malloc: tcache double free check

* malloc/malloc.c (tcache_entry): Add key field.
(tcache_put): Set it.
(tcache_get): Likewise.
(_int_free): Check for double free in tcache.
* malloc/tst-tcfree1.c: New.
* malloc/tst-tcfree2.c: New.
* malloc/Makefile: Run the new tests.
* manual/probes.texi: Document memory_tcache_double_free probe.

* dlfcn/dlerror.c (check_free): Prevent double frees.

(cherry picked from commit bcdaad21d4)

malloc: tcache: Validate tc_idx before checking for double-frees [BZ #23907]

The previous check could read beyond the end of the tcache entry
array.  If the e->key == tcache cookie check happened to pass, this
would result in crashes.

(cherry picked from commit affec03b71)

ChangeLog

@@ -1,3 +1,23 @@
+2018-11-26  Florian Weimer  <fweimer@redhat.com>
+
+	[BZ #23907]
+	* malloc/malloc.c (_int_free): Validate tc_idx before checking for
+	double-frees.
+
+
+2018-11-20  DJ Delorie  <dj@redhat.com>
+
+	* malloc/malloc.c (tcache_entry): Add key field.
+	(tcache_put): Set it.
+	(tcache_get): Likewise.
+	(_int_free): Check for double free in tcache.
+	* malloc/tst-tcfree1.c: New.
+	* malloc/tst-tcfree2.c: New.
+	* malloc/Makefile: Run the new tests.
+	* manual/probes.texi: Document memory_tcache_double_free probe.
+
+	* dlfcn/dlerror.c (check_free): Prevent double frees.
+
 2018-11-27  Florian Weimer  <fweimer@redhat.com>
 
 	[BZ #23927]

dlfcn/dlerror.c

@@ -197,7 +197,10 @@ check_free (struct dl_action_result *rec)
       Dl_info info;
       if (_dl_addr (check_free, &info, &map, NULL) != 0 && map->l_ns == 0)
 #endif
-	free ((char *) rec->errstring);
+	{
+	  free ((char *) rec->errstring);
+	  rec->errstring = NULL;
+	}
     }
 }
 

malloc/Makefile

@@ -37,6 +37,7 @@ tests := mallocbug tst-malloc tst-valloc tst-calloc tst-obstack \
 	 tst-malloc-tcache-leak \
 	 tst-malloc_info \
 	 tst-malloc-too-large \
+	 tst-tcfree1 tst-tcfree2 \
 
 tests-static := \
 	 tst-interpose-static-nothread \

malloc/malloc.c

@@ -2904,6 +2904,8 @@ mremap_chunk (mchunkptr p, size_t new_size)
 typedef struct tcache_entry
 {
   struct tcache_entry *next;
+  /* This field exists to detect double frees.  */
+  struct tcache_perthread_struct *key;
 } tcache_entry;
 
 /* There is one of these for each thread, which contains the
@@ -2927,6 +2929,11 @@ tcache_put (mchunkptr chunk, size_t tc_idx)
 {
   tcache_entry *e = (tcache_entry *) chunk2mem (chunk);
   assert (tc_idx < TCACHE_MAX_BINS);
+
+  /* Mark this chunk as "in the tcache" so the test in _int_free will
+     detect a double free.  */
+  e->key = tcache;
+
   e->next = tcache->entries[tc_idx];
   tcache->entries[tc_idx] = e;
   ++(tcache->counts[tc_idx]);
@@ -2942,6 +2949,7 @@ tcache_get (size_t tc_idx)
   assert (tcache->entries[tc_idx] > 0);
   tcache->entries[tc_idx] = e->next;
   --(tcache->counts[tc_idx]);
+  e->key = NULL;
   return (void *) e;
 }
 
@@ -4165,13 +4173,33 @@ _int_free (mstate av, mchunkptr p, int have_lock)
 #if USE_TCACHE
   {
     size_t tc_idx = csize2tidx (size);
-
-    if (tcache
-	&& tc_idx < mp_.tcache_bins
-	&& tcache->counts[tc_idx] < mp_.tcache_count)
+    if (tcache != NULL && tc_idx < mp_.tcache_bins)
       {
-	tcache_put (p, tc_idx);
-	return;
+	/* Check to see if it's already in the tcache.  */
+	tcache_entry *e = (tcache_entry *) chunk2mem (p);
+
+	/* This test succeeds on double free.  However, we don't 100%
+	   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))
+	  {
+	    tcache_entry *tmp;
+	    LIBC_PROBE (memory_tcache_double_free, 2, e, tc_idx);
+	    for (tmp = tcache->entries[tc_idx];
+		 tmp;
+		 tmp = tmp->next)
+	      if (tmp == e)
+		malloc_printerr ("free(): double free detected in tcache 2");
+	    /* If we get here, it was a coincidence.  We've wasted a
+	       few cycles, but don't abort.  */
+	  }
+
+	if (tcache->counts[tc_idx] < mp_.tcache_count)
+	  {
+	    tcache_put (p, tc_idx);
+	    return;
+	  }
       }
   }
 #endif

malloc/tst-tcfree1.c

@@ -0,0 +1,42 @@
+/* Test that malloc tcache catches double free.
+   Copyright (C) 2018 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; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <error.h>
+#include <limits.h>
+#include <malloc.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/signal.h>
+
+static int
+do_test (void)
+{
+  /* Do one allocation of any size that fits in tcache.  */
+  char * volatile x = malloc (32);
+
+  free (x); // puts in tcache
+  free (x); // should abort
+
+  printf("FAIL: tcache double free not detected\n");
+  return 1;
+}
+
+#define TEST_FUNCTION do_test
+#define EXPECTED_SIGNAL SIGABRT
+#include <support/test-driver.c>

malloc/tst-tcfree2.c

@@ -0,0 +1,48 @@
+/* Test that malloc tcache catches double free.
+   Copyright (C) 2018 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; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <error.h>
+#include <limits.h>
+#include <malloc.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/signal.h>
+
+static int
+do_test (void)
+{
+  char * volatile ptrs[20];
+  int i;
+
+  /* Allocate enough small chunks so that when we free them all, the tcache
+     is full, and the first one we freed is at the end of its linked list.  */
+#define COUNT 20
+  for (i=0; i<COUNT; i++)
+    ptrs[i] = malloc (20);
+  for (i=0; i<COUNT; i++)
+    free (ptrs[i]);
+  free (ptrs[0]);
+
+  printf("FAIL: tcache double free\n");
+  return 1;
+}
+
+#define TEST_FUNCTION do_test
+#define EXPECTED_SIGNAL SIGABRT
+#include <support/test-driver.c>

manual/probes.texi

@@ -243,6 +243,18 @@ This probe is triggered when the
 value of this tunable.
 @end deftp
 
+@deftp Probe memory_tcache_double_free (void *@var{$arg1}, int @var{$arg2})
+This probe is triggered when @code{free} determines that the memory
+being freed has probably already been freed, and resides in the
+per-thread cache.  Note that there is an extremely unlikely chance
+that this probe will trigger due to random payload data remaining in
+the allocated memory matching the key used to detect double frees.
+This probe actually indicates that an expensive linear search of the
+tcache, looking for a double free, has happened.  Argument @var{$arg1}
+is the memory location as passed to @code{free}, Argument @var{$arg2}
+is the tcache bin it resides in.
+@end deftp
+
 @node Mathematical Function Probes
 @section Mathematical Function Probes