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 <[email protected]>
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
