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Date:   Sat, 31 Dec 2022 13:55:32 +0100
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     Borislav Petkov <bp@alien8.de>, 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 Fri, Dec 30, 2022 at 05:06:55PM -0800, H. Peter Anvin wrote:
> 
> 
> On 12/30/22 14:10, Jason A. Donenfeld wrote:
> > On Fri, Dec 30, 2022 at 01:58:39PM -0800, H. Peter Anvin wrote:
> >> See the other thread fork. They have identified the problem already.
> > 
> > Not sure I follow. Is there another thread where somebody worked out why
> > this 62meg limit was happening?
> > 
> > Note that I sent v2/v3, to fix the original problem in a different way,
> > and if that looks good to the QEMU maintainers, then we can all be happy
> > with that. But I *haven't* addressed and still don't fully understand
> > why the 62meg limit applied to my v1 in the way it does. Did you find a
> > bug there to fix? If so, please do CC me.
> > 
> 
> Yes, you yourself posted the problem:
> 
> > Then build qemu. Run it with `-kernel bzImage`, based on the kernel
> > built with the .config I attached.
> > 
> > You'll see that the CPU triple faults when hitting this line:
> > 
> >         sd = (struct setup_data *)boot_params->hdr.setup_data;
> >         while (sd) {
> >                 unsigned long sd_addr = (unsigned long)sd;
> > 
> >                 kernel_add_identity_map(sd_addr, sd_addr + sizeof(*sd) + sd->len);  <----
> >                 sd = (struct setup_data *)sd->next;
> >         }
> > 
> > , because it dereferences *sd. This does not happen if the decompressed
> > size of the kernel is < 62 megs.
> > 
> > So that's the "big and pretty serious" bug that might be worthy of
> > investigation.
> 
> This needs to be something like:
> 
> kernel_add_identity_map(sd_addr, sd_addr + sizeof(*sd));
> kernel_add_identity_map(sd_addr + sizeof(*sd),
> 	sd_addr + sizeof(*sd) + sd->len);
> 

Oh, right, duh. Thanks for spelling it out.

Jason