Re: [PATCH v4 2/5] x86/alternative: add indirect call patching

[Juergen Gross]

On 14.11.23 13:37, Borislav Petkov wrote:
> On Mon, Oct 30, 2023 at 03:25:05PM +0100, Juergen Gross wrote:
> > + * Rewrite the "call BUG_func" replacement to point to the target of the
> > + * indirect pv_ops call "call *disp(%ip)".
> > + */
> > +static int alt_replace_call(u8 *instr, u8 *insn_buff, struct alt_instr *a)
> > +{
> > +	void *target, *bug = &BUG_func;
> > +
> > +	if (a->replacementlen != 5 || insn_buff[0] != CALL_INSN_OPCODE) {
> > +		pr_err("Alternative: ALT_FLAG_CALL set for a non-call replacement instruction\n");
>
> No need for the printk prefix.

Okay.

> > +		pr_err("  Ignoring the flag for the instruction at %pS (%px)\n", instr, instr);
>
> No ignoring - BUG

Okay.

> > +		return 5;
> > +	}
> > +
> > +	if (a->instrlen != 6 || instr[0] != 0xff || instr[1] != 0x15) {
> > +		pr_err("Alternative: ALT_FLAG_CALL set for unrecognized indirect call\n");
> > +		pr_err("  Not replacing the instruction at %pS (%px)\n", instr, instr);
> > +		return -1;
>
> Ditto.

Okay.

> > +	}
> > +
> > +#ifdef CONFIG_X86_64
> > +	/* ff 15 00 00 00 00   call   *0x0(%rip) */
> > +	target = *(void **)(instr + a->instrlen + *(s32 *)(instr + 2));
> > +#else
> > +	/* ff 15 00 00 00 00   call   *0x0 */
> > +	target = *(void **)(*(s32 *)(instr + 2));
> > +#endif
> > +	if (!target)
> > +		target = bug;
> > +
> > +	/* (BUG_func - .) + (target - BUG_func) := target - . */
> > +	*(s32 *)(insn_buff + 1) += target - bug;
>
> If I squint hard enough, this looks like it is replacing one call with
> another. We have a C macro alternative_call() which does exactly that.
> Why can't you define an asm version ALTERNATIVE_CALL and use it
> instead of using adding a new flag? We have 16 possible ones in total.

It is replacing an _indirect_ call with a _direct_ one, taking the call target
from the pointer used by the indirect call.


Juergen

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