linux-kernel - Re: [PATCH v6 RESEND] x86/boot/KASLR: Restrict kernel to be randomized in mirror regions

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Message-ID: <20170804112325.GB8187@codeblueprint.co.uk>
Date:   Fri, 4 Aug 2017 12:23:25 +0100
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Baoquan He <bhe@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, keescook@...omium.org, tglx@...utronix.de,
        hpa@...or.com, izumi.taku@...fujitsu.com, fanc.fnst@...fujitsu.com,
        thgarnie@...gle.com, n-horiguchi@...jp.nec.com,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-efi@...r.kernel.org
Subject: Re: [PATCH v6 RESEND] x86/boot/KASLR: Restrict kernel to be
 randomized in mirror regions

On Fri, 28 Jul, at 07:26:03PM, Baoquan He wrote:
> Hi Matt,
> 
> On 07/28/17 at 11:55am, Ingo Molnar wrote:
> > 
> > * Matt Fleming <matt@...eblueprint.co.uk> wrote:
> > 
> > > On Fri, 21 Jul, at 09:19:56PM, Baoquan He wrote:
> > > >
> > > > There are places where the efi map is getting and used like this. E.g
> > > > in efi_high_alloc() of drivers/firmware/efi/libstub/efi-stub-helper.c.
> > > > EFI developers worry the size of efi_memory_desc_t could not be the same
> > > > as e->efi_memdesc_size?
> > > > 
> > > > Hi Matt,
> > > > 
> > > > Could you help have a look at this?
> > > 
> > > You're exactly right. The code guards against the size of the
> > > efi_memory_desc_t struct changing. The UEFI spec says to traverse the
> > > memory map this way.
> > 
> > This is not obvious and looks pretty ugly as well, and open coded in several 
> > places.
> > 
> > At minimum we should have an efi_memdesc_ptr(efi, i) wrapper inline (or so) that 
> > gives us the entry pointer, plus a comment that points out that ->memdesc_size 
> > might not be equal to sizeof(efi_memory_memdesc_t).
> 
> I can make a efi_memdesc_ptr(efi, i) wrapper as Ingo suggested and use
> it here if you agree. Seems it might be not good to add another
> for_each_efi_memory_desc_xxxx wrapper since there are different memmap
> data structures in x86 boot and in general efi libstub. Or any other
> idea?

I think adding a wrapper is fine, but I'd suggest including the word
"early" (or something similar) to explain that it should only be used
during bootup -- we want everyone else to use the
for_each_efi_memory_*() API.