[PATCH v2 0/4] kasan: Fix ordering between MTE tag colouring and page->flags

[Catalin Marinas]

Hi,

That's a second attempt on fixing the race race between setting the
allocation (in-memory) tags in a page and the corresponding logical tag
in page->flags. Initial version here:

https://lore.kernel.org/r/20220517180945.756303-1-catalin.marinas@arm.com

This new series does not introduce any new GFP flags but instead always
skips unpoisoning of the user pages (we already skip the poisoning on
free). Any unpoisoned page will have the page->flags tag reset.

For the background:

On a system with MTE and KASAN_HW_TAGS enabled, when a page is allocated
kasan_unpoison_pages() sets a random tag and saves it in page->flags so
that page_to_virt() re-creates the correct tagged pointer. We need to
ensure that the in-memory tags are visible before setting the
page->flags:

P0 (__kasan_unpoison_range):    P1 (access via virt_to_page):
  Wtags=x                         Rflags=x
    |                               |
    | DMB                           | address dependency
    V                               V
  Wflags=x                        Rtags=x

The first patch changes the order of page unpoisoning with the tag
storing in page->flags. page_kasan_tag_set() has the right barriers
through try_cmpxchg().

If a page is mapped in user-space with PROT_MTE, the architecture code
will set the allocation tag to 0 and a subsequent page_to_virt()
dereference will fault. We currently try to fix this by resetting the
tag in page->flags so that it is 0xff (match-all, not faulting).
However, setting the tags and flags can race with another CPU reading
the flags (page_to_virt()) and barriers can't help, e.g.:

P0 (mte_sync_page_tags):        P1 (memcpy from virt_to_page):
                                  Rflags!=0xff
  Wflags=0xff
  DMB (doesn't help)
  Wtags=0
                                  Rtags=0   // fault

Since clearing the flags in the arch code doesn't work, to do this at
page allocation time when __GFP_SKIP_KASAN_UNPOISON is passed.

Thanks.

Catalin Marinas (4):
  mm: kasan: Ensure the tags are visible before the tag in page->flags
  mm: kasan: Skip unpoisoning of user pages
  mm: kasan: Skip page unpoisoning only if __GFP_SKIP_KASAN_UNPOISON
  arm64: kasan: Revert "arm64: mte: reset the page tag in page->flags"

 arch/arm64/kernel/hibernate.c |  5 -----
 arch/arm64/kernel/mte.c       |  9 ---------
 arch/arm64/mm/copypage.c      |  9 ---------
 arch/arm64/mm/fault.c         |  1 -
 arch/arm64/mm/mteswap.c       |  9 ---------
 include/linux/gfp.h           |  2 +-
 mm/kasan/common.c             |  3 ++-
 mm/page_alloc.c               | 19 ++++++++++---------
 8 files changed, 13 insertions(+), 44 deletions(-)