Re: [PATCH v7 8/8] [DO NOT MERGE] x86/kexec: Add CFI type information to relocate_kernel()

[Josh Poimboeuf]

On Tue, Mar 18, 2025 at 03:56:36PM +0000, David Woodhouse wrote:
> But on the whole, I'm not sure the CFI check is worth it.
> 
> CFI checks that the caller and callee agree about the prototype of the
> function being called. There are two main benefits of this:
> 
>  • to protect against attacks where function pointers are substituted
>    for gadgets.
> 
>  • to protect against genuine bugs, where the caller and the callee
>    disagree about the function arguments.

AFAIK the first one is the main point of CFI.

> For the relocate_kernel() case I don't think we care much about the
> first. Without a CFI prologue, no *other* code can be tricked into
> calling relocate_kernel()

But for FineIBT the hash is checked on the callee side.  So it loses
FineIBT protection.

> — and besides, it's in the kernel's data
> section and isn't executable anyway until the kexec code copies it to a
> page that *is*.

Does the code get copied immediately before getting called, or can it be
initialized earlier during boot when kdump does its initial setup?

-- 
Josh