Re: [PATCH v4 2/4] hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled

[Naresh Kamboju <naresh.kamboju@linaro.org>]

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/


-- 
Linaro LKFT
https://lkft.linaro.org