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Date: Mon, 15 Apr 2024 10:22:53 -0700
From: Kees Cook <keescook@chromium.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Mike Rapoport <rppt@kernel.org>, linux-kernel@vger.kernel.org,
	linux-trace-kernel@vger.kernel.org,
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	Will Deacon <will@kernel.org>
Subject: Re: [POC][RFC][PATCH 1/2] mm/x86: Add wildcard * option as
 memmap=nn*align:name
Message-ID: <202404151017.FC002AA5@keescook>
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On Fri, Apr 12, 2024 at 06:19:40PM -0400, Steven Rostedt wrote:
> On Fri, 12 Apr 2024 23:59:07 +0300
> Mike Rapoport <rppt@kernel.org> wrote:
> 
> > On Tue, Apr 09, 2024 at 04:41:24PM -0700, Kees Cook wrote:
> > > On Tue, Apr 09, 2024 at 07:11:56PM -0400, Steven Rostedt wrote:  
> > > > On Tue, 9 Apr 2024 15:23:07 -0700
> > > > Kees Cook <keescook@chromium.org> wrote:
> > > >   
> > > > > Do we need to involve e820 at all? I think it might be possible to just
> > > > > have pstore call request_mem_region() very early? Or does KASLR make
> > > > > that unstable?  
> > > > 
> > > > Yeah, would that give the same physical memory each boot, and can we
> > > > guarantee that KASLR will not map the kernel over the previous location?  
> > > 
> > > Hm, no, for physical memory it needs to get excluded very early, which
> > > means e820.  
> > 
> > Whatever memory is reserved in arch/x86/kernel/e820.c, that happens after
> > kaslr, so to begin with, a new memmap parameter should be also added to
> > parse_memmap in arch/x86/boot/compressed/kaslr.c to ensure the same
> > physical address will be available after KASLR.
> 
> But doesn't KASLR only affect virtual memory not physical memory?

KASLR for x86 (and other archs, like arm64) do both physical and virtual
base randomization.

> This just makes sure the physical memory it finds will not be used by the
> system. Then ramoops does the mapping via vmap() I believe, to get a
> virtual address to access the physical address.

I was assuming, since you were in the e820 code, that it was
manipulating that before KASLR chose a location. But if not, yeah, Mike
is right -- you need to make sure this is getting done before
decompress_kernel().

-- 
Kees Cook