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SubjectRe: [PATCH v9 09/24] mm: protect mremap() against SPF hanlder
On Tue, 13 Mar 2018, Laurent Dufour wrote:

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 88042d843668..ef6ef0627090 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2189,16 +2189,24 @@ void anon_vma_interval_tree_verify(struct anon_vma_chain *node);
> extern int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin);
> extern int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
> unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert,
> - struct vm_area_struct *expand);
> + struct vm_area_struct *expand, bool keep_locked);
> static inline int vma_adjust(struct vm_area_struct *vma, unsigned long start,
> unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert)
> {
> - return __vma_adjust(vma, start, end, pgoff, insert, NULL);
> + return __vma_adjust(vma, start, end, pgoff, insert, NULL, false);
> }
> -extern struct vm_area_struct *vma_merge(struct mm_struct *,
> +extern struct vm_area_struct *__vma_merge(struct mm_struct *,
> struct vm_area_struct *prev, unsigned long addr, unsigned long end,
> unsigned long vm_flags, struct anon_vma *, struct file *, pgoff_t,
> - struct mempolicy *, struct vm_userfaultfd_ctx);
> + struct mempolicy *, struct vm_userfaultfd_ctx, bool keep_locked);
> +static inline struct vm_area_struct *vma_merge(struct mm_struct *vma,
> + struct vm_area_struct *prev, unsigned long addr, unsigned long end,
> + unsigned long vm_flags, struct anon_vma *anon, struct file *file,
> + pgoff_t off, struct mempolicy *pol, struct vm_userfaultfd_ctx uff)
> +{
> + return __vma_merge(vma, prev, addr, end, vm_flags, anon, file, off,
> + pol, uff, false);
> +}

The first formal to vma_merge() is an mm, not a vma.

This area could use an uncluttering.

> extern struct anon_vma *find_mergeable_anon_vma(struct vm_area_struct *);
> extern int __split_vma(struct mm_struct *, struct vm_area_struct *,
> unsigned long addr, int new_below);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index d6533cb85213..ac32b577a0c9 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -684,7 +684,7 @@ static inline void __vma_unlink_prev(struct mm_struct *mm,
> */
> int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
> unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert,
> - struct vm_area_struct *expand)
> + struct vm_area_struct *expand, bool keep_locked)
> {
> struct mm_struct *mm = vma->vm_mm;
> struct vm_area_struct *next = vma->vm_next, *orig_vma = vma;
> @@ -996,7 +996,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>
> if (next && next != vma)
> vm_raw_write_end(next);
> - vm_raw_write_end(vma);
> + if (!keep_locked)
> + vm_raw_write_end(vma);
>
> validate_mm(mm);
>

This will require a fixup for the following patch where a retval from
anon_vma_close() can also return without vma locked even though
keep_locked == true.

How does vma_merge() handle that error wrt vm_raw_write_begin(vma)?

