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

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Message-ID: <Y7Jwq/TzywGtktJy@zn.tnic>
Date:   Mon, 2 Jan 2023 06:50:35 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>, 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 Sat, Dec 31, 2022 at 07:21:06PM -0800, H. Peter Anvin wrote:
> As far as the decompression itself goes, it should only a problem if we are
> using physical KASLR since otherwise the kernel has a guaranteed safe zone
> already allocated by the boot loader. However, if physical KASLR is in use,

No KASLR in Jason's config AFAICT:

$ grep RANDOMIZE .config
CONFIG_ARCH_HAS_ELF_RANDOMIZE=y
CONFIG_HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET=y
CONFIG_RANDOMIZE_KSTACK_OFFSET=y
# CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT is not set

> then the decompressor needs to know everything there is to know about the
> memory map.

Yeah, we do have that but as you folks establish later in the thread, those
setup_data regions would need to be avoided too. ;-\
 
> However, there also seems to be some kind of interaction with AMD SEV-SNP.
> 
> 
> The bug appears to have been introduced by:
> 
> b57feed2cc2622ae14b2fa62f19e973e5e0a60cf
> x86/compressed/64: Add identity mappings for setup_data entries
> https://lore.kernel.org/r/TYCPR01MB694815CD815E98945F63C99183B49@TYCPR01MB6948.jpnprd01.prod.outlook.com
> 
> ... which was included in version 5.19, so it is relatively recent.

Right. We need that for the CC blob:

b190a043c49a ("x86/sev: Add SEV-SNP feature detection/setup")

> For a small amount of setup_data, the solution of just putting it next to
> the command line makes a lot of sense, and should be safe indefinitely.

Ok.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette