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

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Message-ID: <8959c79b-dd9d-8b1f-87b6-e2c971aa2342@gmail.com>
Date:   Mon, 10 Dec 2018 00:22:56 +0200
From:   Igor Stoppa <igor.stoppa@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
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 06/12/2018 06:44, Matthew Wilcox wrote:
> 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?

No, not that I can think of. It was "just in case", but I can remove it.

>> +	/* XXX make the verification optional? */
> 
> Well, yes.  It seems like debug code to me.

Ok, I was not sure about this, because text_poke() does it as part of 
its normal operations.

>> +	/* 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.

I was actually wondering about the dynamic part.
It's still not clear to me if it's possible to write the code in a 
sufficiently generic way that it could work on all 64 bit architectures.
I'll start with x86-64 as you suggest.

--
igor