[PATCH] Do not leave random talloc magic in free()'ed memory
Andrew Bartlett
abartlet at samba.org
Mon Jan 8 04:38:30 UTC 2018
On Thu, 2017-12-21 at 20:13 +1300, Andrew Bartlett via samba-technical
wrote:
> G'Day,
>
> I've been thinking about ways that our talloc magic protection might be
> avoided and reading the magic from memory that has recently been
> free()ed would be a good attack.
>
> So this patch marks this memory with a fixed magic. All valid use of
> memory still uses the random magic.
>
> This passed a full autobuild.
>
> Please carefully review!
>
> On my re-look it might need to tweak talloc_chunk_from_ptr() a little
> (when other flags could be set), but I would like other thoughts too!
Attached is a revised set of patches, which removes the
talloc_abort_magic() branch as I can't see how it is usefully
triggered.
Clearly this needs very careful review.
Andrew Bartlett
--
Andrew Bartlett
https://samba.org/~abartlet/
Authentication Developer, Samba Team https://samba.org
Samba Development and Support, Catalyst IT
https://catalyst.net.nz/services/samba
-------------- next part --------------
From 6cad7ab0b469984c43a8df2c4c70e995628ef9f1 Mon Sep 17 00:00:00 2001
From: Andrew Bartlett <abartlet at samba.org>
Date: Mon, 8 Jan 2018 17:29:19 +1300
Subject: [PATCH 1/2] talloc: Remove talloc_abort_magic()
The check required for talloc_abort_magic() prevents the 'access after free error'
from being printed.
It is also no longer possible to determine the difference between invalid memory
and a talloc version mismatch as the magic is now random on many platforms.
Signed-off-by: Andrew Bartlett <abartlet at samba.org>
---
lib/talloc/talloc.c | 20 +++++---------------
1 file changed, 5 insertions(+), 15 deletions(-)
diff --git a/lib/talloc/talloc.c b/lib/talloc/talloc.c
index 7721fa4a9c6..3569ba75929 100644
--- a/lib/talloc/talloc.c
+++ b/lib/talloc/talloc.c
@@ -429,11 +429,6 @@ static void talloc_abort(const char *reason)
talloc_abort_fn(reason);
}
-static void talloc_abort_magic(unsigned magic)
-{
- talloc_abort("Bad talloc magic value - wrong talloc version used/mixed");
-}
-
static void talloc_abort_access_after_free(void)
{
talloc_abort("Bad talloc magic value - access after free");
@@ -450,19 +445,14 @@ static inline struct talloc_chunk *talloc_chunk_from_ptr(const void *ptr)
const char *pp = (const char *)ptr;
struct talloc_chunk *tc = discard_const_p(struct talloc_chunk, pp - TC_HDR_SIZE);
if (unlikely((tc->flags & (TALLOC_FLAG_FREE | ~TALLOC_FLAG_MASK)) != talloc_magic)) {
- if ((tc->flags & (~TALLOC_FLAG_MASK)) == talloc_magic) {
- talloc_abort_magic(tc->flags & (~TALLOC_FLAG_MASK));
- return NULL;
- }
-
- if (tc->flags & TALLOC_FLAG_FREE) {
- talloc_log("talloc: access after free error - first free may be at %s\n", tc->name);
- talloc_abort_access_after_free();
- return NULL;
- } else {
+ if ((tc->flags & (~TALLOC_FLAG_MASK)) != talloc_magic) {
talloc_abort_unknown_value();
return NULL;
}
+
+ talloc_log("talloc: access after free error - first free may be at %s\n", tc->name);
+ talloc_abort_access_after_free();
+ return NULL;
}
return tc;
}
--
2.11.0
From 719a67a5f881f1450caccb39179ae53bc69e03c5 Mon Sep 17 00:00:00 2001
From: Andrew Bartlett <abartlet at samba.org>
Date: Mon, 8 Jan 2018 17:34:31 +1300
Subject: [PATCH 2/2] talloc: Do not disclose the random talloc magic in
free()'ed memory
This may help us avoid exploits via memory read attacks on Samba by ensuring that if the read
is on an invalid chunk that the talloc magic disclosed there is not useful
to create a valid chunk and so set a destructor.
