Re: [PATCH] x86/umip: Add emulation/spoofing for SLDT and STR instructions

[Andy Lutomirski]

On Wed, Jun 3, 2020 at 5:12 PM Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, Jun 02, 2020 at 11:42:12AM -0700, Brendan Shanks wrote:
> > Add emulation/spoofing of SLDT and STR for both 32- and 64-bit
> > processes.
> >
> > Wine users have found a small number of Windows apps using SLDT that
> > were crashing when run on UMIP-enabled systems.
> >
> > Reported-by: Andreas Rammhold <andi@xxxxxxxxxxxxx>
> > Originally-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > Signed-off-by: Brendan Shanks <bshanks@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/umip.c | 23 ++++++++++++++---------
> >  1 file changed, 14 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
> > index 8d5cbe1bbb3b..59dfceac5cc0 100644
> > --- a/arch/x86/kernel/umip.c
> > +++ b/arch/x86/kernel/umip.c
> > @@ -64,6 +64,8 @@
> >  #define UMIP_DUMMY_GDT_BASE 0xfffffffffffe0000ULL
> >  #define UMIP_DUMMY_IDT_BASE 0xffffffffffff0000ULL
> >
> > +#define UMIP_DUMMY_TASK_REGISTER_SELECTOR 0x40
> > +
> >  /*
> >   * The SGDT and SIDT instructions store the contents of the global descriptor
> >   * table and interrupt table registers, respectively. The destination is a
> > @@ -244,16 +246,24 @@ static int emulate_umip_insn(struct insn *insn, int umip_inst,
> >               *data_size += UMIP_GDT_IDT_LIMIT_SIZE;
> >               memcpy(data, &dummy_limit, UMIP_GDT_IDT_LIMIT_SIZE);
> >
> > -     } else if (umip_inst == UMIP_INST_SMSW) {
> > -             unsigned long dummy_value = CR0_STATE;
> > +     } else if (umip_inst == UMIP_INST_SMSW || umip_inst == UMIP_INST_SLDT ||
> > +                umip_inst == UMIP_INST_STR) {
> > +             unsigned long dummy_value;
> > +
> > +             if (umip_inst == UMIP_INST_SMSW)
> > +                     dummy_value = CR0_STATE;
> > +             else if (umip_inst == UMIP_INST_STR)
> > +                     dummy_value = UMIP_DUMMY_TASK_REGISTER_SELECTOR;
> > +             else
> > +                     dummy_value = 0;
>
> Perhaps you can return a non-zero value for SLDT if it has an LDT, as
> Andy had suggested. Maybe this can be implemented by looking at
> current->mm->context.ldt
>
> I guess the non-zero value can be (GDT_ENTRY_LDT*8).

You could probably even get away with always returning a nonzero
value.  After all, an empty LDT is quite similar to no LDT.

--Andy