Re: [PATCH v2] arch, mm: promote DEBUG_WX to CHECK_WX

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Sat, Sep 26, 2026 at 12:29:29PM +0300, Mike Rapoport (Microsoft) wrote:
> Verification that the kernel does not have writable + executable
> mappings is about detecting security risks rather than a pure debug
> feature.
>
> Major distribution configurations enable it in their kernels as well as
> defconfigs of most architectures that have ARCH_HAS_DEBUG_WX.
>
> Rename relevant generic configuration options to use CHECK_WX and move
> their definitions from mm/Kconfig.debug to mm/Kconfig.
>
> Rename *debug_checkwx() funcitons and macros to *pgtable_checkwx().
>
> For arm that does not widely enable it, only rename its variants of the
> config options.
>
> Enabling CHECK_WX adds a few kilobytes to the kernel binary and while
> the added size can be slightly reduced with churny updates of
> architecture implementations of ptdump, the core functionality takes
> most of the added size. It cannot be moved to .init.text because the
> verification has to happen after init sections are freed.
>
> With this, make generic CHECK_WX default to STRICT_KERNEL_RWX while
> still leaving users targeting small kernels the possibility to opt-out.
>
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

As with v1, looks sensible to me so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
> Changes in v2:
> - add rename of *debug_checkwx() to *pgtable_checkwx()
> - Link to v1: https://patch.msgid.link/20260925-direct-map-verify-wx-v1-1-7fd2f7d6d23b@kernel.org
> ---
>  arch/arm/Kconfig.debug               |  2 +-
>  arch/arm/configs/aspeed_g4_defconfig |  2 +-
>  arch/arm/configs/aspeed_g5_defconfig |  2 +-
>  arch/arm/configs/shmobile_defconfig  |  2 +-
>  arch/arm/include/asm/ptdump.h        |  6 +++---
>  arch/arm/mm/init.c                   |  2 +-
>  arch/arm64/Kconfig                   |  2 +-
>  arch/powerpc/Kconfig                 |  2 +-
>  arch/powerpc/configs/ppc64_defconfig |  2 +-
>  arch/powerpc/mm/ptdump/ptdump.c      |  2 +-
>  arch/riscv/Kconfig                   |  2 +-
>  arch/s390/Kconfig                    |  2 +-
>  arch/s390/configs/debug_defconfig    |  2 +-
>  arch/s390/configs/defconfig          |  2 +-
>  arch/s390/mm/dump_pagetables.c       |  2 +-
>  arch/x86/Kconfig                     |  2 +-
>  arch/x86/configs/x86_64_defconfig    |  2 +-
>  arch/x86/include/asm/pgtable.h       |  6 +++---
>  arch/x86/mm/pti.c                    |  2 +-
>  include/linux/ptdump.h               |  4 ++--
>  init/main.c                          |  2 +-
>  kernel/configs/debug.config          |  2 +-
>  mm/Kconfig                           | 41 ++++++++++++++++++++++++++++++++++++
>  mm/Kconfig.debug                     | 39 ----------------------------------
>  24 files changed, 68 insertions(+), 66 deletions(-)
>
> diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
> index 366f162e147d..abcf14f10276 100644
> --- a/arch/arm/Kconfig.debug
> +++ b/arch/arm/Kconfig.debug
> @@ -17,7 +17,7 @@ config ARM_PTDUMP_DEBUGFS
>  	  kernel.
