Messages in this thread | | | Date | Wed, 17 Apr 2024 07:59:25 -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 02:38, Borislav Petkov wrote: > On Tue, Apr 16, 2024 at 11:30:01PM -0700, Xin Li (Intel) wrote: >> 3) The FRED kernel entry handler does *NOT* dispatch INT instructions, >> which is of event type EVENT_TYPE_SWINT, so compared with >> do_int80_emulation(), there is no need to do any user mode check. > > What does that mean? > > An event handler doesn't dispatch INT insns? > > /me is confused.
FRED has separate entry flows depending on if the event came from user space or kernel space:
asm_fred_entrypoint_user -> fred_entry_from_user
asm_fred_entrypoint_kernel -> fred_entry_from_kernel
fred_entry_from_kernel does not invoke fred_intx() if the event type is EVENT_TYPE_SWINT, instead it falls through to fred_bad_type(). Perhaps fred_intx() should be renamed fred_intx_user() for additional clarity.
(It might also we worth noting in that function that the reason int $0x03 and int $0x04 are dispatched as INT3 and INTO is to be fully user space compatible with legacy IDT, which behaves similarly.)
Thus, the int $0x80 code is simply not reachable from kernel space; if kernel code were to invoke int $0x80 or any other INT instruction it will error out before getting to this code.
>> +#ifdef CONFIG_X86_FRED >> +/* >> + * A FRED-specific INT80 handler fred_int80_emulation() is required: >> + * >> + * 1) As INT instructions and hardware interrupts are separate event >> + * types, FRED does not preclude the use of vector 0x80 for external >> + * interrupts. As a result the FRED setup code does *NOT* reserve >> + * vector 0x80 and calling int80_is_external() is not merely >> + * suboptimal but actively incorrect: it could cause a system call >> + * to be incorrectly ignored. >> + * >> + * 2) fred_int80_emulation(), only called for handling vector 0x80 of >> + * event type EVENT_TYPE_SWINT, will NEVER be called to handle any >> + * external interrupt (event type EVENT_TYPE_EXTINT). >> + * >> + * 3) The FRED kernel entry handler does *NOT* dispatch INT instructions, >> + * which is of event type EVENT_TYPE_SWINT, so compared with >> + * do_int80_emulation(), there is no need to do any user mode check. >> + * >> + * 4) int80_emulation() does a CLEAR_BRANCH_HISTORY, which is likely >> + * overkill for new x86 CPU implementations that support FRED. >> + * >> + * 5) int $0x80 is the FAST path for 32-bit system calls under FRED. >> + * >> + * A dedicated FRED INT80 handler duplicates quite a bit of the code in >> + * do_int80_emulation(), but it avoids sprinkling more tests and seems >> + * more readable. Just remember that we can always unify common stuff >> + * later if it turns out that it won't diverge anymore, i.e., after the >> + * FRED code settles. >> + */ > > And this is talking about duplication above and that text is duplicated > from the commit message. :) > > I'll zap it when applying. >
I suggested putting it into a comment for future reference. Obviously no need to duplicate it in the commit message :)
-hpa
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