Harden tcache double-free check

The tcache allocator layer uses the tcache pointer as a key to
identify a block that may be freed twice.  Since this is in the
application data area, an attacker exploiting a use-after-free could
potentially get access to the entire tcache structure through this
key.  A detailed write-up was provided by Awarau here:

https://awaraucom.wordpress.com/2020/07/19/house-of-io-remastered/

Replace this static pointer use for key checking with one that is
generated at malloc initialization.  The first attempt is through
getrandom with a fallback to random_bits(), which is a simple
pseudo-random number generator based on the clock.  The fallback ought
to be sufficient since the goal of the randomness is only to make the
key arbitrary enough that it is very unlikely to collide with user
data.

Co-authored-by: Eyal Itkin <eyalit@checkpoint.com>

malloc/arena.c

@@ -287,6 +287,10 @@ extern struct dl_open_hook *_dl_open_hook;
 libc_hidden_proto (_dl_open_hook);
 #endif
 
+#if USE_TCACHE
+static void tcache_key_initialize (void);
+#endif
+
 static void
 ptmalloc_init (void)
 {
@@ -295,6 +299,10 @@ ptmalloc_init (void)
 
   __malloc_initialized = 0;
 
+#if USE_TCACHE
+  tcache_key_initialize ();
+#endif
+
 #ifdef USE_MTAG
   if ((TUNABLE_GET_FULL (glibc, mem, tagging, int32_t, NULL) & 1) != 0)
     {

malloc/malloc.c

@@ -252,6 +252,10 @@
 
 #include <libc-internal.h>
 
+/* For tcache double-free check.  */
+#include <random-bits.h>
+#include <sys/random.h>
+
 /*
   Debugging:
 
@@ -3091,7 +3095,7 @@ typedef struct tcache_entry
 {
   struct tcache_entry *next;
   /* This field exists to detect double frees.  */
-  struct tcache_perthread_struct *key;
+  uintptr_t key;
 } tcache_entry;
 
 /* There is one of these for each thread, which contains the
@@ -3108,6 +3112,31 @@ typedef struct tcache_perthread_struct
 static __thread bool tcache_shutting_down = false;
 static __thread tcache_perthread_struct *tcache = NULL;
 
+/* Process-wide key to try and catch a double-free in the same thread.  */
+static uintptr_t tcache_key;
+
+/* The value of tcache_key does not really have to be a cryptographically
+   secure random number.  It only needs to be arbitrary enough so that it does
+   not collide with values present in applications.  If a collision does happen
+   consistently enough, it could cause a degradation in performance since the
+   entire list is checked to check if the block indeed has been freed the
+   second time.  The odds of this happening are exceedingly low though, about 1
+   in 2^wordsize.  There is probably a higher chance of the performance
+   degradation being due to a double free where the first free happened in a
+   different thread; that's a case this check does not cover.  */
+static void
+tcache_key_initialize (void)
+{
+  if (__getrandom (&tcache_key, sizeof(tcache_key), GRND_NONBLOCK)
+      != sizeof (tcache_key))
+    {
+      tcache_key = random_bits ();
+#if __WORDSIZE == 64
+      tcache_key = (tcache_key << 32) | random_bits ();
+#endif
+    }
+}
+
 /* Caller must ensure that we know tc_idx is valid and there's room
    for more chunks.  */
 static __always_inline void
@@ -3117,7 +3146,7 @@ tcache_put (mchunkptr chunk, size_t tc_idx)
 
   /* Mark this chunk as "in the tcache" so the test in _int_free will
      detect a double free.  */
-  e->key = tcache;
+  e->key = tcache_key;
 
   e->next = PROTECT_PTR (&e->next, tcache->entries[tc_idx]);
   tcache->entries[tc_idx] = e;
@@ -3134,7 +3163,7 @@ tcache_get (size_t tc_idx)
     malloc_printerr ("malloc(): unaligned tcache chunk detected");
   tcache->entries[tc_idx] = REVEAL_PTR (e->next);
   --(tcache->counts[tc_idx]);
-  e->key = NULL;
+  e->key = 0;
   return (void *) e;
 }
 
@@ -4437,7 +4466,7 @@ _int_free (mstate av, mchunkptr p, int have_lock)
 	   trust it (it also matches random payload data at a 1 in
 	   2^<size_t> chance), so verify it's not an unlikely
 	   coincidence before aborting.  */
-	if (__glibc_unlikely (e->key == tcache))
+	if (__glibc_unlikely (e->key == tcache_key))
 	  {
 	    tcache_entry *tmp;
 	    size_t cnt = 0;