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

[Andrew Morton]

On Mon,  7 Dec 2020 10:48:18 +0100 Oscar Salvador <osalvador@xxxxxxx> 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?