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SubjectRe: [PATCH] x86/umip: Add emulation/spoofing for SLDT and STR instructions
On Fri, Jun 05, 2020 at 11:58:13AM -0700, Brendan Shanks wrote:
>
> > On Jun 3, 2020, at 9:39 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >
> > On Wed, Jun 3, 2020 at 5:12 PM Ricardo Neri
> > <ricardo.neri-calderon@linux.intel.com <mailto:ricardo.neri-calderon@linux.intel.com>> 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@notmuch.email>
> >>> Originally-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> >>> Signed-off-by: Brendan Shanks <bshanks@codeweavers.com>
> >>> ---
> >>> 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.
>
>
> Is something like this what you both had in mind?

> I don’t have any software handy to test the LDT-present case though.

Perhaps you can insert a test in the kernel selftest. Something like
this (based on Andreas' test program):

--- a/tools/testing/selftests/x86/ldt_gdt.c
+++ b/tools/testing/selftests/x86/ldt_gdt.c
@@ -220,12 +220,23 @@ static void install_invalid(const struct user_desc *desc, bool oldmode)
}
}

+unsigned long test(void)
+{
+ unsigned char ldtr[5] = "\xef\xbe\xad\xde";
+ unsigned long ldt = 0;
+ asm("sldt %0\n" : "=m" (ldtr));
+ ldt = *((unsigned long *)&ldtr[0]);
+ printf ("LDT base: 0x%lx\n", ldt);
+ return (ldt);
+}
+
static int safe_modify_ldt(int func, struct user_desc *ptr,
unsigned long bytecount)
{
int ret = syscall(SYS_modify_ldt, 0x11, ptr, bytecount);
if (ret < -1)
errno = -ret;
+ test();
return ret;
}

Thanks and BR,
Ricardo
>
> else if (umip_inst == UMIP_INST_STR)
> dummy_value = UMIP_DUMMY_TASK_REGISTER_SELECTOR;
> else if (umip_inst == UMIP_INST_SLDT)
> {
> #ifