Re: [PATCH v3] hw/i386: place setup_data at fixed place in memory

[Michael S. Tsirkin]

On Fri, Aug 05, 2022 at 07:29:29PM +0200, Paolo Bonzini wrote:
> On 8/5/22 13:08, Ard Biesheuvel wrote:
> > > 
> > > Does it work to place setup_data at the end of the cmdline file instead
> > > of having it at the end of the kernel file?  This way the first item
> > > will be at 0x20000 + cmdline_size.
> > > 
> > Does QEMU always allocate the command line statically like that?
> > AFAIK, OVMF never accesses that memory to read the command line, it
> > uses fw_cfg to copy it into a buffer it allocates itself. And I guess
> > that implies that this region could be clobbered by OVMF unless it is
> > told to preserve it.
> 
> No it's not. :(  It also goes to gBS->AllocatePages in the end.
> 
> At this point it seems to me that without extra changes the whole setup_data
> concept is dead on arrival for OVMF.  In principle there's no reason why the
> individual setup_data items couldn't include interior pointers, meaning that
> the setup_data _has_ to be at the address provided in fw_cfg by QEMU.
> 
> One way to "fix" it would be for OVMF to overwrite the pointer to the head
> of the list, so that the kernel ignores the setup data provided by QEMU.
> Another way would be to put it in the command line fw_cfg blob and teach
> OVMF to use a fixed address for the command line.  Both are ugly, and both
> are also broken for new QEMU / old OVMF.
> 
> In any case, I don't think this should be fixed so close to the release.  We
> have two possibilities:
> 
> 1) if we believe "build setup_data in QEMU" is a feasible design that only
> needs more yak shaving, we can keep the code in, but disabled by default,
> and sort it out in 7.2.
> 
> 2) if we go for an alternative design, it needs to be reverted.  For example
> the randomness could be in _another_ fw_cfg file, and the linuxboot DMA can
> patch it in the setup_data.
> 
> 
> With (2) the OVMF breakage would be limited to -dtb, which more or less
> nobody cares about, and we can just look the other way.
> 
> Paolo


So IIUC you retract your pc: add property for Linux setup_data random
number seed then? It's neither of the two options above.

-- 
MST