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Date:   Fri, 30 Dec 2022 16:54:27 +0100
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Borislav Petkov <bp@alien8.de>
Cc:     "H. Peter Anvin" <hpa@zytor.com>, pbonzini@redhat.com,
        ebiggers@kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org,
        qemu-devel@nongnu.org, ardb@kernel.org, kraxel@redhat.com,
        philmd@linaro.org
Subject: Re: [PATCH qemu] x86: don't let decompressed kernel image clobber
 setup_data
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On Thu, Dec 29, 2022 at 01:47:49PM +0100, Borislav Petkov wrote:
> On Wed, Dec 28, 2022 at 11:31:34PM -0800, H. Peter Anvin wrote:
> > As far as a crash... that sounds like a big and a pretty serious one at that.
> > 
> > Could you let me know what kernel you are using and how *exactly* you are booting it?
> 
> Right, with CONFIG_X86_VERBOSE_BOOTUP=y in a guest here, it says:
> 
> early console in extract_kernel
> input_data: 0x000000000be073a8
> input_len: 0x00000000008cfc43
> output: 0x0000000001000000
> output_len: 0x000000000b600a98
> kernel_total_size: 0x000000000ac26000
> needed_size: 0x000000000b800000
> trampoline_32bit: 0x000000000009d000
> 
> so that's a ~9M kernel which gets decompressed at 0x1000000 and the
> output len is, what, ~180M which looks like plenty to me...

I think you might have misunderstood the thread. First, to reproduce the
bug that this patch fixes, you need a kernel with a compressed size of
around 16 megs, not 9. Secondly, that crash is well understood and
doesn't need to be reproduced; this patch fixes it. Rather, the question
now is how to improve this patch to remove the 62 meg limit. I'll follow
up with hpa's request for reproduction info.

Jason