[PATCH] malloc: Fix [BZ #22853] weak randomization on thread arenas.
iripoll
iripoll@disca.upv.es
Mon Mar 23 17:06:49 GMT 2020
Hi!
> Thanks for the work. However, patch discussion is done through the
> email list [1]. Could you re-send it on the libc-alpha?
>
> It seems that alignment restriction was added originally to simplify
> the heap_for_ptr macro and I think it should be added on patch
> description (stating there is no other restriction about arena
> alignment).
Regarding the comments in the code, we don't know what would be the
best way to describe the patch. We don't know if the code shall be
commented with respect to the previous code or if it would be better
to describe the operation of the new heap_for_ptr macro (which can be
tricky). Please, add the comments that you think are better.
> Also, would be possible to add a testcase? I am not sure which
> approach would be better (a maillist discussion might give some
> ideas). Maybe by parametrize the mmap entropy information you
> compiled on arch-specific files (similar to the stackinfo.h), create
> N thread that run M allocation, record the output, and compare the
> histogram with the expected entropy with a certain margin.
The patch (in the mail) contains both the fix and a new test
[tst-arena-aslr.c] that checks the effective randomization of the
malloc on threads and compares it with that of the mmap. That way
the test shall be platform independent.
Although the security issue affects the entropy of the dynamic
memory requested by the threads, it is necessary to launch 1000
processes to have different memory layouts samples. Remember that
ASLR does not change on a per processes basis (fork) but it is
renewed every new binary image (exec).
> Finally, please also add the outcome of the
> bench-malloc-{simple,thread} with and without the patch.
The output of the test is:
-------- If failed (three lines) ------
FAIL: malloc arena ASLR.
Expected value: 29 random bits
But got: 15 random bits
-------- If passed (one line) -------
PASS: malloc arena ASLR.
The test is passed if the random bits of the thread malloc is
greater or equal to the random bits of mmap(). With a small
tolerance. This way (testing greater or equal than) the test will
also pass if additional entropy is add to the heaps in the future.
Important: the test does not estimate the entropy accurately! because
it is not the goal of the test. A more accurate estimation would need
much more samples and a more complex algorithm, which is heavy. In fact,
the actual entropy is 28 bits and 14 in x86_64, but the test gives 29
and 15 for mmap and thread's mallocs respectively.
Regards,
Ismael and Hector.
https://sourceware.org/bugzilla/show_bug.cgi?id=22853
Signed-off-by: Ismael Ripoll <iripoll@disca.upv.es>
Co-Authored-By: Hector Marco-Gisbert <hmarco@hmarco.org>
---
malloc/Makefile | 3 +
malloc/arena.c | 25 ++++-
malloc/tst-arena-aslr.c | 213 ++++++++++++++++++++++++++++++++++++++++
3 files changed, 236 insertions(+), 5 deletions(-)
create mode 100644 malloc/tst-arena-aslr.c
diff --git a/malloc/Makefile b/malloc/Makefile
index 984045b5b9..dc845b1c24 100644
--- a/malloc/Makefile
+++ b/malloc/Makefile
@@ -39,6 +39,8 @@ tests := mallocbug tst-malloc tst-valloc tst-calloc tst-obstack \
tst-malloc-too-large \
tst-malloc-stats-cancellation \
tst-tcfree1 tst-tcfree2 tst-tcfree3 \
+ tst-arena-aslr \
+
tests-static := \
tst-interpose-static-nothread \
@@ -99,6 +101,7 @@ $(objpfx)tst-malloc-thread-exit: $(shared-thread-library)
$(objpfx)tst-malloc-thread-fail: $(shared-thread-library)
$(objpfx)tst-malloc-fork-deadlock: $(shared-thread-library)
$(objpfx)tst-malloc-stats-cancellation: $(shared-thread-library)
+$(objpfx)tst-arena-aslr: $(shared-thread-library)
# Export the __malloc_initialize_hook variable to libc.so.
LDFLAGS-tst-mallocstate = -rdynamic
diff --git a/malloc/arena.c b/malloc/arena.c
index cecdb7f4c4..416559e269 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -122,10 +122,15 @@ int __malloc_initialized = -1;
ptr = arena_get2 ((size), NULL); \
} while (0)
-/* find the heap and corresponding arena for a given ptr */
-
+/* find the heap and corresponding arena for a given ptr. Note that
+ heap_info is not HEAP_MAX_SIZE aligned any more. But a random
+ offset from the expected alignment, known by the process. This way
+ it is fast to get the head of the area and be ASLR friendly.
