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.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13211

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>

diff --git a/lib/talloc/talloc.c b/lib/talloc/talloc.c
index 3569ba759290871f16df0dd64be3165f50a6ddd5..cd159ef89c2eae848ca531cc210585133795b8e6 100644 (file)
--- 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))
@@ -445,13 +488,14 @@ 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_unknown_value();
+               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 +981,7 @@ 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 +1131,7 @@ 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 +1828,22 @@ _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 +1942,7 @@ _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 +1957,7 @@ _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 +1987,24 @@ _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;
        }