[RFC v2 00/10] Hardening page _refcount

[Pasha Tatashin]

From: Pasha Tatashin <tatashin@xxxxxxxxxx>

Changelog:
- use the "fetch" variant instead of "return" of atomic instructions
- allow negative values, as we are using all 32-bits of _refcount.


It is hard to root cause _refcount problems, because they usually
manifest after the damage has occurred.  Yet, they can lead to
catastrophic failures such memory corruptions. There were a number
of refcount related issues discovered recently [1], [2], [3].

Improve debugability by adding more checks that ensure that
page->_refcount never turns negative (i.e. double free does not
happen, or free after freeze etc).

- Check for overflow and underflow right from the functions that
  modify _refcount
- Remove set_page_count(), so we do not unconditionally overwrite
  _refcount with an unrestrained value
- Trace return values in all functions that modify _refcount

Applies against next-20211116.

Previous verions:
RFCv1: https://lore.kernel.org/all/20211026173822.502506-1-pasha.tatashin@xxxxxxxxxx

[1] https://lore.kernel.org/all/xr9335nxwc5y.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
[2] https://lore.kernel.org/all/1582661774-30925-2-git-send-email-akaher@xxxxxxxxxx
[3] https://lore.kernel.org/all/20210622021423.154662-3-mike.kravetz@xxxxxxxxxx

Pasha Tatashin (10):
  mm: page_ref_add_unless() does not trace 'u' argument
  mm: add overflow and underflow checks for page->_refcount
  mm: Avoid using set_page_count() in set_page_recounted()
  mm: remove set_page_count() from page_frag_alloc_align
  mm: avoid using set_page_count() when pages are freed into allocator
  mm: rename init_page_count() -> page_ref_init()
  mm: remove set_page_count()
  mm: simplify page_ref_* functions
  mm: do not use atomic_set_release in page_ref_unfreeze()
  mm: use atomic_cmpxchg_acquire in page_ref_freeze().

 arch/m68k/mm/motorola.c         |   2 +-
 include/linux/mm.h              |   2 +-
 include/linux/page_ref.h        | 159 +++++++++++++++-----------------
 include/trace/events/page_ref.h |  99 +++++++++++++++-----
 mm/debug_page_ref.c             |  30 ++----
 mm/internal.h                   |   6 +-
 mm/page_alloc.c                 |  19 ++--
 7 files changed, 180 insertions(+), 137 deletions(-)

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2.34.0.rc1.387.gb447b232ab-goog