From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sun, 14 Apr 2019 22:09:40 +0000 (-0700)
Subject: Merge branch 'page-refs' (page ref overflow)
X-Git-Url: http://git.cdn.openwrt.org/?a=commitdiff_plain;h=6b3a707736301c2128ca85ce85fb13f60b5e350a;p=openwrt%2Fstaging%2Fblogic.git

Merge branch 'page-refs' (page ref overflow)

Merge page ref overflow branch.

Jann Horn reported that he can overflow the page ref count with
sufficient memory (and a filesystem that is intentionally extremely
slow).

Admittedly it's not exactly easy.  To have more than four billion
references to a page requires a minimum of 32GB of kernel memory just
for the pointers to the pages, much less any metadata to keep track of
those pointers.  Jann needed a total of 140GB of memory and a specially
crafted filesystem that leaves all reads pending (in order to not ever
free the page references and just keep adding more).

Still, we have a fairly straightforward way to limit the two obvious
user-controllable sources of page references: direct-IO like page
references gotten through get_user_pages(), and the splice pipe page
duplication.  So let's just do that.

* branch page-refs:
  fs: prevent page refcount overflow in pipe_buf_get
  mm: prevent get_user_pages() from overflowing page refcount
  mm: add 'try_get_page()' helper function
  mm: make page ref count overflow check tighter and more explicit
---

6b3a707736301c2128ca85ce85fb13f60b5e350a