On 22.08.2025 00:09, Andrew Cooper wrote: > On 15/08/2025 10:03 am, Jan Beulich wrote: >> On 14.08.2025 22:09, Andrew Cooper wrote: >>> On 14/08/2025 4:11 pm, Jan Beulich wrote: >>>> On 08.08.2025 22:23, Andrew Cooper wrote: >>>>> Under FRED, SETSSBSY is unavailable, and we want to be setting up FRED >>>>> prior >>>>> to setting up shadow stacks. As we still need Supervisor Tokens in IDT >>>>> mode, >>>>> we need mode-specific logic to establish SSP. >>>>> >>>>> In FRED mode, write a Restore Token, RSTORSSP it, and discard the >>>>> resulting >>>>> Previous-SSP token. >>>>> >>>>> No change outside of FRED mode. >>>>> >>>>> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com> >>>> Why is it that in patch 17 you could use identical code, but here you >>>> can't? >>> This caught me out at first too. >>> >>> For S3, we're going from "no shadow stack" to "back to where we were on >>> an active shadow stack". All we need to do is get saved_ssp back into >>> the SSP register. >>> >>> Here, we're going from "no shadow stack" to "on a good, empty, shadow >>> stack". For FRED we only need to load a value into SSP, but in IDT mode >>> we must also arrange to create a busy Supervisor Token on the base of >>> the stack. >>> >>> We could in principle conditionally write a busy supervisor token, then >>> unconditionally RSTORSSP, but that's even more complicated to follow IMO. >> Why would the write need to be conditional? > > Because the tokens are different. One has the value &addr, and one has > &addr + 9. > > The Supervisor Shadow Stack Token for IDT needs to survive for the > lifetime of Xen, while the Restore Token for FRED is temporary and > discarded by the logic added in this patch. > >> Can't we write what effectively >> is already there? Or is it more a safety measure to avoid the write when >> it's supposed to be unnecessary, to avoid papering over bugs? > > I genuinely don't understand what you're trying to suggest here.
I think I misunderstood your earlier reply, so the questions probably indeed didn't make a while lot of sense. Jan
