Re: [PATCH v13 26/35] x86/fred: FRED entry/exit and dispatch code

[Andrew Cooper]

On 05/12/2023 10:50 am, Xin Li wrote:
> diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
> new file mode 100644
> index 000000000000..215883e90f94
> --- /dev/null
> +++ b/arch/x86/entry/entry_fred.c
> @@ -0,0 +1,230 @@
> ...
> +static noinstr void fred_intx(struct pt_regs *regs)
> +{
> +	switch (regs->fred_ss.vector) {
> +	/* INT0 */

INTO (for overflow), not INT-zero.  However...

> +	case X86_TRAP_OF:
> +		exc_overflow(regs);
> +		return;
> +
> +	/* INT3 */
> +	case X86_TRAP_BP:
> +		exc_int3(regs);
> +		return;

... neither OF nor BP will ever enter fred_intx() because they're type
SWEXC not SWINT.

SWINT is strictly the INT $imm8 instruction.

> ...
> +static noinstr void fred_extint(struct pt_regs *regs)
> +{
> +	unsigned int vector = regs->fred_ss.vector;
> +
> +	if (WARN_ON_ONCE(vector < FIRST_EXTERNAL_VECTOR))
> +		return;
> +
> +	if (likely(vector >= FIRST_SYSTEM_VECTOR)) {
> +		irqentry_state_t state = irqentry_enter(regs);
> +
> +		instrumentation_begin();
> +		sysvec_table[vector - FIRST_SYSTEM_VECTOR](regs);

array_index_mask_nospec()

This is easy for an attacker to abuse, to install non-function-pointer
targets into the indirect predictor.

> +		instrumentation_end();
> +		irqentry_exit(regs, state);
> +	} else {
> +		common_interrupt(regs, vector);
> +	}
> +}
> +
> +static noinstr void fred_exception(struct pt_regs *regs, unsigned long error_code)
> +{
> +	/* Optimize for #PF. That's the only exception which matters performance wise */
> +	if (likely(regs->fred_ss.vector == X86_TRAP_PF)) {
> +		exc_page_fault(regs, error_code);
> +		return;
> +	}
> +
> +	switch (regs->fred_ss.vector) {
> +	case X86_TRAP_DE: return exc_divide_error(regs);
> +	case X86_TRAP_DB: return fred_exc_debug(regs);
> +	case X86_TRAP_BP: return exc_int3(regs);
> +	case X86_TRAP_OF: return exc_overflow(regs);

Depending on what you want to do with BP/OF vs fred_intx(), this may
need adjusting.

If you are cross-checking type and vector, then these should be rejected
for not being of type HWEXC.

> +	case X86_TRAP_BR: return exc_bounds(regs);
> +	case X86_TRAP_UD: return exc_invalid_op(regs);
> +	case X86_TRAP_NM: return exc_device_not_available(regs);
> +	case X86_TRAP_DF: return exc_double_fault(regs, error_code);
> +	case X86_TRAP_TS: return exc_invalid_tss(regs, error_code);
> +	case X86_TRAP_NP: return exc_segment_not_present(regs, error_code);
> +	case X86_TRAP_SS: return exc_stack_segment(regs, error_code);
> +	case X86_TRAP_GP: return exc_general_protection(regs, error_code);
> +	case X86_TRAP_MF: return exc_coprocessor_error(regs);
> +	case X86_TRAP_AC: return exc_alignment_check(regs, error_code);
> +	case X86_TRAP_XF: return exc_simd_coprocessor_error(regs);
> +
> +#ifdef CONFIG_X86_MCE
> +	case X86_TRAP_MC: return fred_exc_machine_check(regs);
> +#endif
> +#ifdef CONFIG_INTEL_TDX_GUEST
> +	case X86_TRAP_VE: return exc_virtualization_exception(regs);
> +#endif
> +#ifdef CONFIG_X86_KERNEL_IBT

CONFIG_X86_CET

Userspace can use CET even if the kernel isn't compiled with IBT, so
this exception needs handling.

> +	case X86_TRAP_CP: return exc_control_protection(regs, error_code);
> +#endif
> +	default: return fred_bad_type(regs, error_code);
> +	}
> +}
> +
> +__visible noinstr void fred_entry_from_user(struct pt_regs *regs)
> +{
> +	unsigned long error_code = regs->orig_ax;
> +
> +	/* Invalidate orig_ax so that syscall_get_nr() works correctly */
> +	regs->orig_ax = -1;
> +
> +	switch (regs->fred_ss.type) {
> +	case EVENT_TYPE_EXTINT:
> +		return fred_extint(regs);
> +	case EVENT_TYPE_NMI:
> +		return fred_exc_nmi(regs);
> +	case EVENT_TYPE_SWINT:
> +		return fred_intx(regs);
> +	case EVENT_TYPE_HWEXC:
> +	case EVENT_TYPE_SWEXC:
> +	case EVENT_TYPE_PRIV_SWEXC:
> +		return fred_exception(regs, error_code);

PRIV_SWEXC should have it's own function and not fall into fred_exception().

It is strictly only the ICEBP (INT1) instruction at the moment, so
should fall into bad_type() for any vector other than X86_TRAP_DB.

> +	case EVENT_TYPE_OTHER:
> +		return fred_other(regs);
> +	default:
> +		return fred_bad_type(regs, error_code);
> +	}
> +}

~Andrew