Re: [PATCH] mm: vmalloc: Support more granular vrealloc() sizing

[Danilo Krummrich <dakr@kernel.org>]

On Wed, Apr 23, 2025 at 07:31:23PM -0700, Kees Cook wrote:
> Introduce struct vm_struct::requested_size so that the requested
> (re)allocation size is retained separately from the allocated area
> size. This means that KASAN will correctly poison the correct spans
> of requested bytes. This also means we can support growing the usable
> portion of an allocation that can already be supported by the existing
> area's existing allocation.
> 
> Reported-by: Erhard Furtner <erhard_f@mailbox.org>
> Closes: https://lore.kernel.org/all/20250408192503.6149a816@outsider.home/
> Fixes: 3ddc2fefe6f3 ("mm: vmalloc: implement vrealloc()")
> Signed-off-by: Kees Cook <kees@kernel.org>

Good catch!

One question below, otherwise

	Reviewed-by: Danilo Krummrich <dakr@kernel.org>

> @@ -4088,14 +4093,27 @@ void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
>  	 * would be a good heuristic for when to shrink the vm_area?
>  	 */
>  	if (size <= old_size) {
> -		/* Zero out spare memory. */
> -		if (want_init_on_alloc(flags))
> +		/* Zero out "freed" memory. */
> +		if (want_init_on_free())
>  			memset((void *)p + size, 0, old_size - size);
> +		vm->requested_size = size;
>  		kasan_poison_vmalloc(p + size, old_size - size);
>  		kasan_unpoison_vmalloc(p, size, KASAN_VMALLOC_PROT_NORMAL);
>  		return (void *)p;
>  	}
>  
> +	/*
> +	 * We already have the bytes available in the allocation; use them.
> +	 */
> +	if (size <= alloced_size) {
> +		kasan_unpoison_vmalloc(p, size, KASAN_VMALLOC_PROT_NORMAL);
> +		/* Zero out "alloced" memory. */
> +		if (want_init_on_alloc(flags))
> +			memset((void *)p + old_size, 0, size - old_size);
> +		vm->requested_size = size;
> +		kasan_poison_vmalloc(p + size, alloced_size - size);

Do we need this? We know that old_size < size <= alloced_size. And since
previously [p + old_size, p + alloced_size) must have been poisoned,
[p + size, p + alloced_size) must be poisoned already?

Maybe there was a reason, since in the above (size <= old_size) case
kasan_unpoison_vmalloc() seems unnecessary too.