Re: [PATCH] mm,memory_failure: Always pin the page in madvise_inject_error

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Mon, Dec 07, 2020 at 06:22:00PM -0800, Andrew Morton wrote:
> On Mon,  7 Dec 2020 10:48:18 +0100 Oscar Salvador <osalvador@suse.de> wrote:
> 
> > madvise_inject_error() uses get_user_pages_fast to translate the
> > address we specified to a page.
> > After [1], we drop the extra reference count for memory_failure() path.
> > That commit says that memory_failure wanted to keep the pin in order
> > to take the page out of circulation.
> > 
> > The truth is that we need to keep the page pinned, otherwise the
> > page might be re-used after the put_page() and we can end up messing
> > with someone else's memory.
> > 
> > E.g:
> > 
> > CPU0
> > process X					CPU1
> >  madvise_inject_error
> >   get_user_pages
> >    put_page
> > 					page gets reclaimed
> > 					process Y allocates the page
> >   memory_failure
> >    // We mess with process Y memory
> > 
> > madvise() is meant to operate on a self address space, so messing with
> > pages that do not belong to us seems the wrong thing to do.
> > To avoid that, let us keep the page pinned for memory_failure as well.
> > 
> > Pages for DAX mappings will release this extra refcount in
> > memory_failure_dev_pagemap.
> 
> Does the bug have any known user-visible effects?  Is a deliberate
> exploit conceivable?
> 
> IOW, cc:stable and if so, why?

This interface is a testing feature and only available only for privileged
(CAP_SYS_ADMIN) users, so I don't think that this bug is critical. But if
someone think it need to go to stable, I'm fine with that.

Thanks,
Naoya Horiguchi