On 01/09/2021 14:08, Jan Beulich wrote: >>>> Restricting execute permissions is something unique to virt. It doesn't >>>> exist in a non-virtualised system, as I and D side reads are >>>> indistinguishable outside of the core. >>>> >>>> Furthermore, it is inexpressible on some systems/configurations. >>>> >>>> Introspection is the only technology which should be restricting execute >>>> permissions in the p2m, and only when it takes responsibility for >>>> dealing with the fallout. >>> IOW are you saying that the calls to set_identity_p2m_entry() >>> (pre-dating XSA-378) were wrong to use p2m_access_rw? >> Yes. >> >>> Because that's >>> what's getting the way here. >> On a real machine, you really can write some executable code into an >> E820 reserved region and jump to it. You can also execute code from >> real BARs is you happen to know that they are prefetchable (or you're a >> glutton for UC reads...) >> >> And there is the WPBT ACPI table which exists specifically to let >> firmware inject drivers/applications into a windows environment, and may >> come out of the SPI ROM in the first place. >> >> >> Is it sensible to execute an E820 reserved region, or unmarked BAR? >> Probably not. >> >> Should it work, because that's how real hardware behaves? Absolutely. >> >> Any restrictions beyond that want handling by some kind of introspection >> agent which has a policy of what to do with legal-but-dodgy-looking actions. > IOW you suggest we remove p2m_access_t parameters from various functions, > going with just default access?
p2m_access_t was very obviously a bodge when introduced, and I doubt it would pass today's review standards. It is definitely a mis-design, given its ill-specified and overlapping semantics with respect to the p2m type. > Of course, as pointed out in another > reply, we'll need to split p2m_mmio_direct into multiple types then, at > the very least to honor the potential r/o restriction of AMD IOMMU unity > mapped regions. (FAOD all of this isn't a short term plan anyway, at least > afaic.) I don't think that will be necessary. IVMDs are almost non-existent, and given how many other areas of the AMD IOMMU spec are totally unused, I wouldn't be surprised if r/o unity mappings were in that category too. There's no obvious usecase for r/o, as anything critical enough in the platform to have an IVMD in the first place will also be non-trivial enough to require bidirectional communication somehow. The unity mapping only says "this device requires read-only access". It doesn't say "this must be mapped read-only", and it is legitimate to map a r/o unity mapping as r/w. If such a case actually exists, there's clearly one agent in the system with r/w access into the r/o range, and mapping it r/w is equivalent to the "IOMMU not enabled in the first place" case which is the default case for most software for the past decade-and-a-bit. In other words, I don't think the r/o unit maps on their own are a good enough reasons to split the type. If we gain other reasons then fine, but this seems like chunk of complexity with no real users. ~Andrew
