[PATCH 0/3] kasan: Fix ordering between MTE tag colouring and page->flags

[Catalin Marinas <catalin.marinas@arm.com>]

Hi,

That's more of an RFC to get a discussion started. I plan to eventually
apply the third patch reverting the page_kasan_tag_reset() calls under
arch/arm64 since they don't cover all cases (the race is rare and we
haven't hit anything yet but it's possible).

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 such page is mapped in user-space with PROT_MTE, the architecture
code will set the 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, try to do this
at page allocation time by a new flag added to GFP_USER. Could we
instead add __GFP_SKIP_KASAN_UNPOISON rather than a new flag?

Thanks.

Catalin Marinas (3):
  mm: kasan: Ensure the tags are visible before the tag in page->flags
  mm: kasan: Reset the tag on pages intended for user
  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           | 10 +++++++---
 mm/kasan/common.c             |  3 ++-
 mm/page_alloc.c               |  9 ++++++---
 8 files changed, 15 insertions(+), 40 deletions(-)