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

Luck, Tony
2021-01-11T22:21:05Z

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@intel.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