Re: [PATCH] x86/alternative: Optimize single-byte NOPs at an arbitrary position

[Peter Zijlstra]

On Tue, Jun 01, 2021 at 11:21:25PM +0200, Borislav Petkov wrote:
> From: Borislav Petkov <bp@xxxxxxx>
> 
> Up until now the assumption was that an alternative patching site would
> have some instructions at the beginning and trailing single-byte NOPs
> (0x90) padding. Therefore, the patching machinery would go and optimize
> those single-byte NOPs into longer ones.
> 
> However, this assumption is broken on 32-bit when code like
> hv_do_hypercall() in hyperv_init() would use the ratpoline speculation
> killer CALL_NOSPEC. The 32-bit version of that macro would align certain
> insns to 16 bytes, leading to the compiler issuing a one or more
> single-byte NOPs, depending on the holes it needs to fill for alignment.

Who again insisted that wouldn't happen? :-)

> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 6974b5174495..7baf13b11952 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -182,42 +182,68 @@ recompute_jump(struct alt_instr *a, u8 *orig_insn, u8 *repl_insn, u8 *insn_buff)
>  		n_dspl, (unsigned long)orig_insn + n_dspl + repl_len);
>  }
>  
> +/*
> + * @instr: instruction byte stream
> + * @instrlen: length of the above
> + * @off: offset within @instr where the first NOP has been detected
> + */

That's almost a proper comment, might as well finish it

/*
 * optimize_nops_range() - Optimize a sequence of single byte NOPs (0x90)
 * @instr: instruction byte stream
 * @instrlen: length of the above
 * @off: offset within @instr where the first NOP has been detected
 *
 * Return: number of NOPs found (and replaced)
 */
> +static __always_inline int optimize_nops_range(u8 *instr, u8 instrlen, int off)
> +{
> +	unsigned long flags;
> +	int i = off, nnops;
> +
> +	while (i < instrlen) {
> +		if (instr[i] != 0x90)
> +			break;
> +
> +		i++;
> +	}

	for (; i < instrlen && instr[i] == 0x90; i++)
		;

perhaps? (possibly too dense, up to you)

> +
> +	nnops = i - off;
> +
> +	if (nnops <= 1)
> +		return nnops;

!nnops would be an error, no?

> +
> +	local_irq_save(flags);
> +	add_nops(instr + off, nnops);
> +	local_irq_restore(flags);
> +
> +	DUMP_BYTES(instr, instrlen, "%px: [%d:%d) optimized NOPs: ",
> +		   instr, off, i);
> +
> +	return nnops;
> +}
> +
> +

We really needs that extra line?

>  /*
>   * "noinline" to cause control flow change and thus invalidate I$ and
>   * cause refetch after modification.
>   */
>  static void __init_or_module noinline optimize_nops(struct alt_instr *a, u8 *instr)
>  {
>  	struct insn insn;
> +	int i = 0;
>  
>  	/*
> +	 * Jump over the non-NOP insns and optimize single-byte NOPs into bigger
> +	 * ones.
>  	 */
>  	for (;;) {
>  		if (insn_decode_kernel(&insn, &instr[i]))
>  			return;
>  
> +		/*
> +		 * See if this and any potentially following NOPs can be
> +		 * optimized.
> +		 */
>  		if (insn.length == 1 && insn.opcode.bytes[0] == 0x90)
> +			i += optimize_nops_range(instr, a->instrlen, i);
> +		else
> +			i += insn.length;
>  
> +		if (i >= a->instrlen)
>  			return;
>  	}
>  }

Anyway, irrespective of nits:

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>