linux-kernel - Re: [PATCH 2/2] ARM: Clamp MAX_DMA

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Date:   Thu, 24 Mar 2022 21:31:56 +0100
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>,
        Nicolas Pitre <nico@...xnic.net>,
        Catalin Marinas <catalin.marinas@....com>,
        open list <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 2/2] ARM: Clamp MAX_DMA_ADDRESS to 32-bit

Hi Florian,

On Thu, Mar 24, 2022 at 6:54 PM Florian Fainelli <f.fainelli@...il.com> wrote:
> MAX_DMA_ADDRESS is a virtual address, therefore it needs to fit within a
> 32-bit unsigned quantity. Platforms defining a DMA zone size in
> their machine descriptor can easily overflow this quantity depending on
> the DMA zone size and/or the PAGE_OFFSET setting.
>
> In most cases this is harmless, however in the case of a
> CONFIG_DEBUG_VIRTUAL enabled, __virt_addr_valid() will be unable to
> return that MAX_DMA_ADDRESS is valid because the value passed to that
> function is an unsigned long which has already overflowed.
>
> Fixes: e377cd8221eb ("ARM: 8640/1: Add support for CONFIG_DEBUG_VIRTUAL")
> Fixes: 2fb3ec5c9503 ("ARM: Replace platform definition of ISA_DMA_THRESHOLD/MAX_DMA_ADDRESS")
> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>

> --- a/arch/arm/include/asm/dma.h
> +++ b/arch/arm/include/asm/dma.h
> @@ -8,10 +8,12 @@
>  #ifndef CONFIG_ZONE_DMA
>  #define MAX_DMA_ADDRESS        0xffffffffUL
>  #else
> +#include <linux/minmax.h>
>  #define MAX_DMA_ADDRESS        ({ \
>         extern phys_addr_t arm_dma_zone_size; \
>         arm_dma_zone_size && arm_dma_zone_size < (0x10000000 - PAGE_OFFSET) ? \

"arm_dma_zone_size < (0x10000000 - PAGE_OFFSET)" looks
like an overflow-avoiding version of
"PAGE_OFFSET + arm_dma_zone_size < 0x10000000".
However, PAGE_OFFSET is always larger than 0x10000000,
so "0x10000000 - PAGE_OFFSET" is a rather large number?

> -               (PAGE_OFFSET + arm_dma_zone_size - 1) : 0xffffffffUL; })
> +               min_t(phys_addr_t, (PAGE_OFFSET + arm_dma_zone_size - 1), 0xffffffffUL) : \
> +               0xffffffffUL; })
>  #endif
>
>  #ifdef CONFIG_ISA_DMA_API

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