Drivers: hv: vmbus: Track decrypted status in vmbus_gpadl

In CoCo VMs it is possible for the untrusted host to cause
set_memory_encrypted() or set_memory_decrypted() to fail such that an
error is returned and the resulting memory is shared. Callers need to
take care to handle these errors to avoid returning decrypted (shared)
memory to the page allocator, which could lead to functional or security
issues.

In order to make sure callers of vmbus_establish_gpadl() and
vmbus_teardown_gpadl() don't return decrypted/shared pages to
allocators, add a field in struct vmbus_gpadl to keep track of the
decryption status of the buffers. This will allow the callers to
know if they should free or leak the pages.
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/r/20240311161558.1310-3-mhklinux@outlook.comSigned-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20240311161558.1310-3-mhklinux@outlook.com>

drivers/hv/channel.c

@@ -436,9 +436,18 @@ static int __vmbus_establish_gpadl(struct vmbus_channel *channel,
 		(atomic_inc_return(&vmbus_connection.next_gpadl_handle) - 1);
 
 	ret = create_gpadl_header(type, kbuffer, size, send_offset, &msginfo);
-	if (ret)
+	if (ret) {
+		gpadl->decrypted = false;
 		return ret;
+	}
 
+	/*
+	 * Set the "decrypted" flag to true for the set_memory_decrypted()
+	 * success case. In the failure case, the encryption state of the
+	 * memory is unknown. Leave "decrypted" as true to ensure the
+	 * memory will be leaked instead of going back on the free list.
+	 */
+	gpadl->decrypted = true;
 	ret = set_memory_decrypted((unsigned long)kbuffer,
 				   PFN_UP(size));
 	if (ret) {
@@ -527,9 +536,15 @@ static int __vmbus_establish_gpadl(struct vmbus_channel *channel,
 
 	kfree(msginfo);
 
-	if (ret)
-		set_memory_encrypted((unsigned long)kbuffer,
-				     PFN_UP(size));
+	if (ret) {
+		/*
+		 * If set_memory_encrypted() fails, the decrypted flag is
+		 * left as true so the memory is leaked instead of being
+		 * put back on the free list.
+		 */
+		if (!set_memory_encrypted((unsigned long)kbuffer, PFN_UP(size)))
+			gpadl->decrypted = false;
+	}
 
 	return ret;
 }
@@ -850,6 +865,8 @@ int vmbus_teardown_gpadl(struct vmbus_channel *channel, struct vmbus_gpadl *gpad
 	if (ret)
 		pr_warn("Fail to set mem host visibility in GPADL teardown %d.\n", ret);
 
+	gpadl->decrypted = ret;
+
 	return ret;
 }
 EXPORT_SYMBOL_GPL(vmbus_teardown_gpadl);

include/linux/hyperv.h

@@ -832,6 +832,7 @@ struct vmbus_gpadl {
 	u32 gpadl_handle;
 	u32 size;
 	void *buffer;
+	bool decrypted;
 };
 
 struct vmbus_channel {