linux-kernel - Re: [PATCH v2] efi: Ignore unrealistically large option roms

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Date:   Tue, 1 May 2018 15:57:54 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] efi: Ignore unrealistically large option roms

On 1 May 2018 at 15:52, David Laight <David.Laight@...lab.com> wrote:
> From: Ard Biesheuvel
>> Sent: 28 April 2018 07:41
>> On 27 April 2018 at 23:35, Hans de Goede <hdegoede@...hat.com> wrote:
>> > setup_efi_pci() tries to save a copy of each PCI option ROM as this may
>> > be necessary for the device driver for the PCI device to have access too.
>> >
>> > On some systems the efi_pci_io_protocol_64's romimage and romsize fields
>> > contain invalid data, which looks a bit like pointers pointing back into
>> > other EFI code or data. Interpreting these pointers as romsize leads to
>> > a very large value and if we then try to alloc this amount of memory to
>> > save a copy the alloc call fails.
>> >
>> > This leads to a "Failed to alloc mem for rom" error being printed on the
>> > EFI console for each PCI device.
>> >
>> > This commit avoids the printing of these errors, by checking romsize
>> > before doing the alloc and if it is larger then 256M silently ignore the
>> > ROM fields instead of trying to alloc mem and fail.
>> >
>>
>> The UEFI spec limits the size of option ROMs to 16 MiB, so I'd prefer
>> we use that as the upper bound instead.
>
> Copying even 16MB of rom data into physical memory on the 'off chance' that
> the kernel might need it seems a waste of memory.
>

16 MB is the smallest number we can use that still guarantees that we
will not misidentify a valid option ROM as bogus. But in reality,
option ROMs are rarely that large.

> I can't help feeling that some kind of caching would be more appropriate.
>

Could you elaborate?