Re: [PATCH v2 13/13] s390/scm: Use kmalloc() for SCM information

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Replacing __get_free_page() with kmalloc() breaks the strict 4K-byte hardware alignment requirement for the CHSC command block, leading to a specification exception (kernel panic).
--

commit 9a0607c813ec0f02b4147f95093917e38b21c342
Author: Mike Rapoport (Microsoft) <rppt@kernel.org>

s390/scm: Use kmalloc() for SCM information

This commit replaces the page allocator with kmalloc for allocating the
response buffer in scm_update_information(), and introduces scope-based
cleanup using __free(kfree).

Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com

> diff --git a/drivers/s390/cio/scm.c b/drivers/s390/cio/scm.c
> index 171212a6d2d9c..b79fe023ab8d1 100644
> --- a/drivers/s390/cio/scm.c
> +++ b/drivers/s390/cio/scm.c

[ ... ]

> @@ -224,12 +225,12 @@ static int scm_add(struct chsc_scm_info *scm_info, size_t num)
>  
>  int scm_update_information(void)
>  {
> -	struct chsc_scm_info *scm_info;
> +	struct chsc_scm_info *scm_info __free(kfree) = NULL;
>  	u64 token = 0;
>  	size_t num;
>  	int ret;
>  
> -	scm_info = (void *)__get_free_page(GFP_KERNEL | GFP_DMA);
> +	scm_info = kmalloc(PAGE_SIZE, GFP_KERNEL | GFP_DMA);

[Severity: High]
Does switching to kmalloc here introduce a regression by breaking the
hardware alignment requirements for the CHSC command block?

While __get_free_page() guarantees strict page alignment, kmalloc() does not
guarantee 4KB alignment. When debugging options like SLUB_DEBUG or KASAN are
enabled, they add metadata that shifts the returned object away from the page
boundary.

This buffer is eventually passed to the hardware chsc instruction in
chsc_scm_info():

drivers/s390/cio/chsc.c:chsc_scm_info() {
    ...
    ccode = chsc(scm_area);
    ...
}

If the address is unaligned, could the CPU throw a specification exception
resulting in a kernel panic?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260910-s390-cio-ready-v2-0-e931086fdde1@kernel.org?part=13