Re: Do Qualcomm drivers use DMA buffers for request_firmware_into_buf()? - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: linux-efi <linux-efi@vger.kernel.org>,
	Matt Fleming <matt@codeblueprint.co.uk>,
	Will Deacon <will.deacon@arm.com>,
	Michal Hocko <mhocko@kernel.org>,
	David Howells <dhowells@redhat.com>,
	David Brown <david.brown@linaro.org>,
	Peter Jones <pjones@redhat.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	"open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
	linux-security-module <linux-security-module@vger.kernel.org>,
	Nicolas Broeking <nbroeking@me.com>,
	Jonathan Corbet <corbet@lwn.net>,
	the arch/x86 maintainers <x86@kernel.org>,
	"Luis R. Rodriguez" <mcgrof@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Vlastimil Babka <vbabka@suse.cz>,
	Andy Gross <andy.gross@linaro.org>,
	Darren Hart <dvhart@infradead.org>,
	Mimi Zohar <zohar@linux.vnet.ibm.com>,
	platform-driver-x86@vger.kernel.org,
	Arend Van Spriel <arend.vanspriel@broadcom.com>,
	Todd Kjos <tkjos@android.com>, Kees Cook <keescook@>
Subject: Re: Do Qualcomm drivers use DMA buffers for request_firmware_into_buf()?
Date: Thu, 7 Jun 2018 18:33:08 +0200	[thread overview]
Message-ID: <20180607163308.GA18834@kroah.com> (raw)
In-Reply-To: <CAKv+Gu8+Fq7BD4XD-YCyXzZh0mg6Z2k-0styj0cw6_uZfaqy4Q@mail.gmail.com>

On Thu, Jun 07, 2018 at 06:23:01PM +0200, Ard Biesheuvel wrote:
> On 7 June 2018 at 18:18, Bjorn Andersson <bjorn.andersson@linaro.org> wrote:
> > On Wed 06 Jun 13:32 PDT 2018, Luis R. Rodriguez wrote:
> >
> >> On Fri, Jun 01, 2018 at 09:23:46PM +0200, Luis R. Rodriguez wrote:
> >> > On Tue, May 08, 2018 at 03:38:05PM +0000, Luis R. Rodriguez wrote:
> >> > > On Fri, May 04, 2018 at 12:44:37PM -0700, Martijn Coenen wrote:
> >> > > >
> >> > > > I think the Qualcomm folks owning this (Andy, David, Bjorn, already
> >> > > > cc'd here) are better suited to answer that question.
> >> > >
> >> > > Andy, David, Bjorn?
> >> >
> >> > Andy, David, Bjorn?
> >>
> >> A month now with no answer...
> >>
> >
> > The patch at the top of this thread doesn't interest me and you didn't
> > bother sending your question To me.
> >
> > As a matter of fact I'm confused to what the actual question is.
> >
> 
> The actual question is whether it is really required that the firmware
> is loaded by the kernel into a buffer that is already mapped for DMA
> at that point, and thus accessible by the device.
> 
> To me, it is not entirely clear what the nature is of the firmware
> that we are talking about, since it seems to be getting passed to the
> secure world as well?
> 
> In any case, the preferred model in terms of validation/sig checking is
> 
> 1) allocate a CPU accessible buffer
> 
> 2) request the firmware into it (which may include a sig check under the hood)
> 
> 3) map the buffer for DMA to the device so it can load the firmware.
> 
> 4) kick off the DMA transfer.
> 
> The use of dma_alloc_coherent() for this purpose seems unnecessary,
> given that the DMA transfer is not bidirectional. Would it be possible
> to replace it with something like the above sequence?

Why not just use kmalloc, it will always return a DMAable buffer.

Is the problem that vmalloc() might not?

We need to drop the whole DMA zone crud, it confuses everyone who sees
it and was primarily for really really old systems.

greg k-h