Re: [PATCH 0/2] x86: UMIP emulation leaking kernel addresses

[Linus Torvalds <torvalds@linuxfoundation.org>]

On Sat, 9 Dec 2023 at 09:16, Brian Gerst <brgerst@gmail.com> wrote:
>
> A different way to plug this is to harden ptrace (and sigreturn) to
> verify that the segments are code or data type segments instead of
> relying on an IRET fault.

I think that is likely a good idea regardless of this particular issue.

And I don't think you need to even check the segment for any kind of
validity - all you need to check that it's a valid selector.

And we *kind* of do that already, with the x86 ptrace code checking

  static inline bool invalid_selector(u16 value)
  {
        return unlikely(value != 0 && (value & SEGMENT_RPL_MASK) != USER_RPL);
  }

but the thing is, I think we could limit that a lot more.

I think the only valid GDT entries are 0-15 (that includes the default
kernel segments, but they don't contain anything interesting), so we
could tighten that selector check to say that it has to be either a
LDT entry or a selector < 15.

So add some kind of requirement for "(value & 4) || (value < 8*16)", perhaps?

              Linus