linux-kernel - Re: [PATCH qemu] x86: don't let decompressed kernel image clobber setup

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Date:   Wed, 28 Dec 2022 18:13:34 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     pbonzini@...hat.com, ebiggers@...nel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        ardb@...nel.org, kraxel@...hat.com, bp@...en8.de, philmd@...aro.org
Subject: Re: [PATCH qemu] x86: don't let decompressed kernel image clobber
 setup_data

On 12/28/22 15:58, H. Peter Anvin wrote:
> On December 28, 2022 8:57:54 AM PST, "Jason A. Donenfeld" <Jason@...c4.com> wrote:
>> HELLO H. PETER ANVIN,
>> E
>> L
>> L
>> O
>>
>> On Wed, Dec 28, 2022 at 05:30:30PM +0100, Jason A. Donenfeld wrote:
>>>> Fix looks good, glad you figured out the problem.
>>>
>>> I mean, kind of. The solution here sucks, especially given that in the
>>> worst case, setup_data just gets dropped. I'm half inclined to consider
>>> this a kernel bug instead, and add some code to relocate setup_data
>>> prior to decompression, and then fix up all the links. It seems like
>>> this would be a lot more robust.
>>>
>>> I just wish the people who wrote this stuff would chime in. I've had
>>> x86@...nel.org CC'd but so far, no input from them.
>>
>> Apparently you are the x86 boot guru. What do you want to happen here?
>> Your input would be very instrumental.
>>
>> Jason
> 
> Hi!
> 
> Glad you asked.
> 
> So the kernel load addresses are parameterized in the kernel image
> setup header. One of the things that are so parameterized are the
> size and possible realignment of the kernel image in memory.
> 
> I'm very confused where you are getting the 64 MB number from. There
> should not be any such limitation.
> 
> In general, setup_data should be able to go anywhere the initrd can
> go, and so is subject to the same address cap (896 MB for old
> kernels, 4 GB on newer ones; this address too is enumerated in the
> header.)
> 
> If you want to put setup_data above 4 GB, it *should* be ok if and
> only if the kernel supports loading the initrd high, too (again,
> enumerated in the header.
> 
> TL;DR: put setup_data where you put the initrd (before or after
> doesn't matter.)
> 
> To be maximally conservative, link the setup_data list in order from
> lowest to highest address; currently there is no such item of
> relevance, but in the future there may be setup_data items needed by
> the BIOS part of the bootstrap in which case they would have to be <
> 1 MB and precede any items > 1 MB for obvious reasons. That being
> said, with BIOS dying it is not all that likely that such entries
> will ever be needed.
> 

So let me try for an algorithm. Attached as a text file to avoid line 
break damage.

	-hpa
View attachment "kernel-data-addresses.txt" of type "text/plain" (5844 bytes)