linux-kernel - RE: [RFC 0/4] Fix machine check recovery for copy_from

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Date:   Thu, 8 Apr 2021 14:39:09 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Aili Yao <yaoaili@...gsoft.com>, Borislav Petkov <bp@...en8.de>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andy Lutomirski <luto@...nel.org>,
        HORIGUCHI NAOYA( 堀口　直也) 
        <naoya.horiguchi@....com>
Subject: RE: [RFC 0/4] Fix machine check recovery for copy_from_user

> I have one scenario, may you take into account:
>
> If one copyin case occurs, write() returned by your patch, the user process may
> check the return values, for errors, it may exit the process, then the error page
> will be freed, and then the page maybe alloced to other process or to kernel itself,
> then code will initialize it and this will trigger one SRAO, if it's used by kernel,
> we may do nothing for this, and kernel may still touch it, and lead to one panic.

In this case kill_me_never() calls memory_failure() with flags == 0. I think (hope!)
that means that it will unmap the page from the task, but will not send a signal.

When the task exits the PTE for this page has the swap/poison signature, so the
page is not freed for re-use.

-Tony