linux-hardening - Re: [POC][RFC][PATCH 0/2] pstore/mm/x86: Add wildcard memmap to map pstore consistently

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Message-ID: <20240501120904.61c7617c@gandalf.local.home>
Date: Wed, 1 May 2024 12:09:04 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Mike Rapoport <rppt@...nel.org>
Cc: "Guilherme G. Piccoli" <gpiccoli@...lia.com>, "Luck, Tony"
 <tony.luck@...el.com>, Kees Cook <keescook@...omium.org>, Joel Fernandes
 <joel@...lfernandes.org>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "linux-trace-kernel@...r.kernel.org"
 <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, Mathieu
 Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lstoakes@...il.com>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
 <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, "x86@...nel.org"
 <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra
 <peterz@...radead.org>, "linux-hardening@...r.kernel.org"
 <linux-hardening@...r.kernel.org>, Guenter Roeck <linux@...ck-us.net>, Ross
 Zwisler <zwisler@...gle.com>, "wklin@...gle.com" <wklin@...gle.com>,
 Vineeth Remanan Pillai <vineeth@...byteword.org>, Suleiman Souhlal
 <suleiman@...gle.com>, Linus Torvalds <torvalds@...uxfoundation.org>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>
Subject: Re: [POC][RFC][PATCH 0/2] pstore/mm/x86: Add wildcard memmap to map
 pstore consistently

On Wed, 1 May 2024 18:30:40 +0300
Mike Rapoport <rppt@...nel.org> wrote:

> > > /*
> > >  * Parse early_reserve_mem=nn:align:name
> > >  */
> > > static int __init early_reserve_mem(char *p)
> > > {
> > > 	phys_addr_t start, size, align;
> > > 	char *oldp;
> > > 	int err;
> > > 
> > > 	if (!p)
> > > 		return -EINVAL;
> > > 
> > > 	oldp = p;
> > > 	size = memparse(p, &p);
> > > 	if (p == oldp)
> > > 		return -EINVAL;
> > > 
> > > 	if (*p != ':')
> > > 		return -EINVAL;
> > > 
> > > 	align = memparse(p+1, &p);
> > > 	if (*p != ':')
> > > 		return -EINVAL;
> > > 
> > > 	start = memblock_phys_alloc(size, align);  
> > 
> > So this will allocate the same physical location for every boot, if booting
> > the same kernel and having the same physical memory layout?  
> 
> Up to kaslr that might use that location for the kernel image.
> But it's the same as allocating from e820 after kaslr.
> 
> And, TBH, I don't have good ideas how to ensure the same physical location
> with randomization of the physical address of the kernel image.

I'll try it out. Looking at arch/x86/boot/compressed/kaslr.c, if I read the
code correctly, it creates up to a 100 slots to store the kernel.

The method I used was to make sure that the allocation was always done at
the top address of memory, which I think would in most cases never be
assigned by KASLR.

This looks to just grab the next available physical address, which KASLR
can most definitely mess with.

I would still like to get the highest address possible.

-- Steve