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

From: Heiko Carstens <hca@linux.ibm.com>
To: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Alexandre Ghiti <alex@ghiti.fr>, Borislav Petkov <bp@alien8.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	"Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	David Hildenbrand <david@kernel.org>,
	Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	Ingo Molnar <mingo@redhat.com>,
	"Liam R. Howlett" <liam@infradead.org>,
	Lorenzo Stoakes <ljs@kernel.org>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Michal Hocko <mhocko@suse.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <pjw@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Ritesh Harjani <ritesh.list@gmail.com>,
	Russell King <linux@armlinux.org.uk>,
	Shrikanth Hegde <sshegde@linux.ibm.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Thomas Gleixner <tglx@kernel.org>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Vlastimil Babka <vbabka@kernel.org>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
	linux-s390@vger.kernel.org
Subject: Re: [PATCH] arch, mm: promote DEBUG_WX to CHECK_WX
Date: Fri, 25 Sep 2026 15:42:23 +0200	[thread overview]
Message-ID: <20260925134223.23528E0e-hca@linux.ibm.com> (raw)
In-Reply-To: <20260925-direct-map-verify-wx-v1-1-7fd2f7d6d23b@kernel.org>

On Fri, Sep 25, 2026 at 12:53:46PM +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.
> 
> 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>
> ---
...
>  arch/s390/Kconfig                    |  2 +-
>  arch/s390/configs/debug_defconfig    |  2 +-
>  arch/s390/configs/defconfig          |  2 +-
>  arch/s390/mm/dump_pagetables.c       |  2 +-

Acked-by: Heiko Carstens <hca@linux.ibm.com> # s390

> 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),

Hm... looks like there is bug with relocated lowcore handling in the if
condition above the WARN_ONCE(). I'm going to address that, but that
has nothing to do with your patch.