linux-kernel - Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <20210203085232.402e2e35@oasis.local.home>
Date:   Wed, 3 Feb 2021 08:52:32 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Nikolay Borisov <nborisov@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter()
 with nmi_enter()")

On Wed, 3 Feb 2021 22:33:28 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Ah, that is what I worried about. ftrace and kprobes handler usually want to
> know "what is the actual status of the system where the probe hits".
> 
> If the new kernel_exception_enter() for ftrace/kprobes or any other kernel
> instrumention does
> 
>   __preempt_count_add(KEX_OFFSET + NMI_OFFSET + HARDIRQ_OFFSET);
> 
> And we can distinguish the KEX from NMI, and get the original status of the context.
> What would you think about?

Oh, that reminds me about the obvious difference between an NMI and a
ftrace handler. A ftrace handler doesn't disable interrupts nor
preemption. Thus, if you set "in_nmi" to a ftrace handler, and an
interrupt (or NMI) comes in, then any ftrace handlers called by the
interrupt / NMI will be ignored, since it will think it is recursing
from NMI context.

-- Steve