linux-kernel - Re: [PATCH 2/6] __wr_after_init: write rare for static allocation

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Date:   Wed, 5 Dec 2018 20:44:13 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Igor Stoppa <igor.stoppa@...il.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Kees Cook <keescook@...omium.org>, igor.stoppa@...wei.com,
        Nadav Amit <nadav.amit@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-integrity@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] __wr_after_init: write rare for static allocation

On Tue, Dec 04, 2018 at 02:18:01PM +0200, Igor Stoppa wrote:
> +void *__wr_op(unsigned long dst, unsigned long src, __kernel_size_t len,
> +	      enum wr_op_type op)
> +{
> +	temporary_mm_state_t prev;
> +	unsigned long flags;
> +	unsigned long offset;
> +	unsigned long wr_poking_addr;
> +
> +	/* Confirm that the writable mapping exists. */
> +	BUG_ON(!wr_ready);
> +
> +	if (WARN_ONCE(op >= WR_OPS_NUMBER, "Invalid WR operation.") ||
> +	    WARN_ONCE(!is_wr_after_init(dst, len), "Invalid WR range."))
> +		return (void *)dst;
> +
> +	offset = dst - (unsigned long)&__start_wr_after_init;
> +	wr_poking_addr = wr_poking_base + offset;
> +	local_irq_save(flags);

Why not local_irq_disable()?  Do we have a use-case for wanting to access
this from interrupt context?

> +	/* XXX make the verification optional? */

Well, yes.  It seems like debug code to me.

> +	/* Randomize the poking address base*/
> +	wr_poking_base = TASK_UNMAPPED_BASE +
> +		(kaslr_get_random_long("Write Rare Poking") & PAGE_MASK) %
> +		(TASK_SIZE - (TASK_UNMAPPED_BASE + wr_range));

I don't think this is a great idea.  We want to use the same mm for both
static and dynamic wr memory, yes?  So we should have enough space for
all of ram, not splatter the static section all over the address space.

On x86-64 (4 level page tables), we have a 64TB space for all of physmem
and 128TB of user space, so we can place the base anywhere in a 64TB
range.