Messages in this thread | | | Date | Wed, 17 Apr 2024 09:39:13 -0700 | Subject | Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED | From | "H. Peter Anvin" <> |
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On 4/17/24 08:55, Xin Li wrote: > On 4/17/2024 8:07 AM, H. Peter Anvin wrote: >> On 4/17/24 04:02, Nikolay Borisov wrote: >>> >>> On 17.04.24 г. 9:30 ч., Xin Li (Intel) wrote: >>>> 4) int80_emulation() does a CLEAR_BRANCH_HISTORY, which is likely >>>> > overkill for new x86 CPU implementations that support FRED. >>> >>> Well, that's a bit of an overstatement/speculation, because >>> clear_branch_history will only be effective if the machine is >>> susceptible to the given bug and there isn't a better options (i.e >>> using a hardware bit controlling the respective aspect of the CPU). >>>> >> >> It would seem like a huge stretch to expect that a FRED-capable CPU >> would not have such a facility. This is a matter of establishing a >> baseline for FRED-capable hardware. >> >> It would make more sense to me to add it if we turn out to need it; >> note that FRED code is currently only enabled on demand, in order to >> defend against bit rot until we have physical hardware. >> >> Now, if this is still desired, it *probably* belongs better in either >> fred_intx()/fred_other() or asm_fred_entrypoint_user, depending on if >> this ought to be done for all entries from userspace or only system >> calls. > > My bad that I didn't make this a good comment, even neglected that > clear_branch_history is just a nop on machines w/o such security issues. > > > So how about? > > int80_emulation() does a CLEAR_BRANCH_HISTORY, which is IDT-specific. > While FRED will likely take a different approach when it is needed:
s/when/if it is ever needed/
> it *probably* belongs in either fred_intx()/fred_other() or > asm_fred_entrypoint_user(), depending on if this ought to be done for > all entries from userspace or only system calls.
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