Signed-off-by: Andrew Bartlett <abartlet at samba.org>
---
lib/talloc/talloc.c | 121 ++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 90 insertions(+), 31 deletions(-)
diff --git a/lib/talloc/talloc.c b/lib/talloc/talloc.c
index 3569ba75929..b6f81df73dc 100644
--- a/lib/talloc/talloc.c
+++ b/lib/talloc/talloc.c
@@ -75,12 +75,13 @@
#define TALLOC_MAGIC_REFERENCE ((const char *)1)
#define TALLOC_MAGIC_BASE 0xe814ec70
-static unsigned int talloc_magic = (
- ~TALLOC_FLAG_MASK & (
- TALLOC_MAGIC_BASE +
- (TALLOC_BUILD_VERSION_MAJOR << 24) +
- (TALLOC_BUILD_VERSION_MINOR << 16) +
- (TALLOC_BUILD_VERSION_RELEASE << 8)));
+#define TALLOC_MAGIC_NON_RANDOM ( \
+ ~TALLOC_FLAG_MASK & ( \
+ TALLOC_MAGIC_BASE + \
+ (TALLOC_BUILD_VERSION_MAJOR << 24) + \
+ (TALLOC_BUILD_VERSION_MINOR << 16) + \
+ (TALLOC_BUILD_VERSION_RELEASE << 8)))
+static unsigned int talloc_magic = TALLOC_MAGIC_NON_RANDOM;
/* by default we abort when given a bad pointer (such as when talloc_free() is called
on a pointer that came from malloc() */
@@ -332,6 +333,48 @@ _PUBLIC_ int talloc_test_get_magic(void)
return talloc_magic;
}
+static inline void _talloc_chunk_set_free(struct talloc_chunk *tc,
+ const char *location)
+{
+ /*
+ * Mark this memory as free, and also over-stamp the talloc
+ * magic with the old-style magic.
+ *
+ * Why? This tries to avoid a memory read use-after-free from
+ * disclosing our talloc magic, which would then allow an
+ * attacker to prepare a valid header and so run a destructor.
+ *
+ */
+ tc->flags = TALLOC_MAGIC_NON_RANDOM | TALLOC_FLAG_FREE
+ | (tc->flags & TALLOC_FLAG_MASK);
+
+ /* we mark the freed memory with where we called the free
+ * from. This means on a double free error we can report where
+ * the first free came from
+ */
+ if (location) {
+ tc->name = location;
+ }
+}
+
+static inline void _talloc_chunk_set_not_free(struct talloc_chunk *tc)
+{
+ /*
+ * Mark this memory as not free.
+ *
+ * Why? This is memory either in a pool (and so available for
+ * talloc's re-use or after the realloc(). We need to mark
+ * the memory as free() before any realloc() call as we can't
+ * write to the memory after that.
+ *
+ * We put back the normal magic instead of the 'not random'
+ * magic.
+ */
+
+ tc->flags = talloc_magic |
+ ((tc->flags & TALLOC_FLAG_MASK) & ~TALLOC_FLAG_FREE);
+}
+
static void (*talloc_log_fn)(const char *message);
_PUBLIC_ void talloc_set_log_fn(void (*log_fn)(const char *message))
@@ -444,14 +487,16 @@ static inline struct talloc_chunk *talloc_chunk_from_ptr(const void *ptr)
{
const char *pp = (const char *)ptr;
struct talloc_chunk *tc = discard_const_p(struct talloc_chunk, pp - TC_HDR_SIZE);
- if (unlikely((tc->flags & (TALLOC_FLAG_FREE | ~TALLOC_FLAG_MASK)) != talloc_magic)) {
- if ((tc->flags & (~TALLOC_FLAG_MASK)) != talloc_magic) {
- talloc_abort_unknown_value();
+ if (unlikely((tc->flags & (TALLOC_FLAG_FREE | ~TALLOC_FLAG_MASK))
+ != talloc_magic)) {
+ if ((tc->flags & (TALLOC_FLAG_FREE | ~TALLOC_FLAG_MASK))
+ == (TALLOC_MAGIC_NON_RANDOM | TALLOC_FLAG_FREE)) {
+ talloc_log("talloc: access after free error - first free may be at %s\n", tc->name);
+ talloc_abort_access_after_free();
return NULL;
}
- talloc_log("talloc: access after free error - first free may be at %s\n", tc->name);
