Re: [PATCH] kaslr: rename physmem_end and PHYSMEM_END to direct_map_physmem_end

["Gupta, Pankaj" <pankaj.gupta@xxxxxxx>]

On 10/9/2024 4:50 AM, John Hubbard wrote:
> For clarity. It's increasingly hard to reason about the code, when KASLR
> is moving around the boundaries. In this case where KASLR is randomizing
> the location of the kernel image within physical memory, the maximum
> number of address bits for physical memory has not changed.
>
> What has changed is the ending address of memory that is allowed to be
> directly mapped by the kernel.
>
> Let's name the variable, and the associated macro accordingly.
>
> Also, enhance the comment above the direct_map_physmem_end definition,
> to further clarify how this all works.
>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: Jordan Niethe <jniethe@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>

Seems reasonable to me.

Reviewed-by: Pankaj Gupta <pankaj.gupta@xxxxxxx>

> ---
>
> David Hildenbrand, I recall you had an unanswered question in this
> vicinity [1] when tglx's recent kaslr fix was being reviewed. Maybe this
> will help with that.
>
>
> [1] https://lore.kernel.org/linux-mm/ee205448-5fdd-495e-9d7c-c8a2b59f9c9e@xxxxxxxxxxxx/T/#mdf442f077c9023590e144dbed2b04a109793484d
>
> thanks,
> John Hubbard
>
>
>   arch/arm64/include/asm/memory.h         |  2 +-
>   arch/x86/include/asm/page_64.h          |  2 +-
>   arch/x86/include/asm/pgtable_64_types.h |  2 +-
>   arch/x86/mm/init_64.c                   |  2 +-
>   arch/x86/mm/kaslr.c                     | 14 +++++++++-----
>   include/linux/mm.h                      |  6 +++---
>   kernel/resource.c                       |  4 ++--
>   mm/memory_hotplug.c                     |  2 +-
>   mm/sparse.c                             |  2 +-
>   9 files changed, 20 insertions(+), 16 deletions(-)
>
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 0480c61dbb4f..73eaa8c2536a 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -110,7 +110,7 @@
>   #define PAGE_END		(_PAGE_END(VA_BITS_MIN))
>   #endif /* CONFIG_KASAN */
>   
> -#define PHYSMEM_END		__pa(PAGE_END - 1)
> +#define DIRECT_MAP_PHYSMEM_END	__pa(PAGE_END - 1)
>   
>   #define MIN_THREAD_SHIFT	(14 + KASAN_THREAD_SHIFT)
>   
> diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
> index f3d257c45225..d63576608ce7 100644
> --- a/arch/x86/include/asm/page_64.h
> +++ b/arch/x86/include/asm/page_64.h
> @@ -17,7 +17,7 @@ extern unsigned long phys_base;
>   extern unsigned long page_offset_base;
>   extern unsigned long vmalloc_base;
>   extern unsigned long vmemmap_base;
> -extern unsigned long physmem_end;
> +extern unsigned long direct_map_physmem_end;
>   
>   static __always_inline unsigned long __phys_addr_nodebug(unsigned long x)
>   {
> diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
> index a98e53491a4e..ec68f8369bdc 100644
> --- a/arch/x86/include/asm/pgtable_64_types.h
> +++ b/arch/x86/include/asm/pgtable_64_types.h
> @@ -141,7 +141,7 @@ extern unsigned int ptrs_per_p4d;
>   #endif /* CONFIG_DYNAMIC_MEMORY_LAYOUT */
>   
>   #ifdef CONFIG_RANDOMIZE_MEMORY
> -# define PHYSMEM_END		physmem_end
> +# define DIRECT_MAP_PHYSMEM_END	direct_map_physmem_end
>   #endif
>   
>   /*
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index ff253648706f..5a564130b9d0 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -961,7 +961,7 @@ int add_pages(int nid, unsigned long start_pfn, unsigned long nr_pages,
>   	unsigned long end = ((start_pfn + nr_pages) << PAGE_SHIFT) - 1;
>   	int ret;
>   
> -	if (WARN_ON_ONCE(end > PHYSMEM_END))
> +	if (WARN_ON_ONCE(end > DIRECT_MAP_PHYSMEM_END))
>   		return -ERANGE;
>   
>   	ret = __add_pages(nid, start_pfn, nr_pages, params);
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index 230f1dee4f09..70d3353c92fa 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -52,7 +52,7 @@ static __initdata struct kaslr_memory_region {
>   } kaslr_regions[] = {
>   	{
>   		.base	= &page_offset_base,
> -		.end	= &physmem_end,
