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

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Date:   Mon, 2 Jan 2023 07:17:43 +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 Mon, Jan 02, 2023 at 07:01:50AM +0100, Borislav Petkov wrote:
> On Sat, Dec 31, 2022 at 07:31:21PM -0800, H. Peter Anvin wrote:
> > It would probably be a good idea to add a "maximum physical address for
> > initrd/setup_data/cmdline" field to struct kernel_info, though. It appears
> > right now that those fields are being identity-mapped in the decompressor,
> > and that means that if 48-bit addressing is used, physical memory may extend
> > past the addressable range.
> 
> Yeah, we will probably need that too.
> 
> Btw, looka here - it can't get any more obvious than that after dumping
> setup_data too:
> 
> early console in setup code
> early console in extract_kernel
> input_data: 0x00000000040f92bf
> input_len: 0x0000000000f1c325
> output: 0x0000000001000000
> output_len: 0x0000000003c5e7d8
> kernel_total_size: 0x0000000004428000
> needed_size: 0x0000000004600000
> boot_params->hdr.setup_data: 0x00000000010203b0
> trampoline_32bit: 0x000000000009d000
> 
> Decompressing Linux... Parsing ELF... done.
> Booting the kernel.
> <EOF>
> 
> Aligning them vertically:
> 
> output:				0x0000000001000000
> output_len:			0x0000000003c5e7d8
> kernel_total_size:		0x0000000004428000
> needed_size:			0x0000000004600000
> boot_params->hdr.setup_data:	0x00000000010203b0

Ok, waait a minute:

============    ============
Field name:     pref_address
Type:           read (reloc)
Offset/size:    0x258/8
Protocol:       2.10+
============    ============

  This field, if nonzero, represents a preferred load address for the
  kernel.  A relocating bootloader should attempt to load at this
  address if possible.

  A non-relocatable kernel will unconditionally move itself and to run
  at this address.

so a kernel loader (qemu in this case) already knows where the kernel goes:

boot_params->hdr.setup_data: 0x0000000001020450
boot_params->hdr.pref_address: 0x0000000001000000
				^^^^^^^^^^^^^^^^^

now, considering that same kernel loader (qemu) knows how big that kernel is:

kernel_total_size: 0x0000000004428000

should that loader *not* put anything that the kernel will use in the range

pref_addr + kernel_total_size

?

-- 
Regards/Gruss,
    Boris.

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