Re: [PATCH qemu v3] x86: don't let decompressed kernel image clobber setup_data

[Eric Biggers]

On Fri, Dec 30, 2022 at 11:07:25PM +0100, Jason A. Donenfeld wrote:
> The setup_data links are appended to the compressed kernel image. Since
> the kernel image is typically loaded at 0x100000, setup_data lives at
> `0x100000 + compressed_size`, which does not get relocated during the
> kernel's boot process.
> 
> The kernel typically decompresses the image starting at address
> 0x1000000 (note: there's one more zero there than the compressed image
> above). This usually is fine for most kernels.
> 
> However, if the compressed image is actually quite large, then
> setup_data will live at a `0x100000 + compressed_size` that extends into
> the decompressed zone at 0x1000000. In other words, if compressed_size
> is larger than `0x1000000 - 0x100000`, then the decompression step will
> clobber setup_data, resulting in crashes.
> 
> Visually, what happens now is that QEMU appends setup_data to the kernel
> image:
> 
>           kernel image            setup_data
>    |--------------------------||----------------|
> 0x100000                  0x100000+l1     0x100000+l1+l2
> 
> The problem is that this decompresses to 0x1000000 (one more zero). So
> if l1 is > (0x1000000-0x100000), then this winds up looking like:
> 
>           kernel image            setup_data
>    |--------------------------||----------------|
> 0x100000                  0x100000+l1     0x100000+l1+l2
> 
>                                  d e c o m p r e s s e d   k e r n e l
>                      |-------------------------------------------------------------|
>                 0x1000000                                                     0x1000000+l3
> 
> The decompressed kernel seemingly overwriting the compressed kernel
> image isn't a problem, because that gets relocated to a higher address
> early on in the boot process, at the end of startup_64. setup_data,
> however, stays in the same place, since those links are self referential
> and nothing fixes them up.  So the decompressed kernel clobbers it.
> 
> Fix this by appending setup_data to the cmdline blob rather than the
> kernel image blob, which remains at a lower address that won't get
> clobbered.
> 
> This could have been done by overwriting the initrd blob instead, but
> that poses big difficulties, such as no longer being able to use memory
> mapped files for initrd, hurting performance, and, more importantly, the
> initrd address calculation is hard coded in qboot, and it always grows
> down rather than up, which means lots of brittle semantics would have to
> be changed around, incurring more complexity. In contrast, using cmdline
> is simple and doesn't interfere with anything.
> 
> The microvm machine has a gross hack where it fiddles with fw_cfg data
> after the fact. So this hack is updated to account for this appending,
> by reserving some bytes.
> 
> Cc: x86@xxxxxxxxxx
> Cc: Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>

For what it's worth:

Tested-by: Eric Biggers <ebiggers@xxxxxxxxxx>

- Eric