Hi Peter,

On Fri, 19 Feb 2021 at 04:43, Peter Xu peterx@redhat.com wrote:

Huge pmd sharing could bring problem to userfaultfd. The thing is that userfaultfd is running its logic based on the special bits on page table entries, however the huge pmd sharing could potentially share page table entries for different address ranges. That could cause issues on either:

• When sharing huge pmd page tables for an uffd write protected range, the newly mapped huge pmd range will also be write protected unexpectedly, or,

• When we try to write protect a range of huge pmd shared range, we'll first do huge_pmd_unshare() in hugetlb_change_protection(), however that also means the UFFDIO_WRITEPROTECT could be silently skipped for the shared region, which could lead to data loss.

Since at it, a few other things are done altogether:

• Move want_pmd_share() from mm/hugetlb.c into linux/hugetlb.h, because that's definitely something that arch code would like to use too

• ARM64 currently directly check against CONFIG_ARCH_WANT_HUGE_PMD_SHARE when trying to share huge pmd. Switch to the want_pmd_share() helper.

Since at it, move vma_shareable() from huge_pmd_share() into want_pmd_share().

Reviewed-by: Mike Kravetz mike.kravetz@oracle.com Reviewed-by: Axel Rasmussen axelrasmussen@google.com Signed-off-by: Peter Xu peterx@redhat.com

---

arch/arm64/mm/hugetlbpage.c | 3 +-- include/linux/hugetlb.h | 2 ++ include/linux/userfaultfd_k.h | 9 +++++++++ mm/hugetlb.c | 20 ++++++++++++++------ 4 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c index 6e3bcffe2837..58987a98e179 100644 --- a/arch/arm64/mm/hugetlbpage.c +++ b/arch/arm64/mm/hugetlbpage.c @@ -284,8 +284,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma, */ ptep = pte_alloc_map(mm, pmdp, addr); } else if (sz == PMD_SIZE) {

•           if (IS_ENABLED(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) &&
  

•               pud_none(READ_ONCE(*pudp)))
  

•           if (want_pmd_share(vma, addr) && pud_none(READ_ONCE(*pudp)))
  

While building Linux next 20210310 tag for arm64 architecture with

- CONFIG_ARM64_64K_PAGES=y

enabled the build failed due to below errors / warnings

make --silent --keep-going --jobs=8 O=/home/tuxbuild/.cache/tuxmake/builds/1/tmp ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- 'CC=sccache aarch64-linux-gnu-gcc' 'HOSTCC=sccache gcc' aarch64-linux-gnu-ld: Unexpected GOT/PLT entries detected! aarch64-linux-gnu-ld: Unexpected run-time procedure linkages detected! aarch64-linux-gnu-ld: arch/arm64/mm/hugetlbpage.o: in function `huge_pte_alloc': hugetlbpage.c:(.text+0x7d8): undefined reference to `want_pmd_share'

Reported-by: Naresh Kamboju naresh.kamboju@linaro.org

Steps to reproduce: ---------------------------- # TuxMake is a command line tool and Python library that provides # portable and repeatable Linux kernel builds across a variety of # architectures, toolchains, kernel configurations, and make targets. # # TuxMake supports the concept of runtimes. # See https://docs.tuxmake.org/runtimes/, for that to work it requires # that you install podman or docker on your system. # # To install tuxmake on your system globally: # sudo pip3 install -U tuxmake # # See https://docs.tuxmake.org/ for complete documentation.

tuxmake --runtime podman --target-arch arm64 --toolchain gcc-9 --kconfig defconfig --kconfig-add https://builds.tuxbuild.com/1pYCSoc1oGtPWlPgLAJxbHx07kL/config

Build link, https://builds.tuxbuild.com/1pYCSoc1oGtPWlPgLAJxbHx07kL/