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

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Thu, Mar 28, 2019 at 09:02:00AM +0100, Geert Uytterhoeven wrote:
> Hi Marek,
> 
> 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.

Shall I go with the Fixes: tag above ?

Thanks,
Lorenzo

> 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