Re: [PATCH 1/3] x86: Move TSS and LDT to end of the GDT

[Linus Torvalds <torvalds@linuxfoundation.org>]

On Wed, 13 Dec 2023 at 08:34, Brian Gerst <brgerst@gmail.com> wrote:
>
> This will make testing for system segments easier.

It seems to make more sense organizationally too, with the special
non-data/code segments clearly separate at the end.

So I think this is fine conceptually.

HOWEVER, I think that you might want to expand on this a bit more,
because there are other special segments selectors that might not be
thing you want to expose to user space.

We have GDT_ENTRY_PERCPU for example, which is a kernel-only segment.
It also happens to be 32-bit only, it doesn't matter for the thing
you're trying to fix, but that valid_user_selector() thing is then
used on x86-32 too.

So the ESPFIX and per-cpu segments are kernel-only, but then the VDSO
getcpu one is a user segment.

And the PnP and APM BIOS segments are similarly kernel-only.

But then the VDSO getcpu segment is user-visible, in the middle, and
again, it's 32-bit only but that whole GDT_SYSTEM_START thing is
supposed to work there too.

End result: this seems incomplete and not really fully baked.

I wonder if instead of GDT_SYSTEM_START, you'd be better off just
making a trivial constant bitmap of "these are user visible segments
in the GDT". No need to re-order things, just have something like

   #define USER_SEGMENTS_MASK \
        ((1ul << GDT_ENTRY_DEFAULT_USER_CS) |
         ,,,,

and use that for the test (remember to check for GDT_ENTRIES as the max).

Hmm?

             Linus