Re: [PATCH] mm: enforce __must_check on VMA merge and split

[Vlastimil Babka]

On 12/6/24 23:50, Lorenzo Stoakes wrote:
> It is of critical importance to check the return results on VMA merge (and
> split), failure to do so can result in use-after-free's. This bug has
> recurred, so have the compiler enforce this check to prevent any future
> repetition.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/vma.c |  8 +++++---
>  mm/vma.h | 26 +++++++++++++++-----------
>  2 files changed, 20 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index a06747845cac..543c102b4062 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -447,8 +447,9 @@ void unmap_region(struct ma_state *mas, struct vm_area_struct *vma,
>   * has already been checked or doesn't make sense to fail.
>   * VMA Iterator will point to the original VMA.
>   */
> -static int __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> -		       unsigned long addr, int new_below)
> +static __must_check int
> +__split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> +	    unsigned long addr, int new_below)
>  {
>  	struct vma_prepare vp;
>  	struct vm_area_struct *new;
> @@ -710,7 +711,8 @@ static bool can_merge_remove_vma(struct vm_area_struct *vma)
>   * - The caller must hold a WRITE lock on the mm_struct->mmap_lock.
>   * - vmi must be positioned within [@vmg->vma->vm_start, @vmg->vma->vm_end).
>   */
> -static struct vm_area_struct *vma_merge_existing_range(struct vma_merge_struct *vmg)
> +static __must_check struct vm_area_struct *vma_merge_existing_range(
> +		struct vma_merge_struct *vmg)
>  {
>  	struct vm_area_struct *vma = vmg->vma;
>  	struct vm_area_struct *prev = vmg->prev;
> diff --git a/mm/vma.h b/mm/vma.h
> index 295d44ea54db..61ed044b6145 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -139,9 +139,10 @@ void validate_mm(struct mm_struct *mm);
>  #define validate_mm(mm) do { } while (0)
>  #endif
>  
> -int vma_expand(struct vma_merge_struct *vmg);
> -int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
> -	       unsigned long start, unsigned long end, pgoff_t pgoff);
> +__must_check int vma_expand(struct vma_merge_struct *vmg);
> +__must_check int vma_shrink(struct vma_iterator *vmi,
> +		struct vm_area_struct *vma,
> +		unsigned long start, unsigned long end, pgoff_t pgoff);
>  
>  static inline int vma_iter_store_gfp(struct vma_iterator *vmi,
>  			struct vm_area_struct *vma, gfp_t gfp)
> @@ -174,13 +175,14 @@ void unmap_region(struct ma_state *mas, struct vm_area_struct *vma,
>  		struct vm_area_struct *prev, struct vm_area_struct *next);
>  
>  /* We are about to modify the VMA's flags. */
> -struct vm_area_struct *vma_modify_flags(struct vma_iterator *vmi,
> +__must_check struct vm_area_struct
> +*vma_modify_flags(struct vma_iterator *vmi,
>  		struct vm_area_struct *prev, struct vm_area_struct *vma,
>  		unsigned long start, unsigned long end,
>  		unsigned long new_flags);
>  
>  /* We are about to modify the VMA's flags and/or anon_name. */
> -struct vm_area_struct
> +__must_check struct vm_area_struct
>  *vma_modify_flags_name(struct vma_iterator *vmi,
>  		       struct vm_area_struct *prev,
>  		       struct vm_area_struct *vma,
> @@ -190,7 +192,7 @@ struct vm_area_struct
>  		       struct anon_vma_name *new_name);
>  
>  /* We are about to modify the VMA's memory policy. */
> -struct vm_area_struct
> +__must_check struct vm_area_struct
>  *vma_modify_policy(struct vma_iterator *vmi,
>  		   struct vm_area_struct *prev,
>  		   struct vm_area_struct *vma,
> @@ -198,7 +200,7 @@ struct vm_area_struct
>  		   struct mempolicy *new_pol);
>  
>  /* We are about to modify the VMA's flags and/or uffd context. */
> -struct vm_area_struct
> +__must_check struct vm_area_struct
>  *vma_modify_flags_uffd(struct vma_iterator *vmi,
>  		       struct vm_area_struct *prev,
>  		       struct vm_area_struct *vma,
> @@ -206,11 +208,13 @@ struct vm_area_struct
>  		       unsigned long new_flags,
>  		       struct vm_userfaultfd_ctx new_ctx);
>  
> -struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg);
> +__must_check struct vm_area_struct
> +*vma_merge_new_range(struct vma_merge_struct *vmg);
>  
> -struct vm_area_struct *vma_merge_extend(struct vma_iterator *vmi,
> -					struct vm_area_struct *vma,
> -					unsigned long delta);
> +__must_check struct vm_area_struct
> +*vma_merge_extend(struct vma_iterator *vmi,
> +		  struct vm_area_struct *vma,
> +		  unsigned long delta);
>  
>  void unlink_file_vma_batch_init(struct unlink_vma_file_batch *vb);
>