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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Nikolay Borisov <nborisov@suse.com>
Cc: Masami Hiramatsu <masami.hiramatsu@gmail.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	bpf@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()")
Date: Thu, 28 Jan 2021 10:50:14 -0600
Message-ID: <20210128165014.xc77qtun6fl2qfun@treble> (raw)
In-Reply-To: <eb1ec6a3-9e11-c769-84a4-228f23dc5e23@suse.com>

On Thu, Jan 28, 2021 at 06:45:56PM +0200, Nikolay Borisov wrote:
> On 28.01.21 г. 18:12 ч., Nikolay Borisov wrote:
> > On 28.01.21 г. 5:38 ч., Masami Hiramatsu wrote:
> >> Hi,
> > 
> > <snip>
> >>
> >> Alexei, could you tell me what is the concerning situation for bpf?
> > 
> > Another data point masami is that this affects bpf kprobes which are
> > entered via int3, alternatively if the kprobe is entered via
> > kprobe_ftrace_handler it works as expected. I haven't been able to
> > determine why a particular bpf probe won't use ftrace's infrastructure
> > if it's put at the beginning of the function.  An alternative call chain
> > is :
> > 
> >  => __ftrace_trace_stack
> >  => trace_call_bpf
> >  => kprobe_perf_func
> >  => kprobe_ftrace_handler
> >  => 0xffffffffc095d0c8
> >  => btrfs_validate_metadata_buffer
> >  => end_bio_extent_readpage
> >  => end_workqueue_fn
> >  => btrfs_work_helper
> >  => process_one_work
> >  => worker_thread
> >  => kthread
> >  => ret_from_fork
> > 
> >>
> 
> I have a working theory why I'm seeing this. My kernel (broken) was
> compiled with retpolines off and with the gcc that comes with ubuntu
> (both 9 and 10:
> gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
> gcc-10 (Ubuntu 10.2.0-5ubuntu1~20.04) 10.2.0
> )
> 
> this results in CFI being enabled so functions look like:
> 0xffffffff81493890 <+0>: endbr64
> 0xffffffff81493894 <+4>: callq  0xffffffff8104d820 <__fentry__>
> 
> i.e fentry's thunk is not the first instruction on the function hence
> it's not going through the optimized ftrace handler. Instead it's using
> int3 which is broken as ascertained.
> 
> After testing with my testcase I confirm that with cfi off and
> __fentry__ being the first entry bpf starts working. And indeed, even
> with CFI turned on if I use a probe like :
> 
> bpftrace -e 'kprobe:btrfs_sync_file+4 {printf("kprobe: %s\n",
> kstack());}' &>bpf-output &
> 
> 
> 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 ?

For now (and the foreseeable future), CET isn't enabled in the kernel.
So that endbr64 shouldn't be there in the first place.  I can make a
proper patch in a bit.

diff --git a/Makefile b/Makefile
index e0af7a4a5598..5ccc4cdf1fb5 100644
--- a/Makefile
+++ b/Makefile
@@ -948,11 +948,8 @@ KBUILD_CFLAGS   += $(call cc-option,-Werror=designated-init)
 # change __FILE__ to the relative path from the srctree
 KBUILD_CPPFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 
-# ensure -fcf-protection is disabled when using retpoline as it is
-# incompatible with -mindirect-branch=thunk-extern
-ifdef CONFIG_RETPOLINE
+# Intel CET isn't enabled in the kernel
 KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
-endif
 
 # include additional Makefiles when needed
 include-y			:= scripts/Makefile.extrawarn


  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 [this message]
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
     [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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