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From: Nikolay Borisov <nborisov@suse.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>
Cc: Masami Hiramatsu <masami.hiramatsu@gmail.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	bpf@vger.kernel.org, Josh Poimboeuf <jpoimboe@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>
Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()")
Date: Fri, 29 Jan 2021 08:36:30 +0200
Message-ID: <783eafee-681c-45b6-17ef-24473cb33aa1@suse.com> (raw)
In-Reply-To: <20210129013452.njuh3fomws62m4rc@ast-mbp.dhcp.thefacebook.com>



On 29.01.21 г. 3:34 ч., Alexei Starovoitov wrote:
> On Thu, Jan 28, 2021 at 07:24:14PM +0100, Peter Zijlstra wrote:
>> On Thu, Jan 28, 2021 at 06:45:56PM +0200, Nikolay Borisov wrote:
>>> it would be placed on the __fentry__ (and not endbr64) hence it works.
>>> So perhaps a workaround outside of bpf could essentially detect this
>>> scenario and adjust the probe to be on the __fentry__ and not preceding
>>> instruction if it's detected to be endbr64 ?
>>
>> Arguably the fentry handler should also set the nmi context, it can,
>> after all, interrupt pretty much any other context by construction.
> 
> But that doesn't make it a real nmi.
> nmi can actually interrupt anything. Whereas kprobe via int3 cannot
> do nokprobe and noinstr sections. The exposure is a lot different.
> ftrace is even more contained. It's only at the start of the functions.
> It's even smaller subset of places than kprobes.
> So ftrace < kprobe < nmi.
> Grouping them all into nmi doesn't make sense to me.
> That bpf breaking change came unnoticed mostly because people use
> kprobes in the beginning of the functions only, but there are cases
> where kprobes are in the middle too. People just didn't upgrade kernels
> fast enough to notice.

nit: slight correction - I observed while I was putting kprobes at the
beginning of the function but __fentry__ wasn't the first thing in the
function's code. The reason why people haven't observed is because
everyone is running with retpolines enabled which disables CFI/CET.

> imo appropriate solution would be to have some distinction between
> ftrace < kprobe_via_int3 < nmi, so that bpf side can react accordingly.
> That code in trace_call_bpf:
>   if (in_nmi()) /* not supported yet */
> was necessary in the past. Now we have all sorts of protections built in.
> So it's probably ok to just drop it, but I think adding
> called_via_ftrace vs called_via_kprobe_int3 vs called_via_nmi
> is more appropriate solution long term.
> 

  reply index

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <25cd2608-03c2-94b8-7760-9de9935fde64@suse.com>
     [not found] ` <20210128001353.66e7171b395473ef992d6991@kernel.org>
     [not found]   ` <20210128002452.a79714c236b69ab9acfa986c@kernel.org>
     [not found]     ` <a35a6f15-9ab1-917c-d443-23d3e78f2d73@suse.com>
     [not found]       ` <20210128103415.d90be51ec607bb6123b2843c@kernel.org>
2021-01-28  3:38         ` Masami Hiramatsu
2021-01-28  7:11           ` Nikolay Borisov
2021-01-28 16:12           ` Nikolay Borisov
2021-01-28 16:45             ` Nikolay Borisov
2021-01-28 16:50               ` Josh Poimboeuf
2021-01-28 21:52                 ` [PATCH] x86: Disable CET instrumentation in the kernel Josh Poimboeuf
2021-01-29  6:23                   ` Nikolay Borisov
2021-01-29 10:21                   ` Borislav Petkov
     [not found]                     ` <20210129151034.iba4eaa2fuxsipqa@treble>
2021-01-29 16:30                       ` Borislav Petkov
2021-01-29 16:49                         ` Josh Poimboeuf
2021-01-29 16:54                           ` Nikolay Borisov
2021-01-29 17:03                             ` Josh Poimboeuf
2021-01-29 17:07                               ` Borislav Petkov
2021-01-29 17:58                                 ` Seth Forshee
2021-01-28 18:24               ` kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()") Peter Zijlstra
2021-01-29  1:34                 ` Alexei Starovoitov
2021-01-29  6:36                   ` Nikolay Borisov [this message]
     [not found]                   ` <YBPNyRyrkzw2echi@hirez.programming.kicks-ass.net>
     [not found]                     ` <20210129224011.81bcdb3eba1227c414e69e1f@kernel.org>
     [not found]                       ` <20210129105952.74dc8464@gandalf.local.home>
2021-01-29 16:24                         ` Peter Zijlstra
2021-01-29 17:45                           ` Alexei Starovoitov
2021-01-29 17:59                             ` Peter Zijlstra
2021-01-29 19:01                               ` Steven Rostedt
2021-01-29 21:05                                 ` Alexei Starovoitov
2021-01-30  1:41                                   ` Masami Hiramatsu
2021-01-29 21:24                                 ` Steven Rostedt
2021-01-30  8:28                                   ` Peter Zijlstra
2021-01-30 12:44                                     ` Steven Rostedt
2021-02-02 10:45                                       ` Peter Zijlstra
2021-02-02 14:52                                         ` Steven Rostedt
2021-02-02 16:45                                           ` Peter Zijlstra
2021-02-02 16:56                                             ` Steven Rostedt
2021-02-02 18:30                                               ` Peter Zijlstra
2021-02-02 21:05                                                 ` Steven Rostedt
2021-02-03 13:33                                                   ` Masami Hiramatsu
2021-02-03 13:52                                                     ` Steven Rostedt
2021-01-30  2:02                               ` Masami Hiramatsu
2021-01-30  3:08                                 ` Alexei Starovoitov
2021-01-30 12:10                                   ` Masami Hiramatsu

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