Re: [PATCH v3 2/3] mmap: Fix error paths with dup_anon_vma()

From: Suren Baghdasaryan
Date: Tue Oct 03 2023 - 15:40:31 EST

On Tue, Oct 3, 2023 at 11:51 AM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
>
> * Suren Baghdasaryan <surenb@xxxxxxxxxx> [231003 12:21]:
> > On Fri, Sep 29, 2023 at 11:30 AM Liam R. Howlett
> > <Liam.Howlett@xxxxxxxxxx> wrote:
> > >
> > > When the calling function fails after the dup_anon_vma(), the
> > > duplication of the anon_vma is not being undone. Add the necessary
> > > unlink_anon_vma() call to the error paths that are missing them.
> > >
> > > This issue showed up during inspection of the error path in vma_merge()
> > > for an unrelated vma iterator issue.
> > >
> > > Users may experience increased memory usage, which may be problematic as
> > > the failure would likely be caused by a low memory situation.
> > >
> > > Fixes: d4af56c5c7c6 ("mm: start tracking VMAs with maple tree")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Cc: Jann Horn <jannh@xxxxxxxxxx>
> > > Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> > > ---
> > > mm/mmap.c | 30 ++++++++++++++++++++++--------
> > > 1 file changed, 22 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index acb7dea49e23..f9f0a5fe4db4 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -583,11 +583,12 @@ static inline void vma_complete(struct vma_prepare *vp,
> > > * dup_anon_vma() - Helper function to duplicate anon_vma
> > > * @dst: The destination VMA
> > > * @src: The source VMA
> > > + * @dup: Pointer to the destination VMA when successful.
> > > *
> > > * Returns: 0 on success.
> > > */
> > > static inline int dup_anon_vma(struct vm_area_struct *dst,
> > > - struct vm_area_struct *src)
> > > + struct vm_area_struct *src, struct vm_area_struct **dup)
> > > {
> > > /*
> > > * Easily overlooked: when mprotect shifts the boundary, make sure the
> > > @@ -595,9 +596,15 @@ static inline int dup_anon_vma(struct vm_area_struct *dst,
> > > * anon pages imported.
> > > */
> > > if (src->anon_vma && !dst->anon_vma) {
> > > + int ret;
> > > +
> > > vma_assert_write_locked(dst);
> > > dst->anon_vma = src->anon_vma;
> > > - return anon_vma_clone(dst, src);
> > > + ret = anon_vma_clone(dst, src);
> > > + if (ret)
> > > + return ret;
> > > +
> > > + *dup = dst;
> > > }
> > >
> > > return 0;
> > > @@ -624,6 +631,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > > unsigned long start, unsigned long end, pgoff_t pgoff,
> > > struct vm_area_struct *next)
> > > {
> > > + struct vm_area_struct *anon_dup = NULL;
> > > bool remove_next = false;
> > > struct vma_prepare vp;
> > >
> > > @@ -633,7 +641,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >
> > > remove_next = true;
> > > vma_start_write(next);
> > > - ret = dup_anon_vma(vma, next);
> > > + ret = dup_anon_vma(vma, next, &anon_dup);
> > > if (ret)
> > > return ret;
> >
> > Shouldn't the above be changed to a "goto nomem" instead of "return ret" ?
> >
> >
> > > }
> > > @@ -661,6 +669,8 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > > return 0;
> > >
> > > nomem:
> > > + if (anon_dup)
> > > + unlink_anon_vmas(anon_dup);
> > > return -ENOMEM;
> > > }
> > >
> > > @@ -860,6 +870,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > > {
> > > struct vm_area_struct *curr, *next, *res;
> > > struct vm_area_struct *vma, *adjust, *remove, *remove2;
> > > + struct vm_area_struct *anon_dup = NULL;
> > > struct vma_prepare vp;
> > > pgoff_t vma_pgoff;
> > > int err = 0;
> > > @@ -927,18 +938,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > > vma_start_write(next);
> > > remove = next; /* case 1 */
> > > vma_end = next->vm_end;
> > > - err = dup_anon_vma(prev, next);
> > > + err = dup_anon_vma(prev, next, &anon_dup);
> > > if (curr) { /* case 6 */
> > > vma_start_write(curr);
> > > remove = curr;
> > > remove2 = next;
> > > if (!next->anon_vma)
> > > - err = dup_anon_vma(prev, curr);
> > > + err = dup_anon_vma(prev, curr, &anon_dup);
> > > }
> > > } else if (merge_prev) { /* case 2 */
> > > if (curr) {
> > > vma_start_write(curr);
> > > - err = dup_anon_vma(prev, curr);
> > > + err = dup_anon_vma(prev, curr, &anon_dup);
> > > if (end == curr->vm_end) { /* case 7 */
> > > remove = curr;
> > > } else { /* case 5 */
> > > @@ -954,7 +965,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > > vma_end = addr;
> > > adjust = next;
> > > adj_start = -(prev->vm_end - addr);
> > > - err = dup_anon_vma(next, prev);
> > > + err = dup_anon_vma(next, prev, &anon_dup);
> > > } else {
> > > /*
> > > * Note that cases 3 and 8 are the ONLY ones where prev
> > > @@ -968,7 +979,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > > vma_pgoff = curr->vm_pgoff;
> > > vma_start_write(curr);
> > > remove = curr;
> > > - err = dup_anon_vma(next, curr);
> > > + err = dup_anon_vma(next, curr, &anon_dup);
> > > }
> > > }
> > > }
> > > @@ -1018,6 +1029,9 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> > > return res;
> > >
> > > prealloc_fail:
> > > + if (anon_dup)
> > > + unlink_anon_vmas(anon_dup);
> >
> > Maybe a stupid question, but why can't we do this unlinking inside
> > dup_anon_vma() itself when anon_vma_clone() fails? That would
> > eliminate the need for the out parameter in that function. I suspect
> > that there is a reason for that which I'm missing.
>
> It's too late. This is to undo the link when the preallocation for the
> maple tree fails. So we had memory to dup the anon vma, but not to put
> it in the tree.

Ah, I see what I missed now. Sorry for the noise.

>
> >
> > > +
> > > anon_vma_fail:
> > > vma_iter_set(vmi, addr);
> > > vma_iter_load(vmi);
> > > --
> > > 2.40.1
> > >