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

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Date:   Mon, 11 Jan 2021 14:20:57 -0800
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user
 recovery

On Mon, Jan 11, 2021 at 02:11:56PM -0800, Andy Lutomirski wrote:
> 
> > On Jan 11, 2021, at 1:45 PM, Tony Luck <tony.luck@...el.com> wrote:
> > 
> > Recovery action when get_user() triggers a machine check uses the fixup
> > path to make get_user() return -EFAULT.  Also queue_task_work() sets up
> > so that kill_me_maybe() will be called on return to user mode to send a
> > SIGBUS to the current process.
> > 
> > But there are places in the kernel where the code assumes that this
> > EFAULT return was simply because of a page fault. The code takes some
> > action to fix that, and then retries the access. This results in a second
> > machine check.
> > 
> > While processing this second machine check queue_task_work() is called
> > again. But since this uses the same callback_head structure that
> > was used in the first call, the net result is an entry on the
> > current->task_works list that points to itself.
> 
> Is this happening in pagefault_disable context or normal sleepable fault context?  If the latter, maybe we should reconsider finding a way for the machine check code to do its work inline instead of deferring it.

The first machine check is in pagefault_disable() context.

static int get_futex_value_locked(u32 *dest, u32 __user *from)
{
        int ret;

        pagefault_disable();
        ret = __get_user(*dest, from);
        pagefault_enable();

        return (ret == -ENXIO) ? ret : ret ? -EFAULT : 0;
}

-Tony