BUG/MINOR: buffers/threads: always clear a buffer's head before releasing it
A small race exists in buffers with "show sess all". This one wants to show
some information grabbed from the buffer (especially in HTX mode). But the
thread owning this buffer might just be releasing its area, right after a
free() or munmap() call, resulting in a head that is not seen as empty yet
though the area was released. It may then be dereferenced by "show sess all"
causing a crash. Note that in practice it only happens in debug mode with
UAF enabled, but it's tricky enough to fix it right now.
This should be backported to stable versions which support threads and a
store barrier. It's worth noting that by performing the clearing first,
b_free() and b_drop() now become two exact equivalent.
(cherry picked from commit 3b091f80aa66d538ea65a1fd7e60acfb37c5421a)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/include/common/buffer.h b/include/common/buffer.h
index 3d6c97c..34a3e95 100644
--- a/include/common/buffer.h
+++ b/include/common/buffer.h
@@ -109,13 +109,22 @@
return buf;
}
-/* Releases buffer <buf> (no check of emptiness) */
+/* Releases buffer <buf> (no check of emptiness). The buffer's head is marked
+ * empty.
+ */
static inline void __b_drop(struct buffer *buf)
{
- pool_free(pool_head_buffer, buf->area);
+ char *area = buf->area;
+
+ /* let's first clear the area to save an occasional "show sess all"
+ * glancing over our shoulder from getting a dangling pointer.
+ */
+ *buf = BUF_NULL;
+ __ha_barrier_store();
+ pool_free(pool_head_buffer, area);
}
-/* Releases buffer <buf> if allocated. */
+/* Releases buffer <buf> if allocated, and marks it empty. */
static inline void b_drop(struct buffer *buf)
{
if (buf->size)
@@ -126,7 +135,6 @@
static inline void b_free(struct buffer *buf)
{
b_drop(buf);
- *buf = BUF_NULL;
}
/* Ensures that <buf> is allocated. If an allocation is needed, it ensures that