Re: [PATCH 1/2] mm: krealloc: consider spare memory for __GFP_ZERO

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 30 Jul 2024 21:42:05 +0200 Danilo Krummrich <dakr@kernel.org> wrote:

> As long as krealloc() is called with __GFP_ZERO consistently, starting
> with the initial memory allocation, __GFP_ZERO should be fully honored.
> 
> However, if for an existing allocation krealloc() is called with a
> decreased size, it is not ensured that the spare portion the allocation
> is zeroed. Thus, if krealloc() is subsequently called with a larger size
> again, __GFP_ZERO can't be fully honored, since we don't know the
> previous size, but only the bucket size.

Well that's bad.

> Example:
> 
> 	buf = kzalloc(64, GFP_KERNEL);

If this was kmalloc()

> 	memset(buf, 0xff, 64);
> 
> 	buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO);
> 
> 	/* After this call the last 16 bytes are still 0xff. */
> 	buf = krealloc(buf, 64, GFP_KERNEL | __GFP_ZERO);

then this would expose uninitialized kernel memory to kernel code, with
a risk that the kernel code will expose that to userspace, yes?

This does seem rather a trap, and I wonder whether krealloc() should
just zero out any such data by default.

> Fix this, by explicitly setting spare memory to zero, when shrinking an
> allocation with __GFP_ZERO flag set or init_on_alloc enabled.
> 
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1273,6 +1273,13 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
>  
>  	/* If the object still fits, repoison it precisely. */
>  	if (ks >= new_size) {
> +		/* Zero out spare memory. */
> +		if (want_init_on_alloc(flags)) {
> +			kasan_disable_current();
> +			memset((void *)p + new_size, 0, ks - new_size);

Casting away the constness of `*p'.  This is just misleading everyone,
really.  It would be better to make argument `p' have type "void *".

> +			kasan_enable_current();
> +		}
> +
>  		p = kasan_krealloc((void *)p, new_size, flags);