linux-kernel - Re: [PATCH 1/4] arm64: module: create module allocations without exec permissions

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Date:   Tue, 28 May 2019 11:05:33 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Ard Biesheuvel <ard.biesheuvel@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     mark.rutland@....com, marc.zyngier@....com,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Nadav Amit <namit@...are.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        James Morse <james.morse@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH 1/4] arm64: module: create module allocations without exec
 permissions



On 05/23/2019 03:52 PM, Ard Biesheuvel wrote:
> Now that the core code manages the executable permissions of code
> regions of modules explicitly, it is no longer necessary to create

I guess the permission transition for various module sections happen
through module_enable_[ro|nx]() after allocating via module_alloc().

> the module vmalloc regions with RWX permissions, and we can create
> them with RW- permissions instead, which is preferred from a
> security perspective.

Makes sense. Will this be followed in all architectures now ?

> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@....com>
> ---
>  arch/arm64/kernel/module.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
> index 2e4e3915b4d0..88f0ed31d9aa 100644
> --- a/arch/arm64/kernel/module.c
> +++ b/arch/arm64/kernel/module.c
> @@ -41,7 +41,7 @@ void *module_alloc(unsigned long size)
>  
>  	p = __vmalloc_node_range(size, MODULE_ALIGN, module_alloc_base,
>  				module_alloc_base + MODULES_VSIZE,
> -				gfp_mask, PAGE_KERNEL_EXEC, 0,
> +				gfp_mask, PAGE_KERNEL, 0,
>  				NUMA_NO_NODE, __builtin_return_address(0));
>  
>  	if (!p && IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
> @@ -57,7 +57,7 @@ void *module_alloc(unsigned long size)
>  		 */
>  		p = __vmalloc_node_range(size, MODULE_ALIGN, module_alloc_base,
>  				module_alloc_base + SZ_4G, GFP_KERNEL,
> -				PAGE_KERNEL_EXEC, 0, NUMA_NO_NODE,
> +				PAGE_KERNEL, 0, NUMA_NO_NODE,
>  				__builtin_return_address(0));
>  
>  	if (p && (kasan_module_alloc(p, size) < 0)) {
> 

Which just makes sure that PTE_PXN never gets dropped while creating
these mappings.