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

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Date:   Mon, 27 Aug 2018 16:25:43 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH] x86/nmi: Fix some races in NMI uaccess

On Mon, Aug 27, 2018 at 4:12 PM, Jann Horn <jannh@...gle.com> wrote:
> On Tue, Aug 28, 2018 at 1:04 AM Andy Lutomirski <luto@...nel.org> 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.