Messages in this thread | | | Date | Wed, 17 Apr 2024 08:55:46 -0700 | Subject | Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED | From | Xin Li <> |
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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: 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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