Re: [PATCH 5/8] x86/traps: Cleanup and robustify decode_bug()

[Andrew Cooper]

On 05/11/2024 11:39 am, Peter Zijlstra wrote:
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -110,24 +117,37 @@ __always_inline int decode_bug(unsigned
>  		return BUG_NONE;
>  
>  	v = *(u8 *)(addr++);
> -	if (v == SECOND_BYTE_OPCODE_UD2)
> +	if (v == SECOND_BYTE_OPCODE_UD2) {
> +		*len = addr - start;
>  		return BUG_UD2;
> +	}
>  
> -	if (!IS_ENABLED(CONFIG_UBSAN_TRAP) || v != SECOND_BYTE_OPCODE_UD1)
> +	if (v != SECOND_BYTE_OPCODE_UD1)
>  		return BUG_NONE;
>  
> -	/* Retrieve the immediate (type value) for the UBSAN UD1 */
> -	v = *(u8 *)(addr++);
> -	if (X86_MODRM_RM(v) == 4)
> -		addr++;
> -
>  	*imm = 0;
> -	if (X86_MODRM_MOD(v) == 1)
> -		*imm = *(u8 *)addr;
> -	else if (X86_MODRM_MOD(v) == 2)
> -		*imm = *(u32 *)addr;
> -	else
> -		WARN_ONCE(1, "Unexpected MODRM_MOD: %u\n", X86_MODRM_MOD(v));
> +	v = *(u8 *)(addr++);		/* ModRM */
> +
> +	/* Decode immediate, if present */
> +	if (X86_MODRM_MOD(v) != 3) {
> +		if (X86_MODRM_RM(v) == 4)
> +			addr++;		/* Skip SIB byte */
> +
> +		if (X86_MODRM_MOD(v) == 1) {
> +			*imm = *(s8 *)addr;
> +			addr += 1;
> +
> +		} else if (X86_MODRM_MOD(v) == 2) {
> +			*imm = *(s32 *)addr;
> +			addr += 4;
> +		}
> +	}
> +
> +	/* record instruction length */
> +	*len = addr - start;

`ud1 0(%rip),%eax` has something to say about this length calculation[1].

You need the Mod = 0, RM = 5 case wired into addr += 4 without filling
in imm.

~Andrew

[1] or maybe you've got something rude to say about those of us who
encode instructions like that...[2]
[2] It's perhaps fortunate that decode_bug() doesn't know what a REX
prefix is.