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

[Dev Jain <dev.jain@arm.com>]

On 23/07/26 9:33 am, Kees Cook wrote:
> 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?

Thanks. I have written the test, I'll send a v2 with it once the mm people
take a look at this.


> 
> 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
>>
>