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From:   Marco Elver <elver@google.com>
Date:   Mon, 1 Aug 2022 16:05:50 +0200
Message-ID: <CANpmjNNpvbBR6z0T7m1gCp1XoVOHN7CpHoauMKLvtFd5NYJK2Q@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] mm: kfence: apply kmemleak_ignore_phys on early
 allocated pool
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        yee.lee@mediatek.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        "open list:KFENCE" <kasan-dev@googlegroups.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-arm-kernel@lists.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@lists.infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>
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[+x86 maintainers ...]

On Wed, 20 Jul 2022 at 01:22, Dave Hansen <dave.hansen@intel.com> wrote:
> On 7/19/22 16:13, Andrew Morton wrote:
> > On Mon, 18 Jul 2022 16:26:25 +0200 Marco Elver <elver@google.com> wrote:
> >
> >> On Sat, 16 Jul 2022 at 20:43, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> >> [...]
> >>>> - This patch has been accused of crashing the kernel:
> >>>>
> >>>>         https://lkml.kernel.org/r/YsFeUHkrFTQ7T51Q@xsang-OptiPlex-9020
> >>>>
> >>>>   Do we think that report is bogus?
> >>> I think all of this is highly architecture-specific...
> >> The report can be reproduced on i386 with CONFIG_X86_PAE=y. But e.g.
> >> mm/memblock.c:memblock_free() is also guilty of using __pa() on
> >> previously memblock_alloc()'d addresses. Looking at the phys addr
> >> before memblock_alloc() does virt_to_phys(), the result of __pa()
> >> looks correct even on PAE, at least for the purpose of passing it on
> >> to kmemleak(). So I don't know what that BUG_ON(slow_virt_to_phys() !=
> >> phys_addr) is supposed to tell us here.
> >>
> > It's only been nine years, so I'm sure Dave can remember why he added
> > it ;)
> >
> >               BUG_ON(slow_virt_to_phys((void *)x) != phys_addr);
> >
> > in arch/x86/mm/physaddr.c:__phys_addr().
>
> I think I intended it to double check that the linear map is *actually*
> a linear map for 'x'.  Sure, we can use the "x - PAGE_OFFSET" shortcut,
> but did it turn out to be actually accurate for the address it was handed?
>
> I'd be curious what the page tables actually say for the address that's
> causing problems.

test robot just reminded us again:
https://lore.kernel.org/all/YufXncrWhJZH0ifB@xsang-OptiPlex-9020/T/#u

Few things I noticed:

* mm/memblock.c's memblock_free() also uses __pa() to convert back to
physical address. Presumably that's also wrong. What should be used
instead?

* kmemleak happily converts phys_addr_t to unsigned long everywhere,
but with i386 PAE, this will narrow a 64-bit address to a 32-bit
address. Is that correct? Does kmemleak need a "depends on 64BIT ||
!PHYS_ADDR_T_64BIT"?