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

[Andy Lutomirski <luto@kernel.org>]

On Tue, Jan 12, 2021 at 9:16 AM Luck, Tony <tony.luck@intel.com> wrote:
>
> On Tue, Jan 12, 2021 at 09:00:14AM -0800, Andy Lutomirski wrote:
> > > On Jan 11, 2021, at 2:21 PM, Luck, Tony <tony.luck@intel.com> wrote:
> > >
> > > 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);
> >
> > I have very mixed feelings as to whether we should even try to recover
> > from the first failure like this.  If we actually want to try to
> > recover, perhaps we should instead arrange for the second MCE to
> > recover successfully instead of panicking.
>
> Well we obviously have to "recover" from the first machine check
> in order to get to the second. Are you saying you don't like the
> different return value from get_user()?
>
> In my initial playing around with this I just had the second machine
> check simply skip the task_work_add(). This worked for this case, but
> only because there wasn't a third, fourth, etc. access to the poisoned
> data. If the caller keeps peeking, then we'll keep taking more machine
> checks - possibly forever.
>
> Even if we do recover with just one extra machine check ... that's one
> more than was necessary.

Well, we need to do *something* when the first __get_user() trips the
#MC.  It would be nice if we could actually fix up the page tables
inside the #MC handler, but, if we're in a pagefault_disable() context
we might have locks held.  Heck, we could have the pagetable lock
held, be inside NMI, etc.  Skipping the task_work_add() might actually
make sense if we get a second one.

We won't actually infinite loop in pagefault_disable() context -- if
we would, then we would also infinite loop just from a regular page
fault, too.