linux-kernel - Re: [patch V3 03/11] x86/mm/cpa: Rework static

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Date:   Fri, 21 Sep 2018 09:33:08 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
        Bin Yang <bin.yang@...el.com>,
        Mark Gross <mark.gross@...el.com>
Subject: Re: [patch V3 03/11] x86/mm/cpa: Rework static_protections()

On 09/17/2018 07:29 AM, Thomas Gleixner wrote:
> +/*
> + * The kernel text needs to be executable for obvious reasons. This does
> + * not cover __inittext since that is gone after boot. On 64bit we do not
> + * enforce !NX on the low mapping
> + */
> +static pgprotval_t protect_kernel_text(unsigned long address)
> +{
> +	if (within(address, (unsigned long)_text, (unsigned long)_etext))
> +		return _PAGE_NX;
> +	return 0;
> +}

Minor nit: I was scratching my head about how why this works.  It
_reads_ like we are using _PAGE_NX to protect kernel text which doesn't
make any sense of course.

Could we make a connection between the protection and _PAGE_NX in the
comments:

	Protect kernel text against by forbidding _PAGE_NX.  This 	
	protects only the high kernel mapping (_text -> _etext) out of
	which we actually execute.  Do not protect the low mapping.

	This does not cover __inittext since that is gone after boot.

The static_protections() code looks fine because it's totally obvious
that it is dealing with "forbidden" bits, btw:

> +static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
> +					  unsigned long pfn)
> +{
> +	pgprotval_t forbidden;
> +
> +	/* Operate on the virtual address */
> +	forbidden  = protect_kernel_text(address);
> +	forbidden |= protect_kernel_text_ro(address);
> +
> +	/* Check the PFN directly */
> +	forbidden |= protect_pci_bios(pfn);
> +	forbidden |= protect_rodata(pfn);
>  
> -	return prot;
> +	return __pgprot(pgprot_val(prot) & ~forbidden);
>  }

This is more of a, "if you happen to respin these" comment, though, so:

Reviewed-by: Dave Hansen <dave.hansen@...el.com>