Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED

[H. Peter Anvin]

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