Re: [PATCH 3/3] usercopy: Add find_vmap_area_try() to avoid deadlocks

[Uladzislau Rezki]

On Fri, Sep 16, 2022 at 06:59:57AM -0700, Kees Cook wrote:
> The check_object_size() checks under CONFIG_HARDENED_USERCOPY need to be
> more defensive against running from interrupt context. Use a best-effort
> check for VMAP areas when running in interrupt context
> 
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/linux-mm/YyQ2CSdIJdvQPSPO@xxxxxxxxxxxxxxxxxxxx
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Cc: dev@xxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-hardening@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  include/linux/vmalloc.h |  1 +
>  mm/usercopy.c           | 11 ++++++++++-
>  mm/vmalloc.c            | 11 +++++++++++
>  3 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 096d48aa3437..c8a00f181a11 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -216,6 +216,7 @@ void free_vm_area(struct vm_struct *area);
>  extern struct vm_struct *remove_vm_area(const void *addr);
>  extern struct vm_struct *find_vm_area(const void *addr);
>  struct vmap_area *find_vmap_area(unsigned long addr);
> +struct vmap_area *find_vmap_area_try(unsigned long addr);
>  
>  static inline bool is_vm_area_hugepages(const void *addr)
>  {
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index c1ee15a98633..4a371099ac64 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -173,7 +173,16 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
>  	}
>  
>  	if (is_vmalloc_addr(ptr)) {
> -		struct vmap_area *area = find_vmap_area(addr);
> +		struct vmap_area *area;
> +
> +		if (unlikely(in_interrupt())) {
> +			area = find_vmap_area_try(addr);
> +			/* Give up under interrupt to avoid deadlocks. */
> +			if (!area)
> +				return;
> +		} else {
> +			area = find_vmap_area(addr);
> +		}
>  
>  		if (!area)
>  			usercopy_abort("vmalloc", "no area", to_user, 0, n);
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index dd6cdb201195..f14f1902c2f6 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1840,6 +1840,17 @@ struct vmap_area *find_vmap_area(unsigned long addr)
>  	return va;
>  }
>  
> +struct vmap_area *find_vmap_area_try(unsigned long addr)
>
I think it makes sense to add a comment of having such function and for
which context it is supposed to be used. Maybe rename it to something
with "atomic" prefix.

Thanks.

--
Uladzislau Rezki