Re: [PATCH] arm64: mm: Move KPTI helpers to mmu.c

Ryan Roberts
2025-09-10T14:33:39Z

On 10/09/2025 11:44, Kevin Brodsky wrote:
> create_kpti_ng_temp_pgd() is currently defined (as an alias) in
> mmu.c without matching declaration in a header; instead cpufeature.c
> makes its own declaration. This is clearly not pretty, and as commit
> ceca927c86e6 ("arm64: mm: Fix CFI failure due to kpti_ng_pgd_alloc
> function signature") showed, it also makes it very easy for the
> prototypes to go out of sync.
>
> All this would be much simpler if kpti_install_ng_mappings() and
> associated functions lived in mmu.c, where they logically belong.
> This is what this patch does:
> - Move kpti_install_ng_mappings() and associated functions from
> cpufeature.c to mmu.c, add a declaration to <asm/mmu.h>
> - Make create_kpti_ng_temp_pgd() a static function that simply calls
> __create_pgd_mapping_locked() instead of aliasing it
> - Mark all these functions __init
> - Move __initdata after kpti_ng_temp_alloc (as suggested by
> checkpatch)

This is a great clean up IMHO; that alias has caught me out a few times in the
past when hacking in this area. And this code clearly belongs in mmu.c.

>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
> ---
> Note: as things stand, create_kpti_ng_temp_pgd() could be removed,
> but a separate patch [1] will make use of it to add an
> assertion.

I'd vote for removing it and just calling __create_pgd_mapping_locked() direct.
The next version of the other patch can just rebase on top of yours and add the
assert in __kpti_install_ng_mappings().

Either way:

