Re: [PATCH qemu] x86: don't let decompressed kernel image clobber setup_data

From: "H. Peter Anvin" <hpa@zytor.com>
To: "Jason A. Donenfeld" <Jason@zx2c4.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
Date: Fri, 30 Dec 2022 17:06:55 -0800	[thread overview]
Message-ID: <46466e54-25c3-3194-8546-a57cd4a80d9d@zytor.com> (raw)
In-Reply-To: <Y69h6ur79SMhu61F@zx2c4.com>

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);

TThe 62 MB limit mentioned in boot.rst is unrelated, and only applies to 
very, very old kernels that used INT 15h, AH=88h to probe memory.

	-hpa