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

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Date:   Sat, 21 Aug 2021 21:51:11 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Tony Luck <tony.luck@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, Jue Wang <juew@...gle.com>,
        Ding Hui <dinghui@...gfor.com.cn>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>, Oscar Salvador <osalvador@...e.de>,
        Youquan Song <youquan.song@...el.com>, huangcun@...gfor.com.cn,
        X86-ML <x86@...nel.org>,
        Linux Edac Mailing List <linux-edac@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <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 09:51:41PM -0700, Tony Luck wrote:
> On Fri, Aug 20, 2021 at 1:25 PM Luck, Tony <tony.luck@...el.com> wrote:
> > 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.
> 
> Or maybe driver writers are just evil :-(
> 
> for (i = 0; i < len; i++) {
>        tx_wait(10);
>        get_user(dsp56k_host_interface.data.b[1], bin++);
>        get_user(dsp56k_host_interface.data.b[2], bin++);
>        get_user(dsp56k_host_interface.data.b[3], bin++);
> }

Almost any unchecked get_user()/put_user() is a bug.  Fortunately, there's
not a lot of them
<greps>
93 for put_user() and 73 for get_user().  _Some_ of the former variety might
be legitimate, but most should be taken out and shot.

And dsp56k should be taken out and shot, period ;-/  This is far from the
worst in there...