Re: [RFC PATCH 14/28] x86/rethook: Use RIP-relative reference for return address

Linus Torvalds
2024-09-25T16:46:50Z

On Wed, 25 Sept 2024 at 08:16, Ard Biesheuvel <ardb+git@google.com> wrote:
>
> Instead of pushing an immediate absolute address, which is incompatible
> with PIE codegen or linking, use a LEA instruction to take the address
> into a register.

I don't think you can do this - it corrupts %rdi.

Yes, the code uses %rdi later, but that's inside the SAVE_REGS_STRING
/ RESTORE_REGS_STRING area.

And we do have special calling conventions that aren't the regular
ones, so %rdi might actually be used elsewhere. For example,
__get_user_X and __put_user_X all have magical calling conventions:
they don't actually use %rdi, but part of the calling convention is
that the unused registers aren't modified.

Of course, I'm not actually sure you can probe those and trigger this
issue, but it all makes me think it's broken.

And it's entirely possible that I'm wrong for some reason, but this
just _looks_ very very wrong to me.

I think you can do this with a "pushq mem" instead, and put the
relocation into the memory location.

Linus