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Subject[PATCH v3 3/4] mm: don't expose non-hugetlb page to fast gup prematurely
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We don't want to expose a non-hugetlb page to the fast gup running
on a remote CPU before all local non-atomic ops on the page flags
are visible first.

For an anon page that isn't in swap cache, we need to make sure all
prior non-atomic ops, especially __SetPageSwapBacked() in
page_add_new_anon_rmap(), are ordered before set_pte_at() to prevent
the following race:

CPU 1 CPU1
set_pte_at() get_user_pages_fast()
page_add_new_anon_rmap() gup_pte_range()
__SetPageSwapBacked() SetPageReferenced()

This demonstrates a non-fatal scenario. Though haven't been directly
observed, the fatal ones can exist, e.g., PG_lock set by fast gup
caller and then overwritten by __SetPageSwapBacked().

For an anon page that is already in swap cache or a file page, we
don't need smp_wmb() before set_pte_at() because adding to swap or
file cach serves as a valid write barrier. Using non-atomic ops
thereafter is a bug, obviously.

smp_wmb() is added following 11 of total 12 page_add_new_anon_rmap()
call sites, with the only exception being
do_huge_pmd_wp_page_fallback() because of an existing smp_wmb().

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
kernel/events/uprobes.c | 2 ++
mm/huge_memory.c | 6 ++++++
mm/khugepaged.c | 2 ++
mm/memory.c | 10 +++++++++-
mm/migrate.c | 2 ++
mm/swapfile.c | 6 ++++--
mm/userfaultfd.c | 2 ++
7 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 84fa00497c49..7069785e2e52 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -194,6 +194,8 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,

flush_cache_page(vma, addr, pte_pfn(*pvmw.pte));
ptep_clear_flush_notify(vma, addr, pvmw.pte);
+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
set_pte_at_notify(mm, addr, pvmw.pte,
mk_pte(new_page, vma->vm_page_prot));

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index de1f15969e27..21d271a29d96 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -616,6 +616,8 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
mem_cgroup_commit_charge(page, memcg, false, true);
lru_cache_add_active_or_unevictable(page, vma);
pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
mm_inc_nr_ptes(vma->vm_mm);
@@ -1276,7 +1278,9 @@ static vm_fault_t do_huge_pmd_wp_page_fallback(struct vm_fault *vmf,
}
kfree(pages);

+ /* commit non-atomic ops before exposing to fast gup */
smp_wmb(); /* make pte visible before pmd */
+
pmd_populate(vma->vm_mm, vmf->pmd, pgtable);
page_remove_rmap(page, true);
spin_unlock(vmf->ptl);
@@ -1423,6 +1427,8 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
page_add_new_anon_rmap(new_page, vma, haddr, true);
mem_cgroup_commit_charge(new_page, memcg, false, true);
lru_cache_add_active_or_unevictable(new_page, vma);
+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
if (!page) {
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 70ff98e1414d..f2901edce6de 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1074,6 +1074,8 @@ static void collapse_huge_page(struct mm_struct *mm,
count_memcg_events(memcg, THP_COLLAPSE_ALLOC, 1);
lru_cache_add_active_or_unevictable(new_page, vma);
pgtable_trans_huge_deposit(mm, pmd, pgtable);
+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
set_pmd_at(mm, address, pmd, _pmd);
update_mmu_cache_pmd(vma, address, pmd);
spin_unlock(pmd_ptl);
diff --git a/mm/memory.c b/mm/memory.c
index aa86852d9ec2..6dabbc3cd3b7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2367,6 +2367,8 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
* mmu page tables (such as kvm shadow page tables), we want the
* new page to be mapped directly into the secondary page table.
*/
+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
set_pte_at_notify(mm, vmf->address, vmf->pte, entry);
update_mmu_cache(vma, vmf->address, vmf->pte);
if (old_page) {
@@ -2877,7 +2879,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
flush_icache_page(vma, page);
if (pte_swp_soft_dirty(vmf->orig_pte))
pte = pte_mksoft_dirty(pte);
- set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
vmf->orig_pte = pte;

@@ -2886,12 +2887,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
page_add_new_anon_rmap(page, vma, vmf->address, false);
mem_cgroup_commit_charge(page, memcg, false, false);
lru_cache_add_active_or_unevictable(page, vma);
+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
} else {
do_page_add_anon_rmap(page, vma, vmf->address, exclusive);
mem_cgroup_commit_charge(page, memcg, true, false);
activate_page(page);
}

+ set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
swap_free(entry);
if (mem_cgroup_swap_full(page) ||
(vma->vm_flags & VM_LOCKED) || PageMlocked(page))
@@ -3034,6 +3038,8 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
page_add_new_anon_rmap(page, vma, vmf->address, false);
mem_cgroup_commit_charge(page, memcg, false, false);
lru_cache_add_active_or_unevictable(page, vma);
+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
setpte:
set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);

@@ -3297,6 +3303,8 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
page_add_new_anon_rmap(page, vma, vmf->address, false);
mem_cgroup_commit_charge(page, memcg, false, false);
lru_cache_add_active_or_unevictable(page, vma);
+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
} else {
inc_mm_counter_fast(vma->vm_mm, mm_counter_file(page));
page_add_file_rmap(page, false);
diff --git a/mm/migrate.c b/mm/migrate.c
index 9f4ed4e985c1..943d147ecc3e 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2783,6 +2783,8 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
lru_cache_add_active_or_unevictable(page, vma);
get_page(page);

+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
if (flush) {
flush_cache_page(vma, addr, pte_pfn(*ptep));
ptep_clear_flush_notify(vma, addr, ptep);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index dab43523afdd..5c5547053ee0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1880,8 +1880,6 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
get_page(page);
- set_pte_at(vma->vm_mm, addr, pte,
- pte_mkold(mk_pte(page, vma->vm_page_prot)));
if (page == swapcache) {
page_add_anon_rmap(page, vma, addr, false);
mem_cgroup_commit_charge(page, memcg, true, false);
@@ -1889,7 +1887,11 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
page_add_new_anon_rmap(page, vma, addr, false);
mem_cgroup_commit_charge(page, memcg, false, false);
lru_cache_add_active_or_unevictable(page, vma);
+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
}
+ set_pte_at(vma->vm_mm, addr, pte,
+ pte_mkold(mk_pte(page, vma->vm_page_prot)));
swap_free(entry);
/*
* Move the page to the active list so it is not
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index c7ae74ce5ff3..4f92913242a1 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -92,6 +92,8 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
mem_cgroup_commit_charge(page, memcg, false, false);
lru_cache_add_active_or_unevictable(page, dst_vma);

+ /* commit non-atomic ops before exposing to fast gup */
+ smp_wmb();
set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);

/* No need to invalidate - it was non-present before */
--
2.23.0.351.gc4317032e6-goog
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