Re: [PATCH] mm/usercopy: harden bounds checking for vmalloc allocations

[Kees Cook <kees@kernel.org>]

On Wed, Jul 22, 2026 at 02:29:35PM +0000, Dev Jain wrote:
> The vmalloc allocator stores the actual allocation size inside the
> vm_struct structure. We can use this bound in usercopy instead of the
> page-aligned va_end to catch usercopy beyond the actual allocation size.
> 
> For vmap, the requested_size field is always page-aligned since it maps a
> certain number of pages. Same for vm_map_ram (alongwith, not even having
> a vm_struct). So the check is only relevant for vmalloc mappings.
> 
> Because there are early vm areas registered even before vmalloc_init,
> requested_size may be zero. So also check whether the requested_size
> is set.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>

Looks good to me! Can you update the LKDTM tests to check for this more
tightened range check with a new vmalloc-based test?

If other mm folks can double-check this, I'll happily take this via the
hardening tree.

Thanks!

-Kees

> ---
> Applies on mm-new (3d18f3499c48).
> 
>  mm/usercopy.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index 5de7a518b1b1c..772681aff8477 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -176,10 +176,28 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
>  
>  	if (is_vmalloc_addr(ptr) && !pagefault_disabled()) {
>  		struct vmap_area *area = find_vmap_area(addr);
> +		struct vm_struct *vm;
>  
>  		if (!area)
>  			usercopy_abort("vmalloc", "no area", to_user, 0, n);
>  
> +		vm = area->vm;
> +		/*
> +		 * Mappings with a vm_struct track the originally requested
> +		 * size. Check against that rather than the page-rounded
> +		 * vmap_area->va_end so copies cannot reach vmalloc tail
> +		 * padding. vmap mappings are always page aligned.
> +		 */
> +		if (vm && (vm->flags & VM_ALLOC) && vm->requested_size) {
> +			unsigned long size = vm->requested_size;
> +
> +			offset = addr - area->va_start;
> +			if (offset > size || n > size - offset)
> +				usercopy_abort("vmalloc", NULL, to_user,
> +					       offset, n);
> +			return;
> +		}
> +
>  		if (n > area->va_end - addr) {
>  			offset = addr - area->va_start;
>  			usercopy_abort("vmalloc", NULL, to_user, offset, n);
> -- 
> 2.43.0
> 

-- 
Kees Cook