>  	  If in doubt, say "N"
>
> -config ARM_DEBUG_WX
> +config ARM_CHECK_WX
>  	bool "Warn on W+X mappings at boot"
>  	depends on MMU
>  	select ARM_PTDUMP_CORE
> diff --git a/arch/arm/configs/aspeed_g4_defconfig b/arch/arm/configs/aspeed_g4_defconfig
> index f86dd4ce7d0d..2c9d5a644ae9 100644
> --- a/arch/arm/configs/aspeed_g4_defconfig
> +++ b/arch/arm/configs/aspeed_g4_defconfig
> @@ -249,7 +249,7 @@ CONFIG_DEBUG_INFO_REDUCED=y
>  CONFIG_GDB_SCRIPTS=y
>  CONFIG_STRIP_ASM_SYMS=y
>  CONFIG_DEBUG_FS=y
> -CONFIG_ARM_DEBUG_WX=y
> +CONFIG_ARM_CHECK_WX=y
>  CONFIG_SCHED_STACK_END_CHECK=y
>  CONFIG_PANIC_ON_OOPS=y
>  CONFIG_PANIC_TIMEOUT=-1
> diff --git a/arch/arm/configs/aspeed_g5_defconfig b/arch/arm/configs/aspeed_g5_defconfig
> index 45b937419dbd..1327a09e163a 100644
> --- a/arch/arm/configs/aspeed_g5_defconfig
> +++ b/arch/arm/configs/aspeed_g5_defconfig
> @@ -300,7 +300,7 @@ CONFIG_DEBUG_INFO_REDUCED=y
>  CONFIG_GDB_SCRIPTS=y
>  CONFIG_STRIP_ASM_SYMS=y
>  CONFIG_DEBUG_FS=y
> -CONFIG_ARM_DEBUG_WX=y
> +CONFIG_ARM_CHECK_WX=y
>  CONFIG_SCHED_STACK_END_CHECK=y
>  CONFIG_PANIC_ON_OOPS=y
>  CONFIG_PANIC_TIMEOUT=-1
> diff --git a/arch/arm/configs/shmobile_defconfig b/arch/arm/configs/shmobile_defconfig
> index 6f9696e9fe17..cc22e22b989e 100644
> --- a/arch/arm/configs/shmobile_defconfig
> +++ b/arch/arm/configs/shmobile_defconfig
> @@ -225,4 +225,4 @@ CONFIG_CMA_SIZE_MBYTES=64
>  CONFIG_PRINTK_TIME=y
>  CONFIG_DEBUG_KERNEL=y
>  CONFIG_DEBUG_FS=y
> -CONFIG_ARM_DEBUG_WX=y
> +CONFIG_ARM_CHECK_WX=y
> diff --git a/arch/arm/include/asm/ptdump.h b/arch/arm/include/asm/ptdump.h
> index 46a4575146ee..3c5245220ecf 100644
> --- a/arch/arm/include/asm/ptdump.h
> +++ b/arch/arm/include/asm/ptdump.h
> @@ -32,10 +32,10 @@ void ptdump_check_wx(void);
>
>  #endif /* CONFIG_ARM_PTDUMP_CORE */
>
> -#ifdef CONFIG_ARM_DEBUG_WX
> -#define arm_debug_checkwx() ptdump_check_wx()
> +#ifdef CONFIG_ARM_CHECK_WX
> +#define arm_pgtable_checkwx() ptdump_check_wx()
>  #else
> -#define arm_debug_checkwx() do { } while (0)
> +#define arm_pgtable_checkwx() do { } while (0)
>  #endif
>
>  #endif /* __ASM_PTDUMP_H */
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 0cc1bf04686d..c515faf22eeb 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -403,7 +403,7 @@ static int __mark_rodata_ro(void *unused)
>  void mark_rodata_ro(void)
>  {
>  	stop_machine(__mark_rodata_ro, NULL, NULL);
> -	arm_debug_checkwx();
> +	arm_pgtable_checkwx();
>  }
>
>  #else
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b5a51b0ef944..7e120fb21c58 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -11,7 +11,7 @@ config ARM64
>  	select ACPI_MCFG if (ACPI && PCI)
>  	select ACPI_SPCR_TABLE if ACPI
>  	select ACPI_PPTT if ACPI
> -	select ARCH_HAS_DEBUG_WX
> +	select ARCH_HAS_CHECK_WX
>  	select ARCH_BINFMT_ELF_EXTRA_PHDRS
>  	select ARCH_BINFMT_ELF_STATE
>  	select ARCH_ENABLE_HUGEPAGE_MIGRATION if HUGETLB_PAGE && MIGRATION
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 2580e27e4328..7c1fe5b4cd41 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -130,7 +130,7 @@ config PPC
>  	select ARCH_HAS_CURRENT_STACK_POINTER
>  	select ARCH_HAS_DEBUG_VIRTUAL
>  	select ARCH_HAS_DEBUG_VM_PGTABLE
> -	select ARCH_HAS_DEBUG_WX		if STRICT_KERNEL_RWX
> +	select ARCH_HAS_CHECK_WX		if STRICT_KERNEL_RWX
>  	select ARCH_HAS_DEVMEM_IS_ALLOWED
>  	select ARCH_HAS_DMA_MAP_DIRECT 		if PPC_PSERIES
>  	select ARCH_HAS_DMA_OPS			if PPC64
> diff --git a/arch/powerpc/configs/ppc64_defconfig b/arch/powerpc/configs/ppc64_defconfig
> index 1eb8e3457e8b..5c33f0bba0e3 100644
