linux-hardening - Re: [PATCH] arm64: mm: Fix CFI failure due to kpti_ng_pgd

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Message-ID: <6b692d6c-2da6-413b-90fa-c1425c075628@arm.com>
Date: Fri, 29 Aug 2025 17:06:05 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Ard Biesheuvel <ardb@...nel.org>, Kees Cook <kees@...nel.org>,
 Kevin Brodsky <kevin.brodsky@....com>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Oliver Upton <oliver.upton@...ux.dev>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Yue Haibing <yuehaibing@...wei.com>, Mark Rutland <mark.rutland@....com>,
 Marc Zyngier <maz@...nel.org>, Mark Brown <broonie@...nel.org>,
 linux-arm-kernel@...ts.infradead.org,
 Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
 Joey Gouly <joey.gouly@....com>, Yeoreum Yun <yeoreum.yun@....com>,
 James Morse <james.morse@....com>,
 Hardevsinh Palaniya <hardevsinh.palaniya@...iconsignals.io>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Kevin Brodsky <kevin.brodsky@....com>, David Hildenbrand <david@...hat.com>,
 Zhenhua Huang <quic_zhenhuah@...cinc.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Dev Jain <dev.jain@....com>,
 Yicong Yang <yangyicong@...ilicon.com>, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH] arm64: mm: Fix CFI failure due to kpti_ng_pgd_alloc
 function signature

+ Kevin Broadsky.

On 29/08/2025 16:59, Ard Biesheuvel wrote:
> On Fri, 29 Aug 2025 at 17:49, Kees Cook <kees@...nel.org> wrote:
>>
>> Seen during KPTI initialization:
>>
>>   CFI failure at create_kpti_ng_temp_pgd+0x124/0xce8 (target: kpti_ng_pgd_alloc+0x0/0x14; expected type: 0xd61b88b6)
>>
>> The call site is alloc_init_pud() at arch/arm64/mm/mmu.c:
>>
>>   pud_phys = pgtable_alloc(TABLE_PUD);
>>
>> alloc_init_pud() has the prototype:
>>
>>   static void alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
>>                              phys_addr_t phys, pgprot_t prot,
>>                              phys_addr_t (*pgtable_alloc)(enum pgtable_type),
>>                              int flags)
>>
>> where the pgtable_alloc() prototype is declared.
>>
>> The target (kpti_ng_pgd_alloc) is used in arch/arm64/kernel/cpufeature.c:
>>
>>   create_kpti_ng_temp_pgd(kpti_ng_temp_pgd, __pa(alloc), KPTI_NG_TEMP_VA,
>>                           PAGE_SIZE, PAGE_KERNEL, kpti_ng_pgd_alloc, 0);
>>
>> which is an alias for __create_pgd_mapping_locked() with prototype:
>>
>>   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);
>>
>> __create_pgd_mapping_locked() passes the function pointer down:
>>
>>   __create_pgd_mapping_locked() -> alloc_init_p4d() -> alloc_init_pud()
>>
>> But the target function (kpti_ng_pgd_alloc) has the wrong signature:
>>
>>   static phys_addr_t __init kpti_ng_pgd_alloc(int shift);
>>
>> The "int" should be "enum pgtable_type".
>>
>> To make "enum pgtable_type" available to cpufeature.c, move
>> enum pgtable_type definition from arch/arm64/mm/mmu.c to
>> arch/arm64/include/asm/mmu.h.
>>
>> Adjust kpti_ng_pgd_alloc to use "enum pgtable_type" instead of "int".
>> The function behavior remains identical (parameter is unused).
>>
>> Fixes: 47546a1912fc ("arm64: mm: install KPTI nG mappings with MMU enabled")
> 
> Shouldn't this be
> 
> Fixes: c64f46ee1377 ("arm64: mm: use enum to identify pgtable level
> instead of *_SHIFT")

You beat me to it; agreed. I've added Kevin, who made that change incase there
are any other subtlety.

> 
> 
>> Signed-off-by: Kees Cook <kees@...nel.org>
> 
> Acked-by: Ard Biesheuvel <ardb@...nel.org>

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

> 
> 
>> ---
>> Cc: Ard Biesheuvel <ardb@...nel.org>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Oliver Upton <oliver.upton@...ux.dev>
>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>> Cc: Yue Haibing <yuehaibing@...wei.com>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Marc Zyngier <maz@...nel.org>
>> Cc: Mark Brown <broonie@...nel.org>
>> Cc: <linux-arm-kernel@...ts.infradead.org>
>> ---
>>  arch/arm64/include/asm/mmu.h   | 7 +++++++
>>  arch/arm64/kernel/cpufeature.c | 5 +++--
>>  arch/arm64/mm/mmu.c            | 7 -------
>>  3 files changed, 10 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
>> index 6e8aa8e72601..49f1a810df16 100644
>> --- a/arch/arm64/include/asm/mmu.h
>> +++ b/arch/arm64/include/asm/mmu.h
>> @@ -17,6 +17,13 @@
>>  #include <linux/refcount.h>
>>  #include <asm/cpufeature.h>
>>
>> +enum pgtable_type {
>> +       TABLE_PTE,
>> +       TABLE_PMD,
>> +       TABLE_PUD,
>> +       TABLE_P4D,
>> +};
>> +
>>  typedef struct {
>>         atomic64_t      id;
>>  #ifdef CONFIG_COMPAT
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 9ad065f15f1d..e49d142a281f 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -84,6 +84,7 @@
>>  #include <asm/hwcap.h>
>>  #include <asm/insn.h>
>>  #include <asm/kvm_host.h>
>> +#include <asm/mmu.h>
>>  #include <asm/mmu_context.h>
>>  #include <asm/mte.h>
>>  #include <asm/hypervisor.h>
>> @@ -1945,11 +1946,11 @@ static bool has_pmuv3(const struct arm64_cpu_capabilities *entry, int scope)
>>  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)(int), int flags);
>> +                            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(int shift)
>> +static phys_addr_t __init kpti_ng_pgd_alloc(enum pgtable_type type)
>>  {
>>         kpti_ng_temp_alloc -= PAGE_SIZE;
>>         return kpti_ng_temp_alloc;
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 34e5d78af076..183801520740 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -47,13 +47,6 @@
>>  #define NO_CONT_MAPPINGS       BIT(1)
>>  #define NO_EXEC_MAPPINGS       BIT(2)  /* assumes FEAT_HPDS is not used */
>>
>> -enum pgtable_type {
>> -       TABLE_PTE,
>> -       TABLE_PMD,
>> -       TABLE_PUD,
>> -       TABLE_P4D,
>> -};
>> -
>>  u64 kimage_voffset __ro_after_init;
>>  EXPORT_SYMBOL(kimage_voffset);
>>
>> --
>> 2.34.1
>>