On Fri, 14 Dec 2012, Konstantin Khlebnikov wrote:
> Content of non-uptodate pages completely random, we cannot expose them into
> userspace. This leads to information leak and will crash userspace for sure.
Good find, yes, it's very wrong as is. But, sorry, I don't like your fix
- better than ignoring the issue as at present, but not the right answer.
> Probably we can reuse hwpoison entries here, but tmpfs already too complex.
HWpoison entries? They're for when that page of RAM is bad, but this is
quite a different case: the page is fine and can perfectly well be freed
and reused - what's bad is the data currently in it.
>
> Signed-off-by: Konstantin Khlebnikov <khlebni...@openvz.org>
> Original-patch-by: Alexey Kuznetsov <kuz...@ms2.inr.ac.ru>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Hugh Dickins <hu...@google.com>
> Cc: Andi Kleen <a...@firstfloor.org>
> ---
> mm/swapfile.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index e97a0e5..98fc2fd 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1127,6 +1127,22 @@ int try_to_unuse(unsigned int type, bool frontswap,
> wait_on_page_writeback(page);
>
> /*
> + * If read failed we cannot map not-uptodate page to
> + * user space. Actually, we are in serious troubles,
> + * we do not even know what process to kill. So, the only
try_to_unuse() is all about locating exactly where this page belongs;
and if the user is lucky, the page in question won't even be needed again
before the process exits, so nothing should be killed at this point.
> + * variant remains: to stop swapoff() and allow someone
> + * to kill processes to zap invalid pages.
No, we should not abort swapoff: there's every reason to continue,
to make sure that this unreliable area can be taken out of service.
> + *
> + * TODO replace page with hwpoison entry in pte and shmem.
Instead of blindly going ahead and inserting ptes pointing to the
!PageUptodate page, unuse_pte() and shmem_unuse_inode() should insert
a substitute bad swapentry, to generate SIGBUS if it's accessed.
swp_entry(1, 0) might serve, but there's probably a few mods needed
here and there; and getting the details right (e.g. memcg charges)
will need care.
Not as straightforward as your block below, I admit. I wonder if you
posted that just to stir me to do better: or can you take it further?
Thanks,
Hugh
> + */
> + if (unlikely(!PageUptodate(page))) {
> + unlock_page(page);
> + page_cache_release(page);
> + retval = -EIO;
> + break;
> + }
> +
> + /*
> * Remove all references to entry.
> */
> swcount = *swap_map;
--
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