (e->key);
+	  uint64_t k = (uint64_t) e->key;
+	  uint64_t hash = ht_key_hash (k);
 	  used--;
-	  *ht_lookup (ht, e->key, hash) = *e;
+	  *ht_lookup (ht, k, hash) = *e;
 	}
     }
   ht_tab_free (oldtab, oldlen);
@@ -191,48 +191,61 @@ ht_unreserve (struct ht *ht)
 }
 
 static bool
-ht_add (struct ht *ht, uint64_t key, void *value)
+ht_add (struct ht *ht, void *key, void *value)
 {
+  uint64_t k = (uint64_t) key;
+  uint64_t hash = ht_key_hash (k);
+  assert (k != 0 && k != -1);
+
   __libc_lock_lock (ht->mutex);
   assert (ht->reserve > 0);
   ht->reserve--;
-  uint64_t hash = ht_key_hash (key);
-  struct htentry *e = ht_lookup (ht, key, hash);
+  struct htentry *e = ht_lookup (ht, k, hash);
   bool r = false;
   if (!htentry_isused (e))
     {
       if (htentry_isempty (e))
         ht->fill++;
       ht->used++;
-      e->key = key;
       r = true;
     }
+  e->key = key;
   e->value = value;
   __libc_lock_unlock (ht->mutex);
   return r;
 }
 
 static bool
-ht_del (struct ht *ht, uint64_t key)
+ht_del (struct ht *ht, void *key)
 {
+  uint64_t k = (uint64_t) key;
+  uint64_t hash = ht_key_hash (k);
+  assert (k != 0 && k != -1);
+
   __libc_lock_lock (ht->mutex);
-  struct htentry *e = ht_lookup (ht, key, ht_key_hash (key));
+  struct htentry *e = ht_lookup (ht, k, hash);
   bool r = htentry_isused (e);
   if (r)
     {
+      r = __builtin_cheri_equal_exact(e->key, key);
       ht->used--;
-      e->key = -1;
+      e->key = (void *) -1;
+      e->value = NULL;
     }
   __libc_lock_unlock (ht->mutex);
   return r;
 }
 
 static void *
-ht_get (struct ht *ht, uint64_t key)
+ht_get (struct ht *ht, void *key)
 {
+  uint64_t k = (uint64_t) key;
+  uint64_t hash = ht_key_hash (k);
+  assert (k != 0 && k != -1);
+
   __libc_lock_lock (ht->mutex);
-  struct htentry *e = ht_lookup (ht, key, ht_key_hash (key));
-  void *v = htentry_isused (e) ? e->value : NULL;
+  struct htentry *e = ht_lookup (ht, k, hash);
+  void *v = __builtin_cheri_equal_exact(e->key, key) ? e->value : NULL;
   __libc_lock_unlock (ht->mutex);
   return v;
 }
@@ -317,10 +330,9 @@ __libc_cap_align (size_t n)
 static __always_inline void *
 __libc_cap_narrow (void *p, size_t n)
 {
-  assert (p != NULL);
-  uint64_t key = (uint64_t)(uintptr_t) p;
-  assert (ht_add (&__libc_cap_ht, key, p));
   void *narrow = __builtin_cheri_bounds_set_exact (p, n);
+  assert (__builtin_cheri_tag_get (narrow));
+  assert (ht_add (&__libc_cap_ht, narrow, p));
   return narrow;
 }
 
@@ -329,9 +341,7 @@ __libc_cap_narrow (void *p, size_t n)
 static __always_inline void *
 __libc_cap_widen (void *p)
 {
-  assert (__builtin_cheri_tag_get (p) && __builtin_cheri_offset_get (p) == 0);
-  uint64_t key = (uint64_t)(uintptr_t) p;
-  void *cap = ht_get (&__libc_cap_ht, key);
+  void *cap = ht_get (&__libc_cap_ht, p);
   assert (cap == p);
   return cap;
 }
@@ -351,9 +361,13 @@ __libc_cap_unreserve (void)
 static __always_inline void
 __libc_cap_drop (void *p)
 {
-  assert (p != NULL);
-  uint64_t key = (uint64_t)(uintptr_t) p;
-  assert (ht_del (&__libc_cap_ht, key));
+  assert (ht_del (&__libc_cap_ht, p));
+}
+
+static __always_inline void
+__libc_cap_put_back (void *p, void *narrow)
+{
+  assert (ht_add (&__libc_cap_ht, narrow, p));
 }
 
 #endif

sysdeps/generic/libc-cap.h

@@ -39,5 +39,6 @@ void __libc_cap_link_error (void);
 #define __libc_cap_reserve(p) __libc_cap_fail (bool)
 #define __libc_cap_unreserve(p) __libc_cap_fail (void)
 #define __libc_cap_drop(p) __libc_cap_fail (void)
+#define __libc_cap_put_back(p, q) __libc_cap_fail (void)
 
 #endif