[RFC v2 00/10] Hardening page _refcount

From: Pasha Tatashin
Date: Tue Nov 16 2021 - 20:21:18 EST

From: Pasha Tatashin <tatashin@xxxxxxxxxx>

- 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(-)