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

[H. Peter Anvin]

On December 13, 2023 10:51:11 AM PST, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx> wrote:
>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

Somewhat unrelated: X86_BUG_ESPFIX should just be deleted, as we aren't actually ever cleaning it. All current x86 processors have that problem (until FRED).