[PATCH V2] vmci: Prevent the dispatching of uninitialized payloads

[Lizhi Xu <lizhi.xu@windriver.com>]

The reproducer executes the host's unlocked_ioctl call in two different
tasks. When init_context fails, the struct vmci_event_ctx is not fully
initialized when executing vmci_datagram_dispatch() to send events to all
vm contexts. This affects the datagram taken from the datagram queue of
its context by another task, because the datagram payload is not initialized
according to the size payload_size, which causes the kernel data to leak
to the user space.

Before dispatching the datagram, and before setting the payload content,
explicitly set the payload content to 0 to avoid data leakage caused by
incomplete payload initialization.

To avoid the oob check failure when executing __compiletime_lessthan()
in memset(), directly use the address of the vmci_event_ctx instance ev
to replace ev.msg.hdr, because their addresses are the same.

Fixes: 28d6692cd8fb ("VMCI: context implementation.")
Reported-by: syzbot+9b9124ae9b12d5af5d95@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9b9124ae9b12d5af5d95
Tested-by: syzbot+9b9124ae9b12d5af5d95@syzkaller.appspotmail.com
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
---
V1 -> V2: fix building warning reported by Stephen Rothwell

 drivers/misc/vmw_vmci/vmci_context.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/misc/vmw_vmci/vmci_context.c b/drivers/misc/vmw_vmci/vmci_context.c
index f22b44827e92..d566103caa27 100644
--- a/drivers/misc/vmw_vmci/vmci_context.c
+++ b/drivers/misc/vmw_vmci/vmci_context.c
@@ -251,6 +251,8 @@ static int ctx_fire_notification(u32 context_id, u32 priv_flags)
 		ev.msg.hdr.src = vmci_make_handle(VMCI_HYPERVISOR_CONTEXT_ID,
 						  VMCI_CONTEXT_RESOURCE_ID);
 		ev.msg.hdr.payload_size = sizeof(ev) - sizeof(ev.msg.hdr);
+		memset((char*)&ev + sizeof(ev.msg.hdr), 0,
+			ev.msg.hdr.payload_size);
 		ev.msg.event_data.event = VMCI_EVENT_CTX_REMOVED;
 		ev.payload.context_id = context_id;
 
-- 
2.43.0