linux-kernel - Re: [PATCH] kprobes, x86/alternatives: use text_mutex to protect smp_alt

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Date:   Sat, 28 Oct 2017 09:26:56 +0800
From:   zhouchengming <zhouchengming1@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Borislav Petkov <bp@...e.de>, <mhiramat@...nel.org>,
        <ananth@...ux.vnet.ibm.com>, <anil.s.keshavamurthy@...el.com>,
        <davem@...emloft.net>, <hpa@...or.com>, <tglx@...utronix.de>,
        <jkosina@...e.cz>, <rostedt@...dmis.org>, <mjurczyk@...gle.com>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kprobes, x86/alternatives: use text_mutex to protect
 smp_alt_modules

On 2017/10/27 22:15, Peter Zijlstra wrote:
> On Fri, Oct 27, 2017 at 02:33:48PM +0200, Borislav Petkov wrote:
>> On Fri, Oct 27, 2017 at 07:42:45PM +0800, zhouchengming wrote:
>>> This is a real bug happened on one of our machines, below is the calltrace.
>>> We can see the trigger is at alternatives_text_reserved+0x20/0x80, and
>>> encounter a deleted (poisoned) list_head.
>> Looks like some out-of-tree, old kernel thing. We don't have
>> mlx4_stats_sysfs_create() upstream and looking at the boot timestamps,
>> it could be that register_jprobe() is not ready yet.
>>
>> Looking at the Code, though:
>>
>>    20:   74 59                   je     0x7b
>>    22:   66 0f 1f 84 00 00 00    nopw   0x0(%rax,%rax,1)
>>    29:   00 00
>>    2b:*  48 3b 71 20             cmp    0x20(%rcx),%rsi<-- trapping instruction
>>    2f:   72 3a                   jb     0x6b
>>    31:   48 3b 79 28             cmp    0x28(%rcx),%rdi
>>    35:   77 34                   ja     0x6b
>>
>> %rcx is 0xdead0000000000d0 and that is POISON_POINTER_DELTA + 0xd0 so
>> that looks more like smp_alt_modules is not initialized yet but I could
>> could very well be wrong because this is an old kernel. So trigger that
>> with the upstream kernel without out of tree modules.
> Not to mention that we're about (or just have) yanked jprobes out of the
> kernel entirely.

Well... but this is a bug of alternatives_text_reserved(), it traverse the list without holding
the smp_alt mutex. So all users of it, like kprobes, will still have this problem. Maybe I could
think of a way to get rid of the mutex entirely.

Thanks.

> .
>