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

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Date:   Sun, 1 Jan 2023 06:55:42 +0200
From:   Mika Penttilä <mika.penttila@...sol.com>
To:     "H. Peter Anvin" <hpa@...or.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Borislav Petkov <bp@...en8.de>
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, philmd@...aro.org
Subject: Re: [PATCH qemu] x86: don't let decompressed kernel image clobber
 setup_data



On 1.1.2023 6.33, H. Peter Anvin wrote:
> 
> 
> On 12/31/22 10:22, Jason A. Donenfeld wrote:
>> On Sat, Dec 31, 2022 at 03:24:32PM +0100, Borislav Petkov wrote:
>>> On Sat, Dec 31, 2022 at 02:51:28PM +0100, Jason A. Donenfeld wrote:
>>>> That failure is unrelated to the ident mapping issue Peter and
>>>> I discussed. The original failure is described in the commit message:
>>>> decompression clobbers the data, so sd->next points to garbage.
>>>
>>> Right
>>
>> So with that understanding confirmed, I'm confused at your surprise that
>> hpa's unrelated fix to the different issue didn't fix this issue.
>>
> 
> If decompression does clobber the data, then we *also* need to figure 
> out why that is. There are basically three possibilities:
> 
> 1. If physical KASLR is NOT used:
> 
>      a. The boot loader doesn't honor the kernel safe area properly;
>      b. Somewhere in the process a bug in the calculation of the
>         kernel safe area has crept in.
> 
> 2. If physical KASLR IS used:
> 
>      The decompressor doesn't correctly keep track of nor relocate
>      all the keep-out zones before picking a target address.

Seems setup_data is not included in those mem_avoid regions.

> 
> One is a bootloader bug, two is a kernel bug. My guess is (2) is the 
> culprit, but (1b) should be checked, too.
> 
>      -hpa
> 


--Mika