Re: [PATCH 1/3] x86: Move TSS and LDT to end of the GDT
From: Linus Torvalds
Date: Wed Dec 13 2023 - 13:51:34 EST
On Wed, 13 Dec 2023 at 08:34, Brian Gerst <brgerst@xxxxxxxxx> 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