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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Tue, 7 Jan 2020 18:51:45 +0100
Message-ID: <CAKv+Gu-djB=3zTxjEbyjJXXpw=8NE6YA82hMW-JYyAQ2TSywtQ@mail.gmail.com>
Subject: Re: [PATCH v4 4/4] efi: Fix handling of multiple efi_fake_mem= entries
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>,
        Michael Weiser <michael@weiser.dinsnail.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        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 06:19, Dave Young <dyoung@redhat.com> wrote:
>
> On 01/06/20 at 08:16pm, Dan Williams wrote:
> > On Mon, Jan 6, 2020 at 8:04 PM Dave Young <dyoung@redhat.com> wrote:
> > >
> > > On 01/06/20 at 04:40pm, Dan Williams wrote:
> > > > Dave noticed that when specifying multiple efi_fake_mem= entries only
> > > > the last entry was successfully being reflected in the efi memory map.
> > > > This is due to the fact that the efi_memmap_insert() is being called
> > > > multiple times, but on successive invocations the insertion should be
> > > > applied to the last new memmap rather than the original map at
> > > > efi_fake_memmap() entry.
> > > >
> > > > Rework efi_fake_memmap() to install the new memory map after each
> > > > efi_fake_mem= entry is parsed.
> > > >
> > > > This also fixes an issue in efi_fake_memmap() that caused it to litter
> > > > emtpy entries into the end of the efi memory map. An empty entry causes
> > > > efi_memmap_insert() to attempt more memmap splits / copies than
> > > > efi_memmap_split_count() accounted for when sizing the new map. When
> > > > that happens efi_memmap_insert() may overrun its allocation, and if you
> > > > are lucky will spill over to an unmapped page leading to crash
> > > > signature like the following rather than silent corruption:
> > > >
> > > >     BUG: unable to handle page fault for address: ffffffffff281000
> > > >     [..]
> > > >     RIP: 0010:efi_memmap_insert+0x11d/0x191
> > > >     [..]
> > > >     Call Trace:
> > > >      ? bgrt_init+0xbe/0xbe
> > > >      ? efi_arch_mem_reserve+0x1cb/0x228
> > > >      ? acpi_parse_bgrt+0xa/0xd
> > > >      ? acpi_table_parse+0x86/0xb8
> > > >      ? acpi_boot_init+0x494/0x4e3
> > > >      ? acpi_parse_x2apic+0x87/0x87
> > > >      ? setup_acpi_sci+0xa2/0xa2
> > > >      ? setup_arch+0x8db/0x9e1
> > > >      ? start_kernel+0x6a/0x547
> > > >      ? secondary_startup_64+0xb6/0xc0
> > > >
> > > > Commit af1648984828 "x86/efi: Update e820 with reserved EFI boot
> > > > services data to fix kexec breakage" is listed in Fixes: since it
> > > > introduces more occurrences where efi_memmap_insert() is invoked after
> > > > an efi_fake_mem= configuration has been parsed. Previously the side
> > > > effects of vestigial empty entries were benign, but with commit
> > > > af1648984828 that follow-on efi_memmap_insert() invocation triggers
> > > > efi_memmap_insert() overruns.
> > > >
> > > > Fixes: 0f96a99dab36 ("efi: Add 'efi_fake_mem' boot option")
> > > > Fixes: af1648984828 ("x86/efi: Update e820 with reserved EFI boot services...")
> > >
> > > A nitpick for the Fixes flags, as I replied in the thread below:
> > > https://lore.kernel.org/linux-efi/CAPcyv4jLxqPaB22Ao9oV31Gm=b0+Phty+Uz33Snex4QchOUb0Q@mail.gmail.com/T/#m2bb2dd00f7715c9c19ccc48efef0fcd5fdb626e7
> > >
> > > I reproduced two other panics without the patches applied, so this issue
> > > is not caused by either of the commits, maybe just drop the Fixes.
> >
> > Just the "Fixes: af1648984828", right? No objection from me. I'll let
> > Ingo say if he needs a resend for that.
> >
> > The "Fixes: 0f96a99dab36" is valid because the original implementation
> > failed to handle the multiple argument case from the beginning.
>
> Agreed, thanks!
>

I'll queue this but without the fixes tags. The -stable maintainers
are far too trigger happy IMHO, and this really needs careful review
before being backported. efi_fake_mem is a debug feature anyway, so I
don't see an urgent need to get this fixed retroactively in older
kernels.