> --- a/arch/powerpc/configs/ppc64_defconfig
> +++ b/arch/powerpc/configs/ppc64_defconfig
> @@ -393,7 +393,7 @@ CONFIG_MAGIC_SYSRQ=y
>  CONFIG_PAGE_OWNER=y
>  CONFIG_PAGE_POISONING=y
>  CONFIG_DEBUG_RODATA_TEST=y
> -CONFIG_DEBUG_WX=y
> +CONFIG_CHECK_WX=y
>  CONFIG_DEBUG_STACK_USAGE=y
>  CONFIG_DEBUG_VM=y
>  # CONFIG_DEBUG_VM_PGTABLE is not set
> diff --git a/arch/powerpc/mm/ptdump/ptdump.c b/arch/powerpc/mm/ptdump/ptdump.c
> index 0d499aebee72..3451351b756b 100644
> --- a/arch/powerpc/mm/ptdump/ptdump.c
> +++ b/arch/powerpc/mm/ptdump/ptdump.c
> @@ -191,7 +191,7 @@ static void note_prot_wx(struct pg_state *st, unsigned long addr)
>  	if (!pte_write(pte) || !pte_exec(pte))
>  		return;
>
> -	WARN_ONCE(IS_ENABLED(CONFIG_DEBUG_WX),
> +	WARN_ONCE(IS_ENABLED(CONFIG_CHECK_WX),
>  		  "powerpc/mm: Found insecure W+X mapping at address %p/%pS\n",
>  		  (void *)st->start_address, (void *)st->start_address);
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index d6c2dbf8455c..05e33d4d5efa 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -29,7 +29,7 @@ config RISCV
>  	select ARCH_HAS_CURRENT_STACK_POINTER
>  	select ARCH_HAS_DEBUG_VIRTUAL if MMU
>  	select ARCH_HAS_DEBUG_VM_PGTABLE
> -	select ARCH_HAS_DEBUG_WX
> +	select ARCH_HAS_CHECK_WX
>  	select ARCH_HAS_DELAY_TIMER
>  	select ARCH_HAS_ELF_CORE_EFLAGS if BINFMT_ELF && ELF_CORE
>  	select ARCH_HAS_FAST_MULTIPLIER
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index 4b51bc6e8948..11b76f2b0f80 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -92,7 +92,7 @@ config S390
>  	select ARCH_HAS_CURRENT_STACK_POINTER
>  	select ARCH_HAS_DEBUG_VIRTUAL
>  	select ARCH_HAS_DEBUG_VM_PGTABLE
> -	select ARCH_HAS_DEBUG_WX
> +	select ARCH_HAS_CHECK_WX
>  	select ARCH_HAS_DEVMEM_IS_ALLOWED
>  	select ARCH_HAS_DMA_OPS if PCI
>  	select ARCH_HAS_ELF_RANDOMIZE
> diff --git a/arch/s390/configs/debug_defconfig b/arch/s390/configs/debug_defconfig
> index 3dae71474333..68d53c0bc8db 100644
> --- a/arch/s390/configs/debug_defconfig
> +++ b/arch/s390/configs/debug_defconfig
> @@ -841,7 +841,7 @@ CONFIG_DEBUG_PAGEALLOC=y
>  CONFIG_SLUB_DEBUG_ON=y
>  CONFIG_PAGE_OWNER=y
>  CONFIG_DEBUG_RODATA_TEST=y
> -CONFIG_DEBUG_WX=y
> +CONFIG_CHECK_WX=y
>  CONFIG_PTDUMP_DEBUGFS=y
>  CONFIG_DEBUG_OBJECTS=y
>  CONFIG_DEBUG_OBJECTS_SELFTEST=y
> diff --git a/arch/s390/configs/defconfig b/arch/s390/configs/defconfig
> index 6f5722634b4d..8e5cfc695121 100644
> --- a/arch/s390/configs/defconfig
> +++ b/arch/s390/configs/defconfig
> @@ -820,7 +820,7 @@ CONFIG_DEBUG_INFO_DWARF4=y
>  CONFIG_GDB_SCRIPTS=y
>  CONFIG_DEBUG_SECTION_MISMATCH=y
>  CONFIG_MAGIC_SYSRQ=y
> -CONFIG_DEBUG_WX=y
> +CONFIG_CHECK_WX=y
>  CONFIG_PTDUMP_DEBUGFS=y
>  CONFIG_DEBUG_MEMORY_INIT=y
>  CONFIG_PANIC_ON_OOPS=y
> diff --git a/arch/s390/mm/dump_pagetables.c b/arch/s390/mm/dump_pagetables.c
> index 89badbe72ae7..a23a0bd4d8a8 100644
> --- a/arch/s390/mm/dump_pagetables.c
> +++ b/arch/s390/mm/dump_pagetables.c
> @@ -86,7 +86,7 @@ static void note_prot_wx(struct pg_state *st, unsigned long addr)
>  	 */
>  	if (addr == PAGE_SIZE && (nospec_uses_trampoline() || !cpu_has_bear()))
>  		return;
> -	WARN_ONCE(IS_ENABLED(CONFIG_DEBUG_WX),
> +	WARN_ONCE(IS_ENABLED(CONFIG_CHECK_WX),
>  		  "s390/mm: Found insecure W+X mapping at address %pS\n",
>  		  (void *)st->start_address);
>  	st->wx_pages += (addr - st->start_address) / PAGE_SIZE;
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 15fd9ec5ecac..170cfcb06174 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -110,7 +110,7 @@ config X86
