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Message-ID: <843f059f-dd54-4481-b46a-e87e56274db3@lucifer.local>
Date: Fri, 29 Sep 2023 23:28:41 +0100
From: Lorenzo Stoakes <lstoakes@...il.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>,
Suren Baghdasaryan <surenb@...gle.com>,
Matthew Wilcox <willy@...radead.org>, stable@...r.kernel.org
Subject: Re: [PATCH v3 2/3] mmap: Fix error paths with dup_anon_vma()
On Fri, Sep 29, 2023 at 02:30:40PM -0400, Liam R. Howlett 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@...r.kernel.org
> Cc: Jann Horn <jannh@...gle.com>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
> 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.
Being a bit nitpicky/refactory here, but anon_vma_clone() appears to have
two possible return values - 0 for success, and -ENOMEM.
As a result, it's not really gaining us much passing through this value.
It'd be nice if dup_anon_vma() and anon_vma_clone() were therefore updated
to instead return NULL on ENOMEM and the dst otherwise.
Then we could de-clunk this whole code path, and the quite natural fact of
'thing didn't return a pointer therefore had no memory to allocate it' fals
out.
But this isn't exactly an earth-shattering concern :)
> */
> 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;
> }
> @@ -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);
> +
> anon_vma_fail:
> vma_iter_set(vmi, addr);
> vma_iter_load(vmi);
> --
> 2.40.1
>
Other than the nice-to-have, this looks good to me:
Reviewed-by: Lorenzo Stoakes <lstoakes@...il.com>
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