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

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Date:   Fri, 29 Apr 2022 16:07:52 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        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

On Thu, Mar 24, 2022 at 09:31:56PM +0100, Geert Uytterhoeven wrote:
> 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?

This, to me, looks like it should have been:

	arm_dma_zone_size && arm_dma_zone_size < (0x100000000 - PAGE_OFFSET) ? \

since that is the only thing that would make sense - it's the virtual
space remaining between PAGE_OFFSET and the top of addressable space.

However, 0x100000000 isn't correct - since the vectors live at
0xffff0000, and we have the vmalloc space etc.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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