linux-kernel - Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery

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Message-ID: <YSJg/tCG5/YRSZIQ@zn.tnic>
Date:   Sun, 22 Aug 2021 16:36:46 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Jue Wang <juew@...gle.com>, Ding Hui <dinghui@...gfor.com.cn>,
        naoya.horiguchi@....com, osalvador@...e.de,
        Youquan Song <youquan.song@...el.com>, huangcun@...gfor.com.cn,
        x86@...nel.org, linux-edac@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user
 recovery

On Fri, Aug 20, 2021 at 01:23:46PM -0700, Luck, Tony wrote:
> To recover we need to have some other place to jump to (besides the
> normal extable error return ... which isn't working if we find ourselves
> in this situation) when we hit a fault covered by an extable entry. And
> also know how many machine checks is "normal" before taking the other path.

Hohumm, we're on the same page here.

...

> Bottom line is that I don't think this panic can actually happen unless
> there is some buggy kernel code that retries get_user() or copyin()
> indefinitely.

You know how such statements of "well, this should not really happen in
practice" get disproved by, well, practice. :-)

I guess we'll see anyway what actually happens in practice.

> Probably the same for the two different addresses case ... though I'm
> not 100% confident about that. There could be some ioctl() that peeks
> at two parts of a passed in structure, and the user might pass in a
> structure that spans across a page boundary with both pages poisoned.
> But that would only hit if the driver code ignored the failure of the
> first get_user() and blindly tried the second. So I'd count that as a
> critically bad driver bug.

Right.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette