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(-)
