CVE-2026-74506: afs: Fix UAF when sending a message

In the Linux kernel, the following vulnerability has been resolved:

afs: Fix UAF when sending a message

In afsmakecall(), there's a race with async call reception and destruction. If a call is dispatched that doesn't have call->writeiter set (used to specify the data content for FS.StoreData), then the first rxrpckernelsenddata() will not set MSGMORE in the msghdr.

Once rxrpcsenddata() queues the last request packet, the response could come in at any time and cause the call to be completed and put. However, afsmakecall() will look at the call again to see it ->writeiter should be handled - something it's only allowed to do if it has its own ref on the call. Whilst this is the case for synchronous calls, it isn't true for async calls such as FS.FetchData.

There's also a potential UAF in afsmakecall() in the event that an asynchronous call is being sent, but the call fails in some way (e.g. it gets aborted from the server). The problem there is that afsmakecall() tries to abort a call if the rxrpc send fails, but the asynchronous notification from rxrpc may have caused the afscall to be torn down.

generic/650 plays games with randomly taking CPUs offline, and can interject a significant delay such that the call is deallocated before afsmakecall() gets to check call->writeiter - and a UAF ensues (caught by KASAN).

BUG: KASAN: slab-use-after-free in afsmakecall+0x1c90/0x2210 [kafs] Read of size 8 at addr ffff888035e050e8 by task fsstress/1409

Fix this by making afsmakeopcall() give the op->call its own ref rather than transferring the caller's ref to it and then dropping the ref when afsmakecall() returns.

This also means that the afsmakecall() func never loses its ref on the call now.