>  	select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
>  	select ARCH_HAS_SYSCALL_WRAPPER
>  	select ARCH_HAS_UBSAN
> -	select ARCH_HAS_DEBUG_WX
> +	select ARCH_HAS_CHECK_WX
>  	select ARCH_HAS_ZONE_DMA_SET if EXPERT
>  	select ARCH_HAVE_NMI_SAFE_CMPXCHG
>  	select ARCH_HAVE_EXTRA_ELF_NOTES
> diff --git a/arch/x86/configs/x86_64_defconfig b/arch/x86/configs/x86_64_defconfig
> index 269f7d808be4..e6896aeb77d8 100644
> --- a/arch/x86/configs/x86_64_defconfig
> +++ b/arch/x86/configs/x86_64_defconfig
> @@ -263,7 +263,7 @@ CONFIG_SECURITY_SELINUX_BOOTPARAM=y
>  CONFIG_PRINTK_TIME=y
>  CONFIG_DEBUG_KERNEL=y
>  CONFIG_MAGIC_SYSRQ=y
> -CONFIG_DEBUG_WX=y
> +CONFIG_CHECK_WX=y
>  CONFIG_DEBUG_STACK_USAGE=y
>  CONFIG_SCHEDSTATS=y
>  CONFIG_BLK_DEV_IO_TRACE=y
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index d551120a7c88..ef0252a09c28 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -41,10 +41,10 @@ void ptdump_walk_user_pgd_level_checkwx(void);
>  #define pgprot_encrypted(prot)	__pgprot(cc_mkenc(pgprot_val(prot)))
>  #define pgprot_decrypted(prot)	__pgprot(cc_mkdec(pgprot_val(prot)))
>
> -#ifdef CONFIG_DEBUG_WX
> -#define debug_checkwx_user()	ptdump_walk_user_pgd_level_checkwx()
> +#ifdef CONFIG_CHECK_WX
> +#define pgtable_checkwx_user()	ptdump_walk_user_pgd_level_checkwx()
>  #else
> -#define debug_checkwx_user()	do { } while (0)
> +#define pgtable_checkwx_user()	do { } while (0)
>  #endif
>
>  extern spinlock_t pgd_lock;
> diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
> index 598f553cc871..31055ee6f1de 100644
> --- a/arch/x86/mm/pti.c
> +++ b/arch/x86/mm/pti.c
> @@ -688,5 +688,5 @@ void pti_finalize(void)
>  	pti_clone_entry_text(true);
>  	pti_clone_kernel_text();
>
> -	debug_checkwx_user();
> +	pgtable_checkwx_user();
>  }
> diff --git a/include/linux/ptdump.h b/include/linux/ptdump.h
> index 240bd3bff18d..af18d1459b2f 100644
> --- a/include/linux/ptdump.h
> +++ b/include/linux/ptdump.h
> @@ -31,9 +31,9 @@ bool ptdump_walk_pgd_level_core(struct seq_file *m,
>  void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm, pgd_t *pgd);
>  bool ptdump_check_wx(void);
>
> -static inline void debug_checkwx(void)
> +static inline void pgtable_checkwx(void)
>  {
> -	if (IS_ENABLED(CONFIG_DEBUG_WX))
> +	if (IS_ENABLED(CONFIG_CHECK_WX))
>  		ptdump_check_wx();
>  }
>
> diff --git a/init/main.c b/init/main.c
> index 31f2bf54976a..a87a3d52f3e2 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1535,7 +1535,7 @@ static void mark_readonly(void)
>  		flush_module_init_free_work();
>  		jump_label_init_ro();
>  		mark_rodata_ro();
> -		debug_checkwx();
> +		pgtable_checkwx();
>  		rodata_test();
>  	} else if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX)) {
>  		pr_info("Kernel memory protection disabled.\n");
> diff --git a/kernel/configs/debug.config b/kernel/configs/debug.config
> index 307c97ac5fa9..ac878669c193 100644
> --- a/kernel/configs/debug.config
> +++ b/kernel/configs/debug.config
> @@ -50,7 +50,7 @@ CONFIG_DEBUG_NET=y
>  # CONFIG_DEBUG_PAGEALLOC is not set
>  # CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF is not set
>  # CONFIG_DEBUG_RODATA_TEST is not set
> -# CONFIG_DEBUG_WX is not set
> +# CONFIG_CHECK_WX is not set
>  # CONFIG_KFENCE is not set
>  # CONFIG_PAGE_POISONING is not set
>  # CONFIG_SLUB_STATS is not set
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 604c58199acb..ffbc641cc31f 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1511,6 +1511,47 @@ config LAZY_MMU_MODE_KUNIT_TEST
>
>  	  If unsure, say N.
