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

From: Peter Xu <[email-masked]>
Date: Wed, 10 Mar 2021 11:57:13 -0500

On Wed, Mar 10, 2021 at 01:18:42PM +0530, Naresh Kamboju wrote:
> Hi Peter,

Hi, Naresh,

> 
> On Fri, 19 Feb 2021 at 04:43, Peter Xu <[email-masked]> 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 <[email-masked]>
> > Reviewed-by: Axel Rasmussen <[email-masked]>
> > Signed-off-by: Peter Xu <[email-masked]>
> > ---
> >  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 <[email-masked]>

Sorry for the issue & thanks for the report.  Would you please check whether
the patch attached could fix the issue?

-- 
Peter Xu
>From 4072042b415d1f78ac1ab017671e345b414ca6ab Mon Sep 17 00:00:00 2001
From: Peter Xu <[email-masked]>
Date: Wed, 10 Mar 2021 11:48:56 -0500
Subject: [PATCH] mm/hugetlb: Fix build with !ARCH_WANT_HUGE_PMD_SHARE

want_pmd_share() is undefined with !ARCH_WANT_HUGE_PMD_SHARE since it's put
by accident into a "#ifdef ARCH_WANT_HUGE_PMD_SHARE" block.  Moving it out
won't work either since vma_shareable() is only defined within the block.
Define it for !ARCH_WANT_HUGE_PMD_SHARE instead.

Fixes: 5b109cc1cdcc ("hugetlb/userfaultfd: forbid huge pmd sharing when uffd enabled")
Cc: Andrew Morton <[email-masked]>
Cc: Mike Kravetz <[email-masked]>
Cc: Axel Rasmussen <[email-masked]>
Reported-by: Naresh Kamboju <[email-masked]>
Signed-off-by: Peter Xu <[email-masked]>
---
 mm/hugetlb.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d58f1456fe27..8dda7e034477 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5469,9 +5469,6 @@ static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
 
 bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr)
 {
-#ifndef CONFIG_ARCH_WANT_HUGE_PMD_SHARE
-	return false;
-#endif
 #ifdef CONFIG_USERFAULTFD
 	if (uffd_disable_huge_pmd_share(vma))
 		return false;
@@ -5616,6 +5613,11 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
 				unsigned long *start, unsigned long *end)
 {
 }
+
+bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr)
+{
+	return false;
+}
 #endif /* CONFIG_ARCH_WANT_HUGE_PMD_SHARE */
 
 #ifdef CONFIG_ARCH_WANT_GENERAL_HUGETLB
-- 
2.26.2