+*/
+static unsigned long __arena_rnd;
#define heap_for_ptr(ptr) \
- ((heap_info *) ((unsigned long) (ptr) & ~(HEAP_MAX_SIZE - 1)))
+ ((heap_info *) ((((unsigned long) ptr-__arena_rnd) & ~(HEAP_MAX_SIZE - 1)) \
+ | __arena_rnd))
#define arena_for_chunk(ptr) \
(chunk_main_arena (ptr) ? &main_arena : heap_for_ptr (ptr)->ar_ptr)
@@ -293,6 +298,11 @@ ptmalloc_init (void)
__malloc_initialized = 0;
+ size_t pagesize = GLRO (dl_pagesize);
+ /* Get the entropy from the already existing ASLR. */
+ __arena_rnd = ((unsigned long)&__arena_rnd) & (HEAP_MAX_SIZE-1) &
+ ~(pagesize-1);
+
#ifdef SHARED
/* In case this libc copy is in a non-default namespace, never use brk.
Likewise if dlopened from statically linked program. */
@@ -490,6 +500,11 @@ new_heap (size_t size, size_t top_pad)
{
p2 = (char *) (((unsigned long) p1 + (HEAP_MAX_SIZE - 1))
& ~(HEAP_MAX_SIZE - 1));
+ /* The heap_info is at a random offset from the alignment to
+ HEAP_MAX_SIZE. */
+ p2 = (char *) ((unsigned long) p2 | __arena_rnd);
+ if (p1 + HEAP_MAX_SIZE <= p2)
+ p2 -= HEAP_MAX_SIZE;
ul = p2 - p1;
if (ul)
__munmap (p1, ul);
@@ -500,12 +515,12 @@ new_heap (size_t size, size_t top_pad)
else
{
/* Try to take the chance that an allocation of only HEAP_MAX_SIZE
- is already aligned. */
+ is already aligned to __arena_rnd. */
p2 = (char *) MMAP (0, HEAP_MAX_SIZE, PROT_NONE, MAP_NORESERVE);
if (p2 == MAP_FAILED)
return 0;
- if ((unsigned long) p2 & (HEAP_MAX_SIZE - 1))
+ if (((unsigned long) p2 & (HEAP_MAX_SIZE - 1)) != __arena_rnd)
{
__munmap (p2, HEAP_MAX_SIZE);
return 0;
diff --git a/malloc/tst-arena-aslr.c b/malloc/tst-arena-aslr.c
new file mode 100644
index 0000000000..c285111e1c
--- /dev/null
+++ b/malloc/tst-arena-aslr.c
@@ -0,0 +1,213 @@
+/* Bug 22853: aslr on thread's malloc weakened due to arena alignment.
+ https://sourceware.org/bugzilla/show_bug.cgi?id=22853
+ Copyright (C) 2020 Free Software Foundation.
+ Copying and distribution of this file, with or without modification,
+ are permitted in any medium without royalty provided the copyright
+ notice and this notice are preserved. This file is offered as-is,
+ without any warranty. */
+
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <libc-diag.h>
+#include <pthread.h>
+#include <string.h>
+#include <sys/mman.h>
+
+/* Bits that are considered to be acceptable: */
+#define TOLERANCE 1
+/* Samples to be run but statisticaly accurate: */
+#define NR_SAMPLES 1000
+
+/* Size to be allocated in the thread's arena pool. */
+#define MALLOC_ALLOCATED_SIZE 400
+
+/* Data structure sent by the sampler to the tester. */
+struct sample_t
+{
+ size_t obj_mmap;
+ size_t obj_arena;
+} sample;
+
+size_t pagesize;
+
+static void *
+thread_main(void * dummy)
+{
+ sample.obj_arena = (size_t) malloc (MALLOC_ALLOCATED_SIZE);
+ pthread_exit(0);
+}
+
+/* Creates a thread to sample malloc and mmap addresses, and writes
+ the result to stdout. */
+static void
+sampler(void)
+{
+ int ret;
+ pthread_t th;
+ pthread_attr_t attrs;
+
+ ret = pthread_attr_init (&attrs);
+ if (0 != ret)
+ {
+ perror ("pthread_attr_init");
+ abort ();
+ }
+ sample.obj_mmap = (size_t)mmap( (void *)0x0, pagesize,
+ PROT_READ | PROT_WRITE,
+ MAP_PRIVATE | MAP_ANONYMOUS, -1 ,0);
+ if (!sample.obj_mmap)
+ {
+ perror ("mmap");
+ abort ();
+ }
