On 7/30/24 9:42 PM, Danilo Krummrich 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.
>
> Example:
>
> buf = kzalloc(64, GFP_KERNEL);
> 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);
>
> Fix this, by explicitly setting spare memory to zero, when shrinking an
> allocation with __GFP_ZERO flag set or init_on_alloc enabled.
>
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> ---
> mm/slab_common.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 40b582a014b8..cff602cedf8e 100644
> --- 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);
> + kasan_enable_current();
If we do kasan_krealloc() first, shouldn't the memset then be legal
afterwards without the disable/enable dance?
> + }
> +
> p = kasan_krealloc((void *)p, new_size, flags);
> return (void *)p;
> }
>
> base-commit: 7c3dd6d99f2df6a9d7944ee8505b195ba51c9b68
