Re: [RFC][PATCH] x86/mm: Disconnect RWX module checking from execmem

[Mike Rapoport <rppt@kernel.org>]

Hi Dave,

On Thu, Oct 01, 2026 at 01:19:52PM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> Frédéric reported:
> 
> 	x86/mm: Checked W+X mappings: FAILED, 7 W+X pages found.
> 
> errors on a build with modules disabled. There are some build
> dependencies in play:
> 
>  * ARCH_HAS_EXECMEM_ROX depends on STRICT_MODULE_RWX
>  * STRICT_MODULE_RWX depends on MODULES
>  * execmem_restore_rox() is a stub without ARCH_HAS_EXECMEM_ROX=y
> 
> execmem_restore_rox() is what the x86 "ITS" code uses to zap its
> W+X mappings during early boot. The error message comes from
> CONFIG_DEBUG_WX which is independent from any of the other options.
> 
> But the nutty part is that there is any connection at all between
> ARCH_HAS_EXECMEM_ROX and STRICT_MODULE_RWX.
> 
> Restore a functional execmem_restore_rox() to all 64-bit builds by
> eliminating the STRICT_MODULE_RWX dependency.
> 
> Then, turn our attention to execmem. 47410d839fcd "x86/Kconfig: only
> enable ROX cache...") was trying to make module memory access more
> permissive unless STRICT_MODULE_RWX is in play. For execmem, never
> consider module memory to be ROX except under STRICT_MODULE_RWX.
> 
> This seems to fix Frédéric's issue, but I'm a bit worried that it
> has other implications in the execmem code. For instance, the
> module_enable_text_rox() loop is now rather weird mem->is_rox is
> now directly dependent on STRICT_MODULE_RWX.
> 
> Mike, do you see anything bad happening on the execmem side if we
> do this?

This could work, but it takes us back to 4k mappings for executable code.

But since x86 always sets STRICT_MODULE_RWX=y when MODULES=y, I think we
can just revert 47410d839fcd.

I also replied on the thread Frederic started.

> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Tested-by: Frederic MARIE-JOSEPH <fredericmariejoseph@gmail.com>
> Reported-by: Frederic MARIE-JOSEPH <fredericmariejoseph@gmail.com>
> Link: https://lore.kernel.org/all/20261001-x86-fault-spurious-rw-v1-1-7fe1189b5efd@imperva.com/
> Fixes: a82b26451de1 ("x86/its: explicitly manage permissions for ITS pages")
> Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: x86@kernel.org
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Luis Chamberlain <mcgrof@kernel.org>
> Cc: Petr Pavlu <petr.pavlu@suse.com>
> Cc: Daniel Gomez <da.gomez@kernel.org>
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Cc: Aaron Tomlin <atomlin@atomlin.com>
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-modules@vger.kernel.org
> ---
> 
>  b/arch/x86/Kconfig     |    2 +-
>  b/kernel/module/main.c |    3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff -puN arch/x86/Kconfig~x86-STRICT_MODULE_RWX arch/x86/Kconfig
> --- a/arch/x86/Kconfig~x86-STRICT_MODULE_RWX	2026-10-01 07:11:50.443595828 -0700
> +++ b/arch/x86/Kconfig	2026-10-01 07:11:50.454595751 -0700
> @@ -85,7 +85,7 @@ config X86
>  	select ARCH_HAS_DMA_OPS			if GART_IOMMU || XEN
>  	select ARCH_HAS_EARLY_DEBUG		if KGDB
>  	select ARCH_HAS_ELF_RANDOMIZE
> -	select ARCH_HAS_EXECMEM_ROX		if X86_64 && STRICT_MODULE_RWX
> +	select ARCH_HAS_EXECMEM_ROX		if X86_64
>  	select ARCH_HAS_FAST_MULTIPLIER
>  	select ARCH_HAS_FORTIFY_SOURCE
>  	select ARCH_HAS_GCOV_PROFILE_ALL
> diff -puN kernel/module/main.c~x86-STRICT_MODULE_RWX kernel/module/main.c
> --- a/kernel/module/main.c~x86-STRICT_MODULE_RWX	2026-10-01 07:11:50.449595786 -0700
> +++ b/kernel/module/main.c	2026-10-01 09:47:20.842360625 -0700
> @@ -1355,7 +1355,8 @@ static int module_memory_alloc(struct mo
>  	if (!ptr)
>  		return -ENOMEM;
>  
> -	mod->mem[type].is_rox = execmem_is_rox(execmem_type);
> +	if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
> +		mod->mem[type].is_rox = execmem_is_rox(execmem_type);
>  
>  	/*
>  	 * The pointer to these blocks of memory are stored on the module
> _

-- 
Sincerely yours,
Mike.