Re: CVE-2023-52466: PCI: Avoid potential out-of-bounds read in pci_dev_for_each_resource()

[Jiri Kosina]

On Tue, 27 Feb 2024, Greg Kroah-Hartman wrote:

> > 09cc90063240 ("PCI: Introduce pci_dev_for_each_resource()") added
> > pci_dev_for_each_resource(), which expands to:
> > 
> >   for (...; res = (&(dev)->resource[(bar)]), bar < PCI_NUM_RESOURCES; ...)
> > 
> > We compute "res" before the bounds-check of "bar", so the pointer may
> > be out-of-bounds, but the body of the pci_dev_for_each_resource() loop
> > is never executed with that out-of-bounds value.
> > 
> > So I don't think this is a security issue, no matter how
> > pci_dev_for_each_resource() is used, unless the mere presence of an
> > invalid address in a register is an issue.
> 
> Ah, yeah, now I remember, stuff like this was fixed up in other loops as
> just reading off into the wild can be a speculation issue and so we had
> to fix up a bunch of places in the kernel where we did have "invalid
> data" in a register.  The code didn't use that, but the processor would
> fetch from there, and boom, speculation mess.  There's a whole research
> paper published on this type of thing somewhere...

Greg, could you please elaborate on this?

Where in this whole construct do you see a potential for *_uncached_* (!) 
memory access that'd cause CPU to speculate into the wild? I just don't 
see it.

Thanks,

-- 
Jiri Kosina
SUSE Labs