linux-kernel - Re: [PATCH] x86/alternatives: check int3 breakpoint physical addresses

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Date:   Mon, 11 Feb 2019 10:57:23 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>
Subject: Re: [PATCH] x86/alternatives: check int3 breakpoint physical
 addresses


On 02/11/2019 10:15 AM, Thomas Gleixner wrote:
> On Mon, 11 Feb 2019, Alexandre Chartre wrote:
>> On 02/10/2019 10:23 PM, Thomas Gleixner wrote:
>>> On Fri, 25 Jan 2019, Alexandre Chartre wrote:
>>>> Note that this issue has been observed and reproduced with a custom kernel
>>>> with some code mapped to different virtual addresses and using jump labels
>>>> As jump labels use text_poke_bp(), crashes were sometimes observed when
>>>> updating jump labels.
>>>
>>> Rightfully so. text_poke_bp() pokes at the kernels text mapping which is
>>> very well defined and unique. Why would you map the same text to different
>>> virtual addresses and then use a randomly chosen one to poke at it?
>>>
>>
>> As an example, we used to have per-CPU SYSCALL entry trampoline [1] where the
>> entry_SYSCALL_64_trampoline function was mapped to a different virtual address
>> for each CPU. So, a syscall would execute entry_SYSCALL_64_trampoline() from a
>> different virtual address depending on the CPU being used. With that code,
>> adding a jump label in entry_SYSCALL_64_trampoline() causes the described
>> issue.
>
> Right, but we knew that and there was no reason to put a jump label into
> that. AFAICT we don't have anything like this in the kernel.

In our particular case, we have introduced a jump label in JMP_NOSPEC (which
is used by entry_SYSCALL_64_trampoline()) to have the option to dynamically
enable/disable retpoline at runtime. So that's when we faced this issue.

> That said, I'm not opposed to apply the patch as is, I just wanted to get a
> better understanding about the background.

Sure. I am aware this is a corner case, and I was precisely looking for feedback
to check if it is worth fixing that issue. So I appreciate your reply, and I would
understand if the patch is rejected because that's a case that we are just not
expecting to happen.

Thanks,

alex.