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Message-Id: <20240730133111.d180e1a6fc63b2883fe99821@linux-foundation.org>
Date: Tue, 30 Jul 2024 13:31:11 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Danilo Krummrich <dakr@...nel.org>
Cc: cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
iamjoonsoo.kim@....com, vbabka@...e.cz, roman.gushchin@...ux.dev,
42.hyeyoo@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] mm: krealloc: consider spare memory for __GFP_ZERO
On Tue, 30 Jul 2024 21:42:05 +0200 Danilo Krummrich <dakr@...nel.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);
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