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

>
> [1] https://lore.kernel.org/all/20250813145607.1612234-3-chaitanyas.prakash@arm.com/
> ---
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Kees Cook <kees@kernel.org>,
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Yeoreum Yun <yeoreum.yun@arm.com>
> ---
> arch/arm64/include/asm/mmu.h | 6 ++
> arch/arm64/kernel/cpufeature.c | 97 ------------------------------
> arch/arm64/mm/mmu.c | 106 ++++++++++++++++++++++++++++++---
> 3 files changed, 103 insertions(+), 106 deletions(-)
>
> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> index 49f1a810df16..624edd6c4964 100644
> --- a/arch/arm64/include/asm/mmu.h
> +++ b/arch/arm64/include/asm/mmu.h
> @@ -104,5 +104,11 @@ static inline bool kaslr_requires_kpti(void)
> return true;
> }
>
> +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> +void kpti_install_ng_mappings(void);
> +#else
> +static inline void kpti_install_ng_mappings(void) {}
> +#endif
> +
> #endif /* !__ASSEMBLY__ */
> #endif
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index ef269a5a37e1..b99eaad48c14 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1940,103 +1940,6 @@ static bool has_pmuv3(const struct arm64_cpu_capabilities *entry, int scope)
> }
> #endif
>
> -#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> -#define KPTI_NG_TEMP_VA (-(1UL << PMD_SHIFT))
> -
> -extern
> -void create_kpti_ng_temp_pgd(pgd_t *pgdir, phys_addr_t phys, unsigned long virt,
> - phys_addr_t size, pgprot_t prot,
> - phys_addr_t (*pgtable_alloc)(enum pgtable_type), int flags);
> -
> -static phys_addr_t __initdata kpti_ng_temp_alloc;
> -
> -static phys_addr_t __init kpti_ng_pgd_alloc(enum pgtable_type type)
> -{
> - kpti_ng_temp_alloc -= PAGE_SIZE;
> - return kpti_ng_temp_alloc;
> -}
> -
> -static int __init __kpti_install_ng_mappings(void *__unused)
> -{
> - typedef void (kpti_remap_fn)(int, int, phys_addr_t, unsigned long);
> - extern kpti_remap_fn idmap_kpti_install_ng_mappings;
> - kpti_remap_fn *remap_fn;
> -
> - int cpu = smp_processor_id();
> - int levels = CONFIG_PGTABLE_LEVELS;
> - int order = order_base_2(levels);
> - u64 kpti_ng_temp_pgd_pa = 0;
> - pgd_t *kpti_ng_temp_pgd;
> - u64 alloc = 0;
> -
> - if (levels == 5 && !pgtable_l5_enabled())
> - levels = 4;
> - else if (levels == 4 && !pgtable_l4_enabled())
> - levels = 3;
> -
> - remap_fn = (void *)__pa_symbol(idmap_kpti_install_ng_mappings);
> -
> - if (!cpu) {
> - alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
> - kpti_ng_temp_pgd = (pgd_t *)(alloc + (levels - 1) * PAGE_SIZE);
> - kpti_ng_temp_alloc = kpti_ng_temp_pgd_pa = __pa(kpti_ng_temp_pgd);
> -
> - //
> - // Create a minimal page table hierarchy that permits us to map
> - // the swapper page tables temporarily as we traverse them.
> - //
> - // The physical pages are laid out as follows:
> - //
> - // +--------+-/-------+-/------ +-/------ +-\\\--------+
> - // : PTE[] : | PMD[] : | PUD[] : | P4D[] : ||| PGD[] :
> - // +--------+-\-------+-\------ +-\------ +-///--------+
> - // ^
> - // The first page is mapped into this hierarchy at a PMD_SHIFT
> - // aligned virtual address, so that we can manipulate the PTE
> - // level entries while the mapping is active. The first entry
> - // covers the PTE[] page itself, the remaining entries are free
> - // to be used as a ad-hoc fixmap.
> - //
> - create_kpti_ng_temp_pgd(kpti_ng_temp_pgd, __pa(alloc),
> - KPTI_NG_TEMP_VA, PAGE_SIZE, PAGE_KERNEL,
> - kpti_ng_pgd_alloc, 0);
> - }
> -
> - cpu_install_idmap();
> - remap_fn(cpu, num_online_cpus(), kpti_ng_temp_pgd_pa, KPTI_NG_TEMP_VA);
> - cpu_uninstall_idmap();
> -
> - if (!cpu) {
> - free_pages(alloc, order);
> - arm64_use_ng_mappings = true;
> - }
> -
> - return 0;
> -}
> -
> -static void __init kpti_install_ng_mappings(void)
> -{
> - /* Check whether KPTI is going to be used */
> - if (!arm64_kernel_unmapped_at_el0())
> - return;
> -
> - /*
> - * We don't need to rewrite the page-tables if either we've done
> - * it already or we have KASLR enabled and therefore have not
> - * created any global mappings at all.
> - */
> - if (arm64_use_ng_mappings)
> - return;
> -
> - stop_machine(__kpti_install_ng_mappings, NULL, cpu_online_mask);
> -}
> -
> -#else
> -static inline void kpti_install_ng_mappings(void)
> -{
> -}
> -#endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
> -