>
> +config ARCH_HAS_CHECK_WX
> +	bool
> +
> +config CHECK_WX
> +	bool "Warn on W+X mappings at boot"
> +	default STRICT_KERNEL_RWX
> +	depends on ARCH_HAS_CHECK_WX
> +	depends on ARCH_HAS_PTDUMP
> +	depends on MMU
> +	select PTDUMP
> +	help
> +	  Generate a warning if any W+X mappings are found at boot.
> +
> +	  This is useful for discovering cases where the kernel is leaving W+X
> +	  mappings after applying NX, as such mappings are a security risk.
> +
> +	  Look for a message in dmesg output like this:
> +
> +	    <arch>/mm: Checked W+X mappings: passed, no W+X pages found.
> +
> +	  or like this, if the check failed:
> +
> +	    <arch>/mm: Checked W+X mappings: failed, <N> W+X pages found.
> +
> +	  Note that even if the check fails, your kernel is possibly
> +	  still fine, as W+X mappings are not a security hole in
> +	  themselves, what they do is that they make the exploitation
> +	  of other unfixed kernel bugs easier.
> +
> +	  There is no runtime or memory usage effect of this option
> +	  once the kernel has booted up - it's a one time check.
> +
> +	  If in doubt, say "Y".
> +
> +config ARCH_HAS_PTDUMP
> +	bool
> +
> +config PTDUMP
> +	bool
> +
> +
>  source "mm/damon/Kconfig"
>
>  endmenu
> diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
> index 15dca19dd07d..75b44e4a6e36 100644
> --- a/mm/Kconfig.debug
> +++ b/mm/Kconfig.debug
> @@ -180,45 +180,6 @@ config DEBUG_RODATA_TEST
>  	help
>  	  This option enables a testcase for the setting rodata read-only.
>
> -config ARCH_HAS_DEBUG_WX
> -	bool
> -
> -config DEBUG_WX
> -	bool "Warn on W+X mappings at boot"
> -	depends on ARCH_HAS_DEBUG_WX
> -	depends on ARCH_HAS_PTDUMP
> -	depends on MMU
> -	select PTDUMP
> -	help
> -	  Generate a warning if any W+X mappings are found at boot.
> -
> -	  This is useful for discovering cases where the kernel is leaving W+X
> -	  mappings after applying NX, as such mappings are a security risk.
> -
> -	  Look for a message in dmesg output like this:
> -
> -	    <arch>/mm: Checked W+X mappings: passed, no W+X pages found.
> -
> -	  or like this, if the check failed:
> -
> -	    <arch>/mm: Checked W+X mappings: failed, <N> W+X pages found.
> -
> -	  Note that even if the check fails, your kernel is possibly
> -	  still fine, as W+X mappings are not a security hole in
> -	  themselves, what they do is that they make the exploitation
> -	  of other unfixed kernel bugs easier.
> -
> -	  There is no runtime or memory usage effect of this option
> -	  once the kernel has booted up - it's a one time check.
> -
> -	  If in doubt, say "Y".
> -
> -config ARCH_HAS_PTDUMP
> -	bool
> -
> -config PTDUMP
> -	bool
> -
>  config PTDUMP_DEBUGFS
>  	bool "Export kernel pagetable layout to userspace via debugfs"
>  	depends on DEBUG_KERNEL
>
> ---
> base-commit: 93f51579e7df248780214094418f205253383cc5
> change-id: 20260925-direct-map-verify-wx-b81bda879781
>
> --
> Sincerely yours,
> Mike.
>

--
Cheers, Lorenzo