linux-kernel - Re: [PATCH v2] ARM: Fix MAX_DMA

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Date:   Wed, 6 Jul 2022 21:46:16 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Russell King <linux@...linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Grygorii Strashko <grygorii.strashko@...com>,
        open list <linux-kernel@...r.kernel.org>,
        Santosh Shilimkar <ssantosh@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>
Subject: Re: [PATCH v2] ARM: Fix MAX_DMA_ADDRESS overflow

On Wed, Jul 6, 2022 at 9:44 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> On Wed, Jul 6, 2022 at 6:46 PM Florian Fainelli <f.fainelli@...il.com> wrote:
> > Commit 26f09e9b3a06 ("mm/memblock: add memblock memory allocation apis")
> > added a check to determine whether arm_dma_zone_size is exceeding the
> > amount of kernel virtual address space available between the upper 4GB
> > virtual address limit and PAGE_OFFSET in order to provide a suitable
> > definition of MAX_DMA_ADDRESS that should fit within the 32-bit virtual
> > address space. The quantity used for comparison was off by a missing
> > trailing 0, leading to MAX_DMA_ADDRESS to be overflowing a 32-bit
> > quantity.
> >
> > This was caught with the bcm2711 platforms which defines a dma_zone_size
> > of 1GB, and using a PAGE_OFFSET of 0xc000_0000 (CONFIG_VMSPLIT_3G) with
> > CONFIG_DEBUG_VIRTUAL enabled would lead to MAX_DMA_ADDRESS being
> > 0x1_0000_0000 which overflows the unsigned long type used throughout
> > __pa() and __virt_addr_valid(). Because the virtual address passed to
> > __virt_addr_valid() would now be 0, the function would loudly warn, thus
> > making the platform unable to boot properly.
> >
> > Fixes: 26f09e9b3a06 ("mm/memblock: add memblock memory allocation apis")
> > Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> > ---
> > Changes in v2:
> >
> > - simplify the patch and drop the first patch that attempted to fix an
> >   off by one in the calculation.
>
> Thanks for the update!
>
> > --- a/arch/arm/include/asm/dma.h
> > +++ b/arch/arm/include/asm/dma.h
> > @@ -10,7 +10,7 @@
> >  #else
> >  #define MAX_DMA_ADDRESS        ({ \
> >         extern phys_addr_t arm_dma_zone_size; \
> > -       arm_dma_zone_size && arm_dma_zone_size < (0x10000000 - PAGE_OFFSET) ? \
>                                                   ^^^^^^^^^^
> 0x10000000ULL, as the constant doesn't fit in 32-bit.
> However, both gcc (9.4.0) and sparse don't seem to complain about
> the missing suffix (anymore?).
>
> > +       arm_dma_zone_size && arm_dma_zone_size < (0x100000000 - PAGE_OFFSET) ? \

Obviously my comment applies to the _new_ line, not to the removed
line...

> >                 (PAGE_OFFSET + arm_dma_zone_size) : 0xffffffffUL; })
> >  #endif

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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