linux-kernel - Re: [PATCH] x86/efi-bgrt: Fix kernel panic when mapping BGRT data

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Message-ID: <20151214150817.GB2571@codeblueprint.co.uk>
Date:	Mon, 14 Dec 2015 15:08:17 +0000
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	"Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>,
	"Neri, Ricardo" <ricardo.neri@...el.com>,
	"Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH] x86/efi-bgrt: Fix kernel panic when mapping BGRT data

On Fri, 11 Dec, at 05:58:09AM, Sai Praneeth Prakhya wrote:
> 
> >The original motivation for efi_lookup_mapped_addr came from
> >early_ioremap printing a warning if used on an address range
> >already mapped as RAM.  Does early_mem* handle that case correctly
> >without a warning?  
> 
> Thanks a lot Josh for letting me know that. I don't think
> early_memremap() does that because early_memremap() and
> early_ioremap() both use __early_ioremap() but with different page
> protections (and I am not sure how those protections effect warning,
> but I will check that). Waiting for comments from Matt and Boris.

Right, early_memremap() was introduced for the purpose of mapping RAM,
so we shouldn't be seeing any warning here.

> >Because not all firmware places the BGRT image in boot services
> >memory; some firmware places the BGRT image variously in BIOS
> >reserved memory, ACPI reclaim >space, or other strange places.
> >
> >- Josh Triplett
> 
> I think we should not support buggy firmware implementations because
> it's same as encouraging them, instead we could let user know that
> he has got a buggy firmware and we skip bgrt code as if bgrt was
> disabled and carry on with normal boot process. One way of detecting
> buggy implementation without doing page table switch in
> efi_bgrt_init() is to enable a chicken bit if we find bgrt header
> and image address in EFI_BOOT_SERVICES_DATA regions while we are
> switching efi runtime services to virtual mode in
> __efi_enter_virtual_mode() and check for the same bit in
> efi_bgrt_init().

Trying to inform firmware developers that we noticed some buggy
behaviour should definitely be discussed as a separate patch (because
it's debatable whether there's any value in that in this scenario).

The changes in this patch look OK to me, and I think all of Josh's
concerns have been addressed. So unless anyone complains soon, I'm
going to apply it and send it to tip.
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