Re: [PATCH 3/4] mm/cma: reject CMA areas that overlap the kernel image

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Tue, Nov 03, 2026 at 09:54:28PM +0530, Radhey Shyam Pandey wrote:
> cma_init_reserved_mem() takes a base chosen by its caller and only requires
> that the range already be reserved, so an overlap with the kernel image is
> indistinguishable from the normal case. cma_activate_area() then clears
> PageReserved on those pages and frees them into MIGRATE_CMA, overriding the
> memblock_reserve() of [_text, _end), so live kernel text reaches userspace.
> kexec reached this until the fix this series builds on; a device tree
> placing a shared-dma-pool over where the bootloader loaded the kernel still
> can. Reject the overlap rather than let such a caller corrupt the kernel.
> 
> Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
> ---
> Changes since RFC:
> - Drop Pratyush reviewed-by tag as there were few changes.
> - Trim the commit description.
> - Drop the redundant comment as suggested by Mike.
> - cma_overlaps_kernel_image() is kept as helpers in asm-generic/sections.h
>   are virtual-address helpers.
> ---
>  mm/cma.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/mm/cma.c b/mm/cma.c
> index 9c53b2b70dc8..3231a53161cd 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -31,6 +31,8 @@
>  #include <linux/kmemleak.h>
>  #include <trace/events/cma.h>
>  
> +#include <asm/sections.h>

You probably don't need this.

> +
>  #include "internal.h"
>  #include "cma.h"
>  #include "mm_init.h"
> @@ -68,6 +70,16 @@ bool cma_has_dynamic_base(const struct cma *cma)
>  	return test_bit(CMA_DYNAMIC_BASE, &cma->flags);
>  }
>  
> +static bool cma_overlaps_kernel_image(phys_addr_t base, phys_addr_t size)
> +{
> +	phys_addr_t end;
> +
> +	if (check_add_overflow(base, size, &end))
> +		return true;
> +
> +	return base < __pa_symbol(_end) && __pa_symbol(_text) < end;
> +}
> +
>  static unsigned long cma_bitmap_aligned_mask(const struct cma *cma,
>  					     unsigned int align_order)
>  {
> @@ -295,6 +307,15 @@ int __init cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
>  	if (!size || !memblock_is_region_reserved(base, size))
>  		return -EINVAL;
>  
> +	if (cma_overlaps_kernel_image(base, size)) {
> +		phys_addr_t text = __pa_symbol(_text);
> +		phys_addr_t end = __pa_symbol(_end);
> +
> +		pr_err("area %s (%pa, size %pa) overlaps kernel image [%pa-%pa]\n",
> +		       name ?: "(unnamed)", &base, &size, &text, &end);

We don't want to expose addresses of the kernel image.
I'd drop text, end, and probably even base from the printout.

> +		return -EBUSY;
> +	}
> +
>  	/*
>  	 * CMA uses CMA_MIN_ALIGNMENT_BYTES as alignment requirement which
>  	 * needs pageblock_order to be initialized. Let's enforce it.
> -- 
> 2.44.4
> 

-- 
Sincerely yours,
Mike.