We received a report that the copy-on-write issue repored by Jann Horn in https://bugs.chromium.org/p/project-zero/issues/detail?id=2045 is still reproducible on 4.14 and 4.19 kernels (the first issue with the reproducer coded in vmsplice.c). I confirmed this and also that the issue was not reproducible with 5.10 kernel. I tracked the fix to the following patch introduced in 5.9 which changes the do_wp_page() logic:
09854ba94c6a 'mm: do_wp_page() simplification'
I backported this patch (#2 in the series) along with 2 prerequisite patches (#1 and #4) that keep the backports clean and two followup fixes to the main patch (#3 and #5). I had to skip the following fix:
feb889fb40fa 'mm: don't put pinned pages into the swap cache'
because it uses page_maybe_dma_pinned() which does not exists in earlier kernels. Because pin_user_pages() does not exist there as well, I *think* we can safely skip this fix on older kernels, but I would appreciate if someone could confirm that claim.
The patchset cleanly applies over: stable linux-4.14.y, tag: v4.14.228
Note: 4.14 and 4.19 backports are very similar, so while I backported only to these two versions I think backports for other versions can be done easily.
Kirill Tkhai (1): mm: reuse only-pte-mapped KSM page in do_wp_page()
Linus Torvalds (2): mm: do_wp_page() simplification mm: fix misplaced unlock_page in do_wp_page()
Nadav Amit (1): mm/userfaultfd: fix memory corruption due to writeprotect
Shaohua Li (1): userfaultfd: wp: add helper for writeprotect check
include/linux/ksm.h | 7 ++++ include/linux/userfaultfd_k.h | 10 ++++++ mm/ksm.c | 30 ++++++++++++++++-- mm/memory.c | 60 ++++++++++++++++------------------- 4 files changed, 73 insertions(+), 34 deletions(-)