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

smack: fix cred UAF in smack file send sigiotask()

When inspecting the credentials of another task, objective credentials (->real cred, accessed with task cred()) must always be used.

Accessing ->cred on a non-current task is forbidden unless that task is being created or destroyed; a task is allowed to change its own ->cred pointer with no synchronization, and changing ->cred should only affect the current syscall.

smack file send sigiotask() was accessing both sets of credentials: First tsk->cred, then task cred(tsk).

Fix it, always access the objective credentials here.

I have tested that this bug can lead to a KASAN-reported UAF of struct cred in smack file send sigiotask(), and that this fix prevents the race.