linux-kernel - Re: [PATCH v2 11/13] arm64: allow kernel Image to be loaded anywhere in physical memory

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Date:	Fri, 8 Jan 2016 16:30:20 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	kernel-hardening@...ts.openwall.com,
	Will Deacon <will.deacon@....com>,
	Mark Rutland <mark.rutland@....com>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Sharma Bhupesh <bhupesh.sharma@...escale.com>,
	Stuart Yoder <stuart.yoder@...escale.com>,
	Marc Zyngier <marc.zyngier@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v2 11/13] arm64: allow kernel Image to be loaded anywhere
 in physical memory

On 8 January 2016 at 16:27, Catalin Marinas <catalin.marinas@....com> wrote:
> On Wed, Dec 30, 2015 at 04:26:10PM +0100, Ard Biesheuvel wrote:
>> +static void __init enforce_memory_limit(void)
>> +{
>> +     const phys_addr_t kbase = round_down(__pa(_text), MIN_KIMG_ALIGN);
>> +     u64 to_remove = memblock_phys_mem_size() - memory_limit;
>> +     phys_addr_t max_addr = 0;
>> +     struct memblock_region *r;
>> +
>> +     if (memory_limit == (phys_addr_t)ULLONG_MAX)
>> +             return;
>> +
>> +     /*
>> +      * The kernel may be high up in physical memory, so try to apply the
>> +      * limit below the kernel first, and only let the generic handling
>> +      * take over if it turns out we haven't clipped enough memory yet.
>> +      */
>> +     for_each_memblock(memory, r) {
>> +             if (r->base + r->size > kbase) {
>> +                     u64 rem = min(to_remove, kbase - r->base);
>> +
>> +                     max_addr = r->base + rem;
>> +                     to_remove -= rem;
>> +                     break;
>> +             }
>> +             if (to_remove <= r->size) {
>> +                     max_addr = r->base + to_remove;
>> +                     to_remove = 0;
>> +                     break;
>> +             }
>> +             to_remove -= r->size;
>> +     }
>> +
>> +     memblock_remove(0, max_addr);
>> +
>> +     if (to_remove)
>> +             memblock_enforce_memory_limit(memory_limit);
>> +}
>
> IIUC, this is changing the user expectations a bit. There are people
> using the mem= limit to hijack some top of the RAM for other needs
> (though they could do it in a saner way like changing the DT memory
> nodes). Your patch first tries to remove the memory below the kernel
> image and only remove the top if additional limitation is necessary.
>
> Can you not remove memory from the top and block the limit if it goes
> below the end of the kernel image, with some warning that memory limit
> was not entirely fulfilled?
>

I'm in the middle of rewriting this code from scratch. The general idea is

static void __init clip_mem_range(u64 min, u64 max);

/*
* Clip memory in order of preference:
* - above the kernel and above 4 GB
* - between 4 GB and the start of the kernel
* - below 4 GB
* Note that tho
*/
clip_mem_range(max(sz_4g, PAGE_ALIGN(__pa(_end))), ULLONG_MAX);
clip_mem_range(sz_4g, round_down(__pa(_text), MIN_KIMG_ALIGN));
clip_mem_range(0, sz_4g);

where clip_mem_range() iterates over the memblocks to remove memory
between min and max iff min < max and the limit has not been met yet.