linux-kernel - Re: [PATCH 2/4] efi: Capsule update support

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Date:	Tue, 29 Mar 2016 15:50:39 +0200
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	joeyli <jlee@...e.com>,
	Kweh Hock Leong <hock.leong.kweh@...el.com>,
	Borislav Petkov <bp@...en8.de>,
	Mark Salter <msalter@...hat.com>,
	Peter Jones <pjones@...hat.com>,
	"Bryan O'Donoghue" <pure.logic@...us-software.ie>
Subject: Re: [PATCH 2/4] efi: Capsule update support

On 29 March 2016 at 14:26, Matt Fleming <matt@...eblueprint.co.uk> wrote:
> On Mon, 21 Mar, at 08:31:59PM, Matt Fleming wrote:
>>
>> Good question. They're not handled in any special way with this patch
>> series, so the firmware will just initiate its own reset inside of
>> UpdateCapsule().
>>
>> That's probably not what we want, because things like on-disk
>> consistency are not guaranteed if the machine spontaneously reboots
>> without assistance from the kernel.
>>
>> The simplest thing to do is to refuse to pass such capsules to the
>> firmware, since it's likely not going to be a common use case. But
>> maybe that's overly restrictive.
>>
>> Let me have a think about that one.
>
> OK, I did think about this, and until someone actually requests the
> ability to handle CAPSULE_FLAGS_INITIATE_RESET, I'm happy to just punt
> on the problem. Anyone got any objections?
>

Nope

> ---
>
> diff --git a/drivers/firmware/efi/capsule.c b/drivers/firmware/efi/capsule.c
> index dac25208ad5e..84450e9cdf41 100644
> --- a/drivers/firmware/efi/capsule.c
> +++ b/drivers/firmware/efi/capsule.c
> @@ -84,6 +84,14 @@ int efi_capsule_supported(efi_guid_t guid, u32 flags, size_t size, int *reset)
>         u64 max_size;
>         int rv = 0;
>
> +       /*
> +        * We do not handle firmware-initiated reset because that
> +        * would require us to prepare the kernel for reboot. Refuse
> +        * to load any capsules with that flag.
> +        */
> +       if (flags & EFI_CAPSULE_INITIATE_RESET)
> +               return -EINVAL;
> +

Should we perhaps whitelist rather than blacklist these flags? If a
'EFI_CAPSULE_INITIATE_RESET_TOO' surfaces at some point, or flags that
do other nasty things, at least we won't be caught off guard.

>         capsule = kmalloc(sizeof(*capsule), GFP_KERNEL);
>         if (!capsule)
>                 return -ENOMEM;