linux-kernel - Re: [PATCH v5 4/4] x86/mm/KASLR: Adjust the padding size for the direct mapping.

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Date:   Fri, 13 Dec 2019 22:54:48 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Masayoshi Mizuma <msys.mizuma@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/4] x86/mm/KASLR: Adjust the padding size for the
 direct mapping.

On 12/13/19 at 03:15pm, Borislav Petkov wrote:
> On Fri, Dec 13, 2019 at 09:28:50PM +0800, Baoquan He wrote:
> > In Documentation/x86/x86_64/mm.rst, the physical memory regions mapping
> > with page_offset is called as the direct mapping of physical memory.
> 
> The fact that it happens to compute the *first* region's size, which
> *happens* to be the direct mapping of all physical memory is immaterial
> here.
> 
> It is actually causing more confusion in an already complex piece of
> code. You can call this function just as well
> 
>   calc_region_size()
> 
> which won't confuse readers. Because all you care about here is the
> region's size - not which region it is.

Won't calc_region_size be too generic? We also have vmalloc and vmemmap,
and here we are specifically calculating the direct mapping of physical
memory. 

> 
> > kernel_randomize_memory() is invoked much earlier than
> > acpi_table_parse_srat().
> 
> And? What are we going to do about that?


void __init setup_arch(char **cmdline_p)
{
...
	kernel_randomize_memory();
...
	init_mem_mapping();
...
	initmem_init();
...
}

In kernel_randomize_memory(), KASLR builds the layout of these regions,
including their starting address and the gap between them. Once
finished, __PAGE_OFFSET, VMALLOC_START, VMEMMAP_START are settled. 
If not knowing the max address to cover all the possible hotplugged
memory, later memory hotplug will fail.

Thanks
Baoquan