summaryrefslogtreecommitdiff
path: root/mm/memory.c
diff options
context:
space:
mode:
authorJason Gunthorpe <jgg@nvidia.com>2020-12-15 06:05:44 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2020-12-15 23:13:39 +0300
commit57efa1fe5957694fa541c9062de0a127f0b9acb0 (patch)
tree4d38c9c0560e072e5cc36f925e89968bf37d4f43 /mm/memory.c
parentc28b1fc70390df32e29991eedd52bd86e7aba080 (diff)
downloadlinux-57efa1fe5957694fa541c9062de0a127f0b9acb0.tar.xz
mm/gup: prevent gup_fast from racing with COW during fork
Since commit 70e806e4e645 ("mm: Do early cow for pinned pages during fork() for ptes") pages under a FOLL_PIN will not be write protected during COW for fork. This means that pages returned from pin_user_pages(FOLL_WRITE) should not become write protected while the pin is active. However, there is a small race where get_user_pages_fast(FOLL_PIN) can establish a FOLL_PIN at the same time copy_present_page() is write protecting it: CPU 0 CPU 1 get_user_pages_fast() internal_get_user_pages_fast() copy_page_range() pte_alloc_map_lock() copy_present_page() atomic_read(has_pinned) == 0 page_maybe_dma_pinned() == false atomic_set(has_pinned, 1); gup_pgd_range() gup_pte_range() pte_t pte = gup_get_pte(ptep) pte_access_permitted(pte) try_grab_compound_head() pte = pte_wrprotect(pte) set_pte_at(); pte_unmap_unlock() // GUP now returns with a write protected page The first attempt to resolve this by using the write protect caused problems (and was missing a barrrier), see commit f3c64eda3e50 ("mm: avoid early COW write protect games during fork()") Instead wrap copy_p4d_range() with the write side of a seqcount and check the read side around gup_pgd_range(). If there is a collision then get_user_pages_fast() fails and falls back to slow GUP. Slow GUP is safe against this race because copy_page_range() is only called while holding the exclusive side of the mmap_lock on the src mm_struct. [akpm@linux-foundation.org: coding style fixes] Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@mail.gmail.com Link: https://lkml.kernel.org/r/2-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com Fixes: f3c64eda3e50 ("mm: avoid early COW write protect games during fork()") Signed-off-by: Jason Gunthorpe <jgg@nvidia.com> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Reviewed-by: John Hubbard <jhubbard@nvidia.com> Reviewed-by: Jan Kara <jack@suse.cz> Reviewed-by: Peter Xu <peterx@redhat.com> Acked-by: "Ahmed S. Darwish" <a.darwish@linutronix.de> [seqcount_t parts] Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> Cc: Christoph Hellwig <hch@lst.de> Cc: Hugh Dickins <hughd@google.com> Cc: Jann Horn <jannh@google.com> Cc: Kirill Shutemov <kirill@shutemov.name> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Leon Romanovsky <leonro@nvidia.com> Cc: Michal Hocko <mhocko@suse.com> Cc: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/memory.c')
-rw-r--r--mm/memory.c13
1 files changed, 12 insertions, 1 deletions
diff --git a/mm/memory.c b/mm/memory.c
index c48f8df6e502..50632c4366b8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1171,6 +1171,15 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE,
0, src_vma, src_mm, addr, end);
mmu_notifier_invalidate_range_start(&range);
+ /*
+ * Disabling preemption is not needed for the write side, as
+ * the read side doesn't spin, but goes to the mmap_lock.
+ *
+ * Use the raw variant of the seqcount_t write API to avoid
+ * lockdep complaining about preemptibility.
+ */
+ mmap_assert_write_locked(src_mm);
+ raw_write_seqcount_begin(&src_mm->write_protect_seq);
}
ret = 0;
@@ -1187,8 +1196,10 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
}
} while (dst_pgd++, src_pgd++, addr = next, addr != end);
- if (is_cow)
+ if (is_cow) {
+ raw_write_seqcount_end(&src_mm->write_protect_seq);
mmu_notifier_invalidate_range_end(&range);
+ }
return ret;
}