Re: [PATCH v3] x86/traps: Enable UBSAN traps on x86

[Kees Cook <kees@kernel.org>]

On Tue, Jun 25, 2024 at 11:37:19AM +0200, Peter Zijlstra wrote:
> Also, wouldn't it be saner to write this something like:
> 
> __always_inline int decode_bug(unsigned long addr, u32 *imm)
> {
> 	u8 v;
> 
> 	if (addr < TASK_SIZE)
> 		return BUG_NONE;
> 
> 	v = *(u8 *)(addr++);
> 	if (v == 0x67)
> 		v = *(u8 *)(addr++);
> 	if (v != 0x0f)
> 		return BUG_NONE;
> 	v = *(u8 *)(addr++);
> 	if (v == 0x0b)
> 		return BUG_UD2;
> 	if (v != 0xb9)
> 		return BUG_NONE;
> 
> 	if (X86_MODRM_RM(v) == 4)
> 		addr++; /* consume SiB */
> 
> 	*imm = 0;
> 	if (X86_MODRM_MOD(v) == 1)
> 		*imm = *(u8 *)addr;
> 	if (X86_MORRM_MOD(v) == 2)
> 		*imm = *(u32 *)addr;
> 
> 	// WARN on MOD(v)==3 ??
> 
> 	return BUG_UD1;
> }

Thanks for the example! (I think it should use macros instead of
open-coded "0x67", "0x0f", etc, but yeah.)

> Why does the thing emit the asop prefix at all through? afaict it
> doesn't affect the immediate you want to get at. And if it does this
> prefix, should we worry about other prefixes? Ideally we'd not accept
> any prefixes.

AFAICT it's because it's a small immediate? For an x86_64 build, this is
how Clang is generating the UD1.

-Kees

-- 
Kees Cook