> @@ -1132,12 +1133,13 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
> * parameter) may establish ptes with the wrong permissions of NNNN
> * instead of the right permissions of XXXX.
> */
> -struct vm_area_struct *vma_merge(struct mm_struct *mm,
> +struct vm_area_struct *__vma_merge(struct mm_struct *mm,
> struct vm_area_struct *prev, unsigned long addr,
> unsigned long end, unsigned long vm_flags,
> struct anon_vma *anon_vma, struct file *file,
> pgoff_t pgoff, struct mempolicy *policy,
> - struct vm_userfaultfd_ctx vm_userfaultfd_ctx)
> + struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
> + bool keep_locked)
> {
> pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
> struct vm_area_struct *area, *next;
> @@ -1185,10 +1187,11 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
> /* cases 1, 6 */
> err = __vma_adjust(prev, prev->vm_start,
> next->vm_end, prev->vm_pgoff, NULL,
> - prev);
> + prev, keep_locked);
> } else /* cases 2, 5, 7 */
> err = __vma_adjust(prev, prev->vm_start,
> - end, prev->vm_pgoff, NULL, prev);
> + end, prev->vm_pgoff, NULL, prev,
> + keep_locked);
> if (err)
> return NULL;
> khugepaged_enter_vma_merge(prev, vm_flags);
> @@ -1205,10 +1208,12 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
> vm_userfaultfd_ctx)) {
> if (prev && addr < prev->vm_end) /* case 4 */
> err = __vma_adjust(prev, prev->vm_start,
> - addr, prev->vm_pgoff, NULL, next);
> + addr, prev->vm_pgoff, NULL, next,
> + keep_locked);
> else { /* cases 3, 8 */
> err = __vma_adjust(area, addr, next->vm_end,
> - next->vm_pgoff - pglen, NULL, next);
> + next->vm_pgoff - pglen, NULL, next,
> + keep_locked);
> /*
> * In case 3 area is already equal to next and
> * this is a noop, but in case 8 "area" has
> @@ -3163,9 +3168,20 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>
> if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent))
> return NULL; /* should never get here */
> - new_vma = vma_merge(mm, prev, addr, addr + len, vma->vm_flags,
> - vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
> - vma->vm_userfaultfd_ctx);
> +
> + /* There is 3 cases to manage here in
> + * AAAA AAAA AAAA AAAA
> + * PPPP.... PPPP......NNNN PPPP....NNNN PP........NN
> + * PPPPPPPP(A) PPPP..NNNNNNNN(B) PPPPPPPPPPPP(1) NULL
> + * PPPPPPPPNNNN(2)
> + * PPPPNNNNNNNN(3)
> + *
> + * new_vma == prev in case A,1,2
> + * new_vma == next in case B,3
> + */

Interleaved tabs and whitespace.

> + new_vma = __vma_merge(mm, prev, addr, addr + len, vma->vm_flags,
> + vma->anon_vma, vma->vm_file, pgoff,
> + vma_policy(vma), vma->vm_userfaultfd_ctx, true);
> if (new_vma) {
> /*
> * Source vma may have been merged into new_vma
> @@ -3205,6 +3221,15 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
> get_file(new_vma->vm_file);
> if (new_vma->vm_ops && new_vma->vm_ops->open)
> new_vma->vm_ops->open(new_vma);
> + /*
> + * As the VMA is linked right now, it may be hit by the
> + * speculative page fault handler. But we don't want it to
> + * to start mapping page in this area until the caller has
> + * potentially move the pte from the moved VMA. To prevent
> + * that we protect it right now, and let the caller unprotect
> + * it once the move is done.
> + */
> + vm_raw_write_begin(new_vma);
> vma_link(mm, new_vma, prev, rb_link, rb_parent);
> *need_rmap_locks = false;
> }
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 049470aa1e3e..8ed1a1d6eaed 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -302,6 +302,14 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> if (!new_vma)
> return -ENOMEM;
>
> + /* new_vma is returned protected by copy_vma, to prevent speculative
> + * page fault to be done in the destination area before we move the pte.
> + * Now, we must also protect the source VMA since we don't want pages
> + * to be mapped in our back while we are copying the PTEs.
> + */
> + if (vma != new_vma)
> + vm_raw_write_begin(vma);
> +
> moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len,
> need_rmap_locks);
> if (moved_len < old_len) {
> @@ -318,6 +326,8 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> */
> move_page_tables(new_vma, new_addr, vma, old_addr, moved_len,
> true);
> + if (vma != new_vma)
> + vm_raw_write_end(vma);
> vma = new_vma;
> old_len = new_len;
> old_addr = new_addr;
> @@ -326,7 +336,10 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> mremap_userfaultfd_prep(new_vma, uf);
> arch_remap(mm, old_addr, old_addr + old_len,
> new_addr, new_addr + new_len);
> + if (vma != new_vma)
> + vm_raw_write_end(vma);
> }
> + vm_raw_write_end(new_vma);

Just do

vm_raw_write_end(vma);
vm_raw_write_end(new_vma);

here.

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