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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Tue, 7 Jan 2020 18:49:24 +0100
Message-ID: <CAKv+Gu8hO5mTbFaqwh9OZOEm9r_e1_ob-pfq4yhH4wJG7yV8MQ@mail.gmail.com>
Subject: Re: [PATCH v4 3/4] efi: Fix efi_memmap_alloc() leaks
To:     Dave Young <dyoung@redhat.com>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Taku Izumi <izumi.taku@jp.fujitsu.com>,
        linux-efi <linux-efi@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kexec Mailing List <kexec@lists.infradead.org>
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On Tue, 7 Jan 2020 at 06:18, Dave Young <dyoung@redhat.com> wrote:
>
> On 01/06/20 at 08:24pm, Dan Williams wrote:
> > On Mon, Jan 6, 2020 at 7:58 PM Dave Young <dyoung@redhat.com> wrote:
> > >
> > > On 01/06/20 at 04:40pm, Dan Williams wrote:
> > > > With efi_fake_memmap() and efi_arch_mem_reserve() the efi table may be
> > > > updated and replaced multiple times. When that happens a previous
> > > > dynamically allocated efi memory map can be garbage collected. Use the
> > > > new EFI_MEMMAP_{SLAB,MEMBLOCK} flags to detect when a dynamically
> > > > allocated memory map is being replaced.
> > > >
> > > > Debug statements in efi_memmap_free() reveal:
> > > >
> > > >  efi: __efi_memmap_free:37: phys: 0x23ffdd580 size: 2688 flags: 0x2
> > > >  efi: __efi_memmap_free:37: phys: 0x9db00 size: 2640 flags: 0x2
> > > >  efi: __efi_memmap_free:37: phys: 0x9e580 size: 2640 flags: 0x2
> > > >
> > > > ...a savings of 7968 bytes on a qemu boot with 2 entries specified to
> > > > efi_fake_mem=.
> > > >
> > > > Cc: Taku Izumi <izumi.taku@jp.fujitsu.com>
> > > > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > > > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > > > ---
> > > >  drivers/firmware/efi/memmap.c |   24 ++++++++++++++++++++++++
> > > >  1 file changed, 24 insertions(+)
> > > >
> > > > diff --git a/drivers/firmware/efi/memmap.c b/drivers/firmware/efi/memmap.c
> > > > index 04dfa56b994b..bffa320d2f9a 100644
> > > > --- a/drivers/firmware/efi/memmap.c
> > > > +++ b/drivers/firmware/efi/memmap.c
> > > > @@ -29,6 +29,28 @@ static phys_addr_t __init __efi_memmap_alloc_late(unsigned long size)
> > > >       return PFN_PHYS(page_to_pfn(p));
> > > >  }
> > > >
> > > > +static void __init __efi_memmap_free(u64 phys, unsigned long size, unsigned long flags)
> > > > +{
> > > > +     if (flags & EFI_MEMMAP_MEMBLOCK) {
> > > > +             if (slab_is_available())
> > > > +                     memblock_free_late(phys, size);
> > > > +             else
> > > > +                     memblock_free(phys, size);
> > > > +     } else if (flags & EFI_MEMMAP_SLAB) {
> > > > +             struct page *p = pfn_to_page(PHYS_PFN(phys));
> > > > +             unsigned int order = get_order(size);
> > > > +
> > > > +             free_pages((unsigned long) page_address(p), order);
> > > > +     }
> > > > +}
> > > > +
> > > > +static void __init efi_memmap_free(void)
> > > > +{
> > > > +     __efi_memmap_free(efi.memmap.phys_map,
> > > > +                     efi.memmap.desc_size * efi.memmap.nr_map,
> > > > +                     efi.memmap.flags);
> > > > +}
> > > > +
> > > >  /**
> > > >   * efi_memmap_alloc - Allocate memory for the EFI memory map
> > > >   * @num_entries: Number of entries in the allocated map.
> > > > @@ -100,6 +122,8 @@ static int __init __efi_memmap_init(struct efi_memory_map_data *data)
> > > >               return -ENOMEM;
> > > >       }
> > > >
> > > > +     efi_memmap_free();
> > > > +
> > >
> > > This seems still not safe,  see below function:
> > > arch/x86/platform/efi/efi.c:
> > > static void __init efi_clean_memmap(void)
> > > It use same memmap for both old and new, and filter out those invalid
> > > ranges in place, if the memory is freed then ..
> >
> > In the efi_clean_memmap() case flags are 0, so efi_memmap_free() is a nop.
> >
> > Would you feel better with an explicit?
> >
> > WARN_ON(efi.memmap.phys_map == data->phys_map && (data->flags &
> > (EFI_MEMMAP_SLAB | EFI_MEMMAP_MEMBLOCK))
> >
> > ...not sure it's worth it.
>
> Ah, yes, sorry I did not see the flags, although it is not very obvious.
> Maybe add some code comment for efi_mem_alloc and efi_mem_init.
>
> Let's defer the suggestion to Ard.
>

A one line comment to remind our future selves of this discussion
would probably be helpful, but beyond that, I don't think we need to
do much here.