- talloc_abort_access_after_free();
+ talloc_abort_unknown_value();
return NULL;
}
return tc;
@@ -937,13 +982,7 @@ static inline void _tc_free_poolmem(struct talloc_chunk *tc,
pool_tc = talloc_chunk_from_pool(pool);
next_tc = tc_next_chunk(tc);
- tc->flags |= TALLOC_FLAG_FREE;
-
- /* we mark the freed memory with where we called the free
- * from. This means on a double free error we can report where
- * the first free came from
- */
- tc->name = location;
+ _talloc_chunk_set_free(tc, location);
TC_INVALIDATE_FULL_CHUNK(tc);
@@ -1093,13 +1132,7 @@ static inline int _tc_free_internal(struct talloc_chunk *tc,
_tc_free_children_internal(tc, ptr, location);
- tc->flags |= TALLOC_FLAG_FREE;
-
- /* we mark the freed memory with where we called the free
- * from. This means on a double free error we can report where
- * the first free came from
- */
- tc->name = location;
+ _talloc_chunk_set_free(tc, location);
if (tc->flags & TALLOC_FLAG_POOL) {
struct talloc_pool_hdr *pool;
@@ -1796,8 +1829,22 @@ _PUBLIC_ void *_talloc_realloc(const void *context, void *ptr, size_t size, cons
}
#endif
- /* by resetting magic we catch users of the old memory */
- tc->flags |= TALLOC_FLAG_FREE;
+ /*
+ * by resetting magic we catch users of the old memory
+ *
+ * We mark this memory as free, and also over-stamp the talloc
+ * magic with the old-style magic.
+ *
+ * Why? This tries to avoid a memory read use-after-free from
+ * disclosing our talloc magic, which would then allow an
+ * attacker to prepare a valid header and so run a destructor.
+ *
+ * What else? We have to re-stamp back a valid normal magic
+ * on this memory once realloc() is done, as it will have done
+ * a memcpy() into the new valid memory. We can't do this in
+ * reverse as that would be a real use-after-free.
+ */
+ _talloc_chunk_set_free(tc, NULL);
#if ALWAYS_REALLOC
if (pool_hdr) {
@@ -1896,7 +1943,7 @@ _PUBLIC_ void *_talloc_realloc(const void *context, void *ptr, size_t size, cons
if (new_chunk_size == old_chunk_size) {
TC_UNDEFINE_GROW_CHUNK(tc, size);
- tc->flags &= ~TALLOC_FLAG_FREE;
+ _talloc_chunk_set_not_free(tc);
tc->size = size;
return ptr;
}
@@ -1911,7 +1958,7 @@ _PUBLIC_ void *_talloc_realloc(const void *context, void *ptr, size_t size, cons
if (space_left >= space_needed) {
TC_UNDEFINE_GROW_CHUNK(tc, size);
- tc->flags &= ~TALLOC_FLAG_FREE;
+ _talloc_chunk_set_not_free(tc);
tc->size = size;
pool_hdr->end = tc_next_chunk(tc);
return ptr;
@@ -1941,12 +1988,24 @@ _PUBLIC_ void *_talloc_realloc(const void *context, void *ptr, size_t size, cons
got_new_ptr:
#endif
if (unlikely(!new_ptr)) {
- tc->flags &= ~TALLOC_FLAG_FREE;
+ /*
+ * Ok, this is a strange spot. We have to put back
+ * the old talloc_magic and any flags, except the
+ * TALLOC_FLAG_FREE as this was not free'ed by the
+ * realloc() call after all
+ */
+ _talloc_chunk_set_not_free(tc);
return NULL;
}
+ /*
+ * tc is now the new value from realloc(), the old memory we
+ * can't access any more and was preemptively marked as
+ * TALLOC_FLAG_FREE before the call. Now we mark it as not
+ * free again
+ */
tc = (struct talloc_chunk *)new_ptr;
- tc->flags &= ~TALLOC_FLAG_FREE;
+ _talloc_chunk_set_not_free(tc);
if (malloced) {
tc->flags &= ~TALLOC_FLAG_POOLMEM;
}
--
2.11.0
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