+ if (pthread_create (&th, &attrs, thread_main, NULL))
+ {
+ perror ("pthread_create");
+ abort ();
+ }
+ if (pthread_join (th, NULL))
+ {
+ perror ("pthread_join");
+ abort ();
+ }
+ if (sizeof (sample) != write (1, &sample, sizeof (sample)))
+ {
+ perror ("write sampler");
+ abort ();
+ }
+ exit(0);
+}
+
+
+struct statitics_t
+{
+ size_t min, max, and, or;
+};
+
+static void
+stats_init (struct statitics_t *st)
+{
+ st->min = st->and = ~0;
+ st->max = st->or = 0;
+}
+
+static void
+stats_account (struct statitics_t *st, size_t value)
+{
+ st->min = value < st->min ? value : st->min;
+ st->max = value > st->max ? value : st->max;
+ st->and &= value;
+ st->or |= value;
+}
+
+/* Flipping bits, this is a "pourman" entroy estimator. There are much
+ better and accurate entropy estimators, but they need more samples
+ and mote time... which do not worth it. */
+static int
+stats_pmentropy (struct statitics_t *st)
+{
+
+ return __builtin_popcountll(st->and ^ st->or);
+}
+
+#if 0
+/* Debugging the test. Not needed for production testing */
+void
+stats_print (struct statitics_t *st)
+{
+ printf ("[%18zx %18zx] %18zx %18zx %18zx %18zx %d\n", st->min, st->max,
+ (st->max >> 2) + (st->min >> 2),
+ st->and,
+ st->or,
+ st->and ^ st->or,
+ stats_pmentropy(st));
+}
+#endif
+
+static int
+do_test (int argc, char **argv)
+{
+ pid_t child;
+ int tube[2];
+ int wstatus;
+ int i, mmap_entropy, arena_entropy;
+ struct sample_t sample;
+ struct statitics_t stats_mmap, stats_arena;
+
+ pagesize = sysconf (_SC_PAGESIZE);
+
+ if ( (2 == argc) && !strcmp (argv[1],"sample"))
+ {
+ sampler(); /* The role of this process is to sample one address. */
+ exit(0);
+ }
+
+ /* The role of this part is to launch NR_SAMPLES samplers and
+ collect the output. */
+ assert (pipe(tube) == 0);
+
+ stats_init (&stats_mmap);
+ stats_init (&stats_arena);
+
+ /* Collect all the samples. */
+ for (i = 0; i < NR_SAMPLES ; i++)
+ {
+ child = fork ();
+ if (child < 0)
+ {
+ perror ("fork");
+ exit (1);
+ }
+ if (0 == child)
+ {
+ assert (dup2(tube[1],1) != -1);
+ execl (argv[0], argv[0], "sample", NULL);
+ perror ("execl");
+ abort ();
+ }
+ if (TEMP_FAILURE_RETRY (read(tube[0], &sample, sizeof (sample)))
+ != sizeof (sample))
+ {
+ printf ("Error: can not read from child process\n");
+ abort ();
+ }
+ if (TEMP_FAILURE_RETRY (waitpid (child, &wstatus, 0) != child) ||
+ (wstatus != 0))
+ {
+ printf ("Error: waitpid for child process\n");
+ abort ();
+ }
+ stats_account (&stats_mmap, sample.obj_mmap);
+ stats_account (&stats_arena, sample.obj_arena);
+ }
+
+ /* Compute the actual entropy and analyze the results. */
+ mmap_entropy = stats_pmentropy (&stats_mmap);
+ arena_entropy = stats_pmentropy (&stats_arena);
+
+ if ((arena_entropy + TOLERANCE) < mmap_entropy)
+ {
+ printf ("FAIL: malloc arena ASLR.\n");
+ printf (" Expected value: %d random bits\n", mmap_entropy);
+ printf (" But got: %d random bits\n", arena_entropy);
+
+ /* stats_print(&stats_mmap);
+ stats_print(&stats_arena); */
+ return 1;
+ }
+ printf ("PASS: malloc arena ASLR.\n");
+ return 0;
+}
+
+#define TEST_FUNCTON_ARGV do_test
+#include "../test-skeleton.c"
+
+/* Local Variables: */
+/* c-file-style: "gnu" */
+/* c-basic-offset: 2 */
+/* End: */
--
2.20.1
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