linux-kernel - Re: [PATCH 4.19 84/86] efi/x86: Handle by-ref arguments covering multiple pages in mixed mode

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Date:   Wed, 11 Mar 2020 14:28:45 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Pavel Machek <pavel@...x.de>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, linux-efi@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4.19 84/86] efi/x86: Handle by-ref arguments covering
 multiple pages in mixed mode

On Wed 2020-03-11 14:13:11, Greg Kroah-Hartman wrote:
> On Wed, Mar 11, 2020 at 02:01:07PM +0100, Pavel Machek wrote:
> > Hi!
> > 
> > > Currently, the mixed mode runtime service wrappers require that all by-ref
> > > arguments that live in the vmalloc space have a size that is a power of 2,
> > > and are aligned to that same value. While this is a sensible way to
> > > construct an object that is guaranteed not to cross a page boundary, it is
> > > overly strict when it comes to checking whether a given object violates
> > > this requirement, as we can simply take the physical address of the first
> > > and the last byte, and verify that they point into the same physical
> > > page.
> > 
> > Dunno. If start passing buffers that _sometime_ cross page boundaries,
> > we'll get hard to debug failures. Maybe original code is better
> > buecause it catches problems earlier?
> > 
> > Furthermore, all existing code should pass aligned, 2^n size buffers,
> > so we should not need this in stable?
> 
> For some crazy reason you cut out the reason I applied this patch to the
> stable tree.  From the changelog text:
> 	Fixes: f6697df36bdf0bf7 ("x86/efi: Prevent mixed mode boot
>corruption with CONFIG_VMAP_STACK=y")

I did not notice that, but reviewing f669 does not really help. If
there is some known code that passes unaligned (but guaranteed
not-to-cross-page) buffers here, then yes, but is it? Having
not-page-crossing guarantees is kind of hard without alignment.

People seem to be adding Fixes: tags even if it is not a bugfix, just
as reminder that this has relation to some other commit...

Best regards,
								Pavel


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