Re: [PATCH] x86/nmi: Fix some races in NMI uaccess

From: Andy Lutomirski
Date: Mon Aug 27 2018 - 19:26:08 EST

On Mon, Aug 27, 2018 at 4:12 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> On Tue, Aug 28, 2018 at 1:04 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> In NMI context, we might be in the middle of context switching or in
>> the middle of switch_mm_irqs_off(). In either case, CR3 might not
>> match current->mm, which could cause copy_from_user_nmi() and
>> friends to read the wrong memory.
>> Fix it by adding a new nmi_uaccess_okay() helper and checking it in
>> copy_from_user_nmi() and in __copy_from_user_nmi()'s callers.
> What about eBPF probes (which I think can be attached to kprobe points
> / tracepoints / perf events) that perform userspace reads / userspace
> writes / kernel reads? Can those run in NMI context, and if so, do
> they also need special handling?

I assume they can run in NMI context, which might be problematic in
and of themselves. For example, does BPF adequately protect against a
BPF program accessing a map while bpf(2) is modifying it? It seems
like bpf_prog_active is intended to serve this purpose.

But I don't see any obvious mechanism for eBPF programs to read user memory.