> static void cpu_enable_kpti(struct arm64_cpu_capabilities const *cap)
> {
> if (__this_cpu_read(this_cpu_vector) == vectors) {
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 183801520740..eff3295393ee 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -27,6 +27,7 @@
> #include <linux/kfence.h>
> #include <linux/pkeys.h>
> #include <linux/mm_inline.h>
> +#include <linux/stop_machine.h>
>
> #include <asm/barrier.h>
> #include <asm/cputype.h>
> @@ -466,14 +467,6 @@ static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
> mutex_unlock(&fixmap_lock);
> }
>
> -#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> -extern __alias(__create_pgd_mapping_locked)
> -void create_kpti_ng_temp_pgd(pgd_t *pgdir, phys_addr_t phys, unsigned long virt,
> - phys_addr_t size, pgprot_t prot,
> - phys_addr_t (*pgtable_alloc)(enum pgtable_type),
> - int flags);
> -#endif
> -
> static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm,
> enum pgtable_type pgtable_type)
> {
> @@ -735,7 +728,102 @@ static void __init declare_vma(struct vm_struct *vma,
> }
>
> #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> -static pgprot_t kernel_exec_prot(void)
> +#define KPTI_NG_TEMP_VA (-(1UL << PMD_SHIFT))
> +
> +static phys_addr_t kpti_ng_temp_alloc __initdata;
> +
> +static phys_addr_t __init kpti_ng_pgd_alloc(enum pgtable_type type)
> +{
> + kpti_ng_temp_alloc -= PAGE_SIZE;
> + return kpti_ng_temp_alloc;
> +}
> +
> +static void __init create_kpti_ng_temp_pgd(pgd_t *pgdir, phys_addr_t phys,
> + unsigned long virt, phys_addr_t size,
> + pgprot_t prot,
> + phys_addr_t (*pgtable_alloc)(enum pgtable_type),
> + int flags)
> +{
> + __create_pgd_mapping_locked(pgdir, phys, virt, size, prot,
> + pgtable_alloc, flags);
> +}
> +
> +static int __init __kpti_install_ng_mappings(void *__unused)
> +{
> + typedef void (kpti_remap_fn)(int, int, phys_addr_t, unsigned long);
> + extern kpti_remap_fn idmap_kpti_install_ng_mappings;
> + kpti_remap_fn *remap_fn;
> +
> + int cpu = smp_processor_id();
> + int levels = CONFIG_PGTABLE_LEVELS;
> + int order = order_base_2(levels);
> + u64 kpti_ng_temp_pgd_pa = 0;
> + pgd_t *kpti_ng_temp_pgd;
> + u64 alloc = 0;
> +
> + if (levels == 5 && !pgtable_l5_enabled())
> + levels = 4;
> + else if (levels == 4 && !pgtable_l4_enabled())
> + levels = 3;
> +
> + remap_fn = (void *)__pa_symbol(idmap_kpti_install_ng_mappings);
> +
> + if (!cpu) {
> + alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
> + kpti_ng_temp_pgd = (pgd_t *)(alloc + (levels - 1) * PAGE_SIZE);
> + kpti_ng_temp_alloc = kpti_ng_temp_pgd_pa = __pa(kpti_ng_temp_pgd);
> +
> + //
> + // Create a minimal page table hierarchy that permits us to map
> + // the swapper page tables temporarily as we traverse them.
> + //
> + // The physical pages are laid out as follows:
> + //
> + // +--------+-/-------+-/------ +-/------ +-\\\--------+
> + // : PTE[] : | PMD[] : | PUD[] : | P4D[] : ||| PGD[] :
> + // +--------+-\-------+-\------ +-\------ +-///--------+
> + // ^
> + // The first page is mapped into this hierarchy at a PMD_SHIFT
> + // aligned virtual address, so that we can manipulate the PTE
> + // level entries while the mapping is active. The first entry
> + // covers the PTE[] page itself, the remaining entries are free
> + // to be used as a ad-hoc fixmap.
> + //
> + create_kpti_ng_temp_pgd(kpti_ng_temp_pgd, __pa(alloc),
> + KPTI_NG_TEMP_VA, PAGE_SIZE, PAGE_KERNEL,
> + kpti_ng_pgd_alloc, 0);
> + }
> +
> + cpu_install_idmap();
> + remap_fn(cpu, num_online_cpus(), kpti_ng_temp_pgd_pa, KPTI_NG_TEMP_VA);
> + cpu_uninstall_idmap();
> +
> + if (!cpu) {
> + free_pages(alloc, order);
> + arm64_use_ng_mappings = true;
> + }
> +
> + return 0;
> +}
> +
> +void __init kpti_install_ng_mappings(void)
> +{
> + /* Check whether KPTI is going to be used */
> + if (!arm64_kernel_unmapped_at_el0())
> + return;
> +
> + /*
> + * We don't need to rewrite the page-tables if either we've done
> + * it already or we have KASLR enabled and therefore have not
> + * created any global mappings at all.
> + */
> + if (arm64_use_ng_mappings)
> + return;
> +
> + stop_machine(__kpti_install_ng_mappings, NULL, cpu_online_mask);
> +}
> +
> +static pgprot_t __init kernel_exec_prot(void)

nit: this change (to add __init) is unrelated; does it deserve it's own patch?

> {
> return rodata_enabled ? PAGE_KERNEL_ROX : PAGE_KERNEL_EXEC;
> }
>
> base-commit: 76eeb9b8de9880ca38696b2fb56ac45ac0a25c6c