Re: [PATCH V4 6/6] PCI: rcar: Fix 64bit MSI message address handling

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Lorenzo,

On Thu, Mar 28, 2019 at 5:28 PM Lorenzo Pieralisi
<lorenzo.pieralisi@arm.com> wrote:
> On Thu, Mar 28, 2019 at 09:02:00AM +0100, Geert Uytterhoeven wrote:
> > On Thu, Mar 28, 2019 at 4:19 AM Marek Vasut <marek.vasut@gmail.com> wrote:
> > > On 3/27/19 1:22 PM, Geert Uytterhoeven wrote:
> > > > On Wed, Mar 27, 2019 at 12:30 PM Simon Horman <horms@verge.net.au> wrote:
> > > >> On Mon, Mar 25, 2019 at 12:41:01PM +0100, marek.vasut@gmail.com wrote:
> > > >>> From: Marek Vasut <marek.vasut+renesas@gmail.com>
> > > >>> The MSI message address in the RC address space can be 64 bit. The
> > > >>> R-Car PCIe RC supports such a 64bit MSI message address as well.
> > > >>> The code currently uses virt_to_phys(__get_free_pages()) to obtain
> > > >>> a reserved page for the MSI message address, and the return value
> > > >>> of which can be a 64 bit physical address on 64 bit system.
> > > >>>
> > > >>> However, the driver only programs PCIEMSIALR register with the bottom
> > > >>> 32 bits of the virt_to_phys(__get_free_pages()) return value and does
> > > >>> not program the top 32 bits into PCIEMSIAUR, but rather programs the
> > > >>> PCIEMSIAUR register with 0x0. This worked fine on older 32 bit R-Car
> > > >>> SoCs, however may fail on new 64 bit R-Car SoCs.
> > > >>>
> > > >>> Since from a PCIe controller perspective, an inbound MSI is a memory
> > > >>> write to a special address (in case of this controller, defined by
> > > >>> the value in PCIEMSIAUR:PCIEMSIALR), which triggers an interrupt, but
> > > >>> never hits the DRAM _and_ because allocation of an MSI by a PCIe card
> > > >>> driver obtains the MSI message address by reading PCIEMSIAUR:PCIEMSIALR
> > > >>> in rcar_msi_setup_irqs(), incorrectly programmed PCIEMSIAUR cannot
> > > >>> cause memory corruption or other issues.
> > > >>>
> > > >>> There is however the possibility that if virt_to_phys(__get_free_pages())
> > > >>> returned address above the 32bit boundary _and_ PCIEMSIAUR was programmed
> > > >>> to 0x0 _and_ if the system had physical RAM at the address matching the
> > > >>> value of PCIEMSIALR, a PCIe card driver could allocate a buffer with a
> > > >>> physical address matching the value of PCIEMSIALR and a remote write to
> > > >>> such a buffer by a PCIe card would trigger a spurious MSI.
> > > >>>
> > > >>> Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
> > > >>> Cc: Geert Uytterhoeven <geert+renesas@glider.be>
> > > >>> Cc: Phil Edworthy <phil.edworthy@renesas.com>
> > > >>> Cc: Simon Horman <horms+renesas@verge.net.au>
> > > >>> Cc: Wolfram Sang <wsa@the-dreams.de>
> > > >>> Cc: linux-renesas-soc@vger.kernel.org
> > > >>> To: linux-pci@vger.kernel.org
> > > >>> Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
> > > >>
> > > >> Does this warrant a Fixes tag?
> > > >
> > > > (digging in old sent email)
> > > > Fixes: 290c1fb358605402 ("PCI: rcar: Add MSI support for PCIe")
> > >
> > > But does it really fix that commit, given that on Gen2 and earlier, it
> > > was not broken as those were 32bit platforms ?
> >
> > It does not fix the bug on that commit, as the bug cannot happen on arm32.
> > It does fix that commit, in that that commit used "unsigned long" for a
> > physical address, which is wrong, even on arm32 (esp. with LPAE).
> > If you insist on having a Fixes tag for a commit where the bug could be
> > seen:
> > Fixes: e015f88c368da1e6 ("PCI: rcar: Add support for R-Car H3 to pcie-rcar")
> >
> > Apart from that, drivers should use the DMA API instead of virt_to_phys().
> > However, now we have a better understanding of how MSI interrupts
> > work, we don't even need to allocate that page. All we need is the
> > physical address of a page that is guaranteed not to be backed by RAM
> > (i.e. not to be a valid target for a legitimate PCI bus mastering
> > transaction).
>
> Agreed but I would merge this patch first since it is a fix
> and update it later.

Sure, definitely.

> Shall I go with the Fixes: tag above ?

Fine for me, thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds