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Date:   Fri, 28 Aug 2020 22:51:13 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     "Eddy_Wu@trendmicro.com" <Eddy_Wu@trendmicro.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "rostedt@goodmis.org" <rostedt@goodmis.org>,
        "naveen.n.rao@linux.ibm.com" <naveen.n.rao@linux.ibm.com>,
        "anil.s.keshavamurthy@intel.com" <anil.s.keshavamurthy@intel.com>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        "cameron@moodycamel.com" <cameron@moodycamel.com>,
        "oleg@redhat.com" <oleg@redhat.com>,
        "will@kernel.org" <will@kernel.org>,
        "paulmck@kernel.org" <paulmck@kernel.org>
Subject: Re: [RFC][PATCH 3/7] kprobes: Remove kretprobe hash
Message-Id: <20200828225113.9541a5f67a3bcb17c4ce930c@kernel.org>
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        <20200827161754.359432340@infradead.org>
        <7df0a1af432040d9908517661c32dc34@trendmicro.com>
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On Fri, 28 Aug 2020 13:11:15 +0000
"Eddy_Wu@trendmicro.com" <Eddy_Wu@trendmicro.com> wrote:

> > -----Original Message-----
> > From: Peter Zijlstra <peterz@infradead.org>
> > Sent: Friday, August 28, 2020 12:13 AM
> > To: linux-kernel@vger.kernel.org; mhiramat@kernel.org
> > Cc: Eddy Wu (RD-TW) <Eddy_Wu@trendmicro.com>; x86@kernel.org; davem@davemloft.net; rostedt@goodmis.org;
> > naveen.n.rao@linux.ibm.com; anil.s.keshavamurthy@intel.com; linux-arch@vger.kernel.org; cameron@moodycamel.com;
> > oleg@redhat.com; will@kernel.org; paulmck@kernel.org; peterz@infradead.org
> > Subject: [RFC][PATCH 3/7] kprobes: Remove kretprobe hash
> >
> > @@ -1935,71 +1932,45 @@ unsigned long __kretprobe_trampoline_han
> >                                         unsigned long trampoline_address,
> >                                         void *frame_pointer)
> >  {
> > // ... removed
> > // NULL here
> > +       first = node = current->kretprobe_instances.first;
> > +       while (node) {
> > +               ri = container_of(node, struct kretprobe_instance, llist);
> >
> > -               orig_ret_address = (unsigned long)ri->ret_addr;
> > -               if (skipped)
> > -                       pr_warn("%ps must be blacklisted because of incorrect kretprobe order\n",
> > -                               ri->rp->kp.addr);
> > +               BUG_ON(ri->fp != frame_pointer);
> >
> > -               if (orig_ret_address != trampoline_address)
> > +               orig_ret_address = (unsigned long)ri->ret_addr;
> > +               if (orig_ret_address != trampoline_address) {
> >                         /*
> >                          * This is the real return address. Any other
> >                          * instances associated with this task are for
> >                          * other calls deeper on the call stack
> >                          */
> >                         break;
> > +               }
> > +
> > +               node = node->next;
> >         }
> >
> 
> Hi, I found a NULL pointer dereference here, where current->kretprobe_instances.first == NULL in these two scenario:

Thanks! that may be what I'm chasing.

> 
> 1) In task "rs:main Q:Reg"
> # insmod samples/kprobes/kretprobe_example.ko func=schedule
> # pkill sddm-greeter
> 
> 2) In task "llvmpipe-10"
> # insmod samples/kprobes/kretprobe_example.ko func=schedule
> login plasmashell session from sddm graphical interface

OK, schedule function will be the key. I guess the senario is..

1) kretporbe replace the return address with kretprobe_trampoline on task1's kernel stack
2) the task1 forks task2 before returning to the kretprobe_trampoline
3) while copying the process with the kernel stack, task2->kretprobe_instances.first = NULL
4) task2 returns to the kretprobe_trampoline
5) Bomb!

Hmm, we need to fixup the kernel stack when copying process. 

Thank you,

> 
> based on Masami's v2 + Peter's lockless patch, I'll try the new branch once I can compile kernel
> 
> Stacktrace may not be really useful here:
> [  402.008630] BUG: kernel NULL pointer dereference, address: 0000000000000018
> [  402.008633] #PF: supervisor read access in kernel mode
> [  402.008642] #PF: error_code(0x0000) - not-present page
> [  402.008644] PGD 0 P4D 0
> [  402.008646] Oops: 0000 [#1] PREEMPT SMP PTI
> [  402.008649] CPU: 7 PID: 1505 Comm: llvmpipe-10 Kdump: loaded Not tainted 5.9.0-rc2-00111-g72091ec08f03-dirty #45
> [  402.008650] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/29/2019
> [  402.008653] RIP: 0010:__kretprobe_trampoline_handler+0xb8/0x17f
> [  402.008655] Code: 65 4c 8b 34 25 80 6d 01 00 4c 89 e2 48 c7 c7 91 6b 85 91 49 8d b6 38 07 00 00 e8 d1 1a f9 ff 48 85 db 74 06 48 3b 5d d0 75 16 <49> 8b 75 18 48 c7 c7 a0 6c 85 91 48
>  8b 56 28 e8 b2 1a f9 ff 0f 0b
> [  402.008655] RSP: 0018:ffffab408147bde0 EFLAGS: 00010246
> [  402.008656] RAX: 0000000000000021 RBX: 0000000000000000 RCX: 0000000000000002
> [  402.008657] RDX: 0000000080000002 RSI: ffffffff9189757d RDI: 00000000ffffffff
> [  402.008658] RBP: ffffab408147be20 R08: 0000000000000001 R09: 000000000000955c
> [  402.008658] R10: 0000000000000004 R11: 0000000000000000 R12: 0000000000000000
> [  402.008659] R13: 0000000000000000 R14: ffff90736d305f40 R15: 0000000000000000
> [  402.008661] FS:  00007f20f6ffd700(0000) GS:ffff9073781c0000(0000) knlGS:0000000000000000
> [  402.008675] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  402.008678] CR2: 0000000000000018 CR3: 00000001ed256006 CR4: 00000000003706e0
> [  402.008684] Call Trace:
> [  402.008689]  ? elfcorehdr_read+0x40/0x40
> [  402.008690]  ? elfcorehdr_read+0x40/0x40
> [  402.008691]  trampoline_handler+0x42/0x60
> [  402.008692]  kretprobe_trampoline+0x2a/0x50
> [  402.008693] RIP: 0010:kretprobe_trampoline+0x0/0x50
> 
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-- 
Masami Hiramatsu <mhiramat@kernel.org>