bpf: Do not allow deleting local storage in NMI

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

bpf: Do not allow deleting local storage in NMI

Currently, local storage may deadlock when deferring freeing selem or

local storage through kfree_rcu(), call_rcu() or call_rcu_tasks_trace()

in NMI or reentrant. Since deleting selem in NMI is an unlikely use

case, partially mitigate it by returning error when calling from

bpf_xxx_storage_delete() helpers in NMI. Note that, it is still possible

to deadlock through reentrant. A full mitigation requires returning

error when irqs_disabled() is true, which, however is too heavy-handed

for bpf_xxx_storage_delete().

The long-term solution requires _nolock versions of call_rcu. Another

possible solution is to defer the free through irq_work [0], but it

would grow the size of selem, which is non-ideal.

The check is only needed in bpf_selem_unlink(), which is used by helpers

and syscalls. bpf_selem_unlink_nofail() is fine as it is called during

map and owner tear down that never run in NMI or reentrant.

[0] https://lore.kernel.org/bpf/[email protected]/