[PATCH v2 2/3] Drivers: hv: Add vmbus_leak_buffer()

[Michael Kelley <mhkelley58@gmail.com>]

If an error case needs to leak the buffer memory allocated by
vmbus_alloc_buffer(), doing so requires knowledge of how
vmbus_free_buffer() works. In a CoCo VM buffers are allocated
differently from a normal VM, and vmbus_free_buffer() handles
the difference.

Encapsulate this knowledge in a new function, vmbus_leak_buffer(),
that error paths can call. After calling vmbus_leak_buffer(), a
subsequent call to vmbus_free_buffer() frees the additional
resources used in the CoCo VM case but does not free the actual
buffer memory. As such, vmbus_leak_buffer() is callable in a
context where accesses to the buffer memory may be in flight.

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
---
 drivers/hv/channel.c   | 26 ++++++++++++++++++++++++++
 include/linux/hyperv.h |  4 ++++
 2 files changed, 30 insertions(+)

diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c
index cc86e8505ad0..389b27cb038d 100644
--- a/drivers/hv/channel.c
+++ b/drivers/hv/channel.c
@@ -648,6 +648,32 @@ void vmbus_free_buffer(void *addr, struct page **chunks, u32 chunk_cnt)
 }
 EXPORT_SYMBOL_GPL(vmbus_free_buffer);
 
+/**
+ * vmbus_leak_buffer - set up a buffer to be leaked by vmbus_free_buffer().
+ *
+ * @addr: buffer address
+ * @chunks: chunks array from vmbus_alloc_buffer()
+ * @chunk_cnt: number of entries in @chunks
+ *
+ * When @chunks is NULL the buffer is a plain vzalloc() allocation and
+ * the buffer is leaked by setting @addr to NULL. Otherwise set
+ * @chunk_cnt to 0 so that vmbus_free_buffer() does not try to re-encrypt
+ * or free the buffer memory, but still releases the vmap address and
+ * the chunks memory.
+ *
+ * This function may be called in a context where the buffer is still
+ * being accessed. It must not remove any kernel virtual addresses of
+ * the buffer or change its encryption status.
+ */
+void vmbus_leak_buffer(void **addr, struct page ***chunks, u32 *chunk_cnt)
+{
+	if (*chunks)
+		*chunk_cnt = 0;
+	else
+		*addr = NULL;
+}
+EXPORT_SYMBOL_GPL(vmbus_leak_buffer);
+
 /**
  * vmbus_alloc_buffer - allocate a host-visible, virtually-contiguous buffer.
  *
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 9e109d91aa14..6f60ce5924ba 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -1220,6 +1220,10 @@ extern void *vmbus_alloc_buffer(struct vmbus_channel *channel,
 
 extern void vmbus_free_buffer(void *addr, struct page **chunks, u32 chunk_cnt);
 
+extern void vmbus_leak_buffer(void **addr,
+			      struct page ***chunks,
+			      u32 *chunk_cnt);
+
 void vmbus_reset_channel_cb(struct vmbus_channel *channel);
 
 extern int vmbus_recvpacket(struct vmbus_channel *channel,
-- 
2.25.1