> +		.end	= &direct_map_physmem_end,
>   	},
>   	{
>   		.base	= &vmalloc_base,
> @@ -62,8 +62,12 @@ static __initdata struct kaslr_memory_region {
>   	},
>   };
>   
> -/* The end of the possible address space for physical memory */
> -unsigned long physmem_end __ro_after_init;
> +/*
> + * The end of the physical address space that can be mapped directly by the
> + * kernel. This starts out at (1<<MAX_PHYSMEM_BITS) - 1), but KASLR may reduce
> + * that in order to increase the available entropy for mapping other regions.
> + */
> +unsigned long direct_map_physmem_end __ro_after_init;
>   
>   /* Get size in bytes used by the memory region */
>   static inline unsigned long get_padding(struct kaslr_memory_region *region)
> @@ -94,7 +98,7 @@ void __init kernel_randomize_memory(void)
>   	BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
>   
>   	/* Preset the end of the possible address space for physical memory */
> -	physmem_end = ((1ULL << MAX_PHYSMEM_BITS) - 1);
> +	direct_map_physmem_end = ((1ULL << MAX_PHYSMEM_BITS) - 1);
>   	if (!kaslr_memory_enabled())
>   		return;
>   
> @@ -145,7 +149,7 @@ void __init kernel_randomize_memory(void)
>   		vaddr += get_padding(&kaslr_regions[i]);
>   		/*
>   		 * KASLR trims the maximum possible size of the
> -		 * direct-map. Update the physmem_end boundary.
> +		 * direct-map. Update the direct_map_physmem_end boundary.
>   		 * No rounding required as the region starts
>   		 * PUD aligned and size is in units of TB.
>   		 */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ecf63d2b0582..92c35fb60d18 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -97,11 +97,11 @@ extern const int mmap_rnd_compat_bits_max;
>   extern int mmap_rnd_compat_bits __read_mostly;
>   #endif
>   
> -#ifndef PHYSMEM_END
> +#ifndef DIRECT_MAP_PHYSMEM_END
>   # ifdef MAX_PHYSMEM_BITS
> -# define PHYSMEM_END	((1ULL << MAX_PHYSMEM_BITS) - 1)
> +# define DIRECT_MAP_PHYSMEM_END	((1ULL << MAX_PHYSMEM_BITS) - 1)
>   # else
> -# define PHYSMEM_END	(((phys_addr_t)-1)&~(1ULL<<63))
> +# define DIRECT_MAP_PHYSMEM_END	(((phys_addr_t)-1)&~(1ULL<<63))
>   # endif
>   #endif
>   
> diff --git a/kernel/resource.c b/kernel/resource.c
> index b730bd28b422..afa3ad09b834 100644
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -1871,7 +1871,7 @@ static resource_size_t gfr_start(struct resource *base, resource_size_t size,
>   	if (flags & GFR_DESCENDING) {
>   		resource_size_t end;
>   
> -		end = min_t(resource_size_t, base->end, PHYSMEM_END);
> +		end = min_t(resource_size_t, base->end, DIRECT_MAP_PHYSMEM_END);
>   		return end - size + 1;
>   	}
>   
> @@ -1888,7 +1888,7 @@ static bool gfr_continue(struct resource *base, resource_size_t addr,
>   	 * @size did not wrap 0.
>   	 */
>   	return addr > addr - size &&
> -	       addr <= min_t(resource_size_t, base->end, PHYSMEM_END);
> +	       addr <= min_t(resource_size_t, base->end, DIRECT_MAP_PHYSMEM_END);
>   }
>   
>   static resource_size_t gfr_next(resource_size_t addr, resource_size_t size,
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 621ae1015106..c43b4e7fb298 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1681,7 +1681,7 @@ struct range __weak arch_get_mappable_range(void)
>   
>   struct range mhp_get_pluggable_range(bool need_mapping)
>   {
> -	const u64 max_phys = PHYSMEM_END;
> +	const u64 max_phys = DIRECT_MAP_PHYSMEM_END;
>   	struct range mhp_range;
>   
>   	if (need_mapping) {
> diff --git a/mm/sparse.c b/mm/sparse.c
> index dc38539f8560..4cb9793f0b52 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -129,7 +129,7 @@ static inline int sparse_early_nid(struct mem_section *section)
>   static void __meminit mminit_validate_memmodel_limits(unsigned long *start_pfn,
>   						unsigned long *end_pfn)
>   {
> -	unsigned long max_sparsemem_pfn = (PHYSMEM_END + 1) >> PAGE_SHIFT;
> +	unsigned long max_sparsemem_pfn = (DIRECT_MAP_PHYSMEM_END + 1) >> PAGE_SHIFT;
>   
>   	/*
>   	 * Sanity checks - do not allow an architecture to pass