[PATCH v7 05/23] mm/shmem: Take care of UFFDIO_COPY_MODE_WP

Peter Xu
2022-03-04T05:18:17Z

Pass wp_copy into shmem_mfill_atomic_pte() through the stack, then apply the
UFFD_WP bit properly when the UFFDIO_COPY on shmem is with UFFDIO_COPY_MODE_WP.
wp_copy lands mfill_atomic_install_pte() finally.

Note: we must do pte_wrprotect() if !writable in mfill_atomic_install_pte(), as
mk_pte() could return a writable pte (e.g., when VM_SHARED on a shmem file).

Signed-off-by: Peter Xu <peterx@redhat.com>
---
include/linux/shmem_fs.h | 4 ++--
mm/shmem.c | 4 ++--
mm/userfaultfd.c | 23 ++++++++++++++++++-----
3 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index ab51d3cd39bd..02d23ce5f979 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -145,11 +145,11 @@ extern int shmem_mfill_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
struct vm_area_struct *dst_vma,
unsigned long dst_addr,
unsigned long src_addr,
- bool zeropage,
+ bool zeropage, bool wp_copy,
struct page **pagep);
#else /* !CONFIG_SHMEM */
#define shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr, \
- src_addr, zeropage, pagep) ({ BUG(); 0; })
+ src_addr, zeropage, wp_copy, pagep) ({ BUG(); 0; })
#endif /* CONFIG_SHMEM */
#endif /* CONFIG_USERFAULTFD */

diff --git a/mm/shmem.c b/mm/shmem.c
index 81a69bd247b4..3f0332c1c1e9 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2317,7 +2317,7 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
struct vm_area_struct *dst_vma,
unsigned long dst_addr,
unsigned long src_addr,
- bool zeropage,
+ bool zeropage, bool wp_copy,
struct page **pagep)
{
struct inode *inode = file_inode(dst_vma->vm_file);
@@ -2390,7 +2390,7 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
goto out_release;

ret = mfill_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
- page, true, false);
+ page, true, wp_copy);
if (ret)
goto out_delete_from_cache;

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e9bb6db002aa..ef418a48b121 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -77,10 +77,19 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
* Always mark a PTE as write-protected when needed, regardless of
* VM_WRITE, which the user might change.
*/
- if (wp_copy)
+ if (wp_copy) {
_dst_pte = pte_mkuffd_wp(_dst_pte);
- else if (writable)
+ writable = false;
+ }
+
+ if (writable)
_dst_pte = pte_mkwrite(_dst_pte);
+ else
+ /*
+ * We need this to make sure write bit removed; as mk_pte()
+ * could return a pte with write bit set.
+ */
+ _dst_pte = pte_wrprotect(_dst_pte);

dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);

@@ -95,7 +104,12 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
}

ret = -EEXIST;
- if (!pte_none(*dst_pte))
+ /*
+ * We allow to overwrite a pte marker: consider when both MISSING|WP
+ * registered, we firstly wr-protect a none pte which has no page cache
+ * page backing it, then access the page.
+ */
+ if (!pte_none_mostly(*dst_pte))
goto out_unlock;

if (page_in_cache) {
@@ -479,11 +493,10 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
err = mfill_zeropage_pte(dst_mm, dst_pmd,
dst_vma, dst_addr);
} else {
- VM_WARN_ON_ONCE(wp_copy);
err = shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
dst_addr, src_addr,
mode != MCOPY_ATOMIC_NORMAL,
- page);
+ wp_copy, page);
}

return err;
--
2.32.0