Re: [PATCH v2] x86,ibt: Use UDB instead of 0xEA

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 02, 2025 at 12:37:26PM -0700, Alexei Starovoitov wrote:

> Well, I mean all these tricky changes are allegedly because
> "Intel/AMD have not blessed using this instruction, it is on
> their 'reserved' opcode list for future use"
> 
> I suspect that 'reserved' opcode will not be used any time soon.
> If 10 years from now the opcode is used in some future CPU that CPU
> is better to be not vulnerable and CFI, FineIBT things will be
> gone from the kernel by then.
> So I would do absolutely nothing and just ignore the lack of blessing.

Ah. CFI is not only about speculation, it is also very much a protection
against pointer hijacking. Note how kCFI was not brought in because of
speculation, but as a hardening against pointer hijacking.

Yes, my interest in Intel CET-IBT is mostly from the speculation
avoidance angle, but CFI as a whole has wider applicability. Even a
'fixed' CPU would want to use CFI.

I would also love AMD to grow support for CET-IBT; they already
implemented CET-SS.

Also, Intel pays me, if they say make it use 0xD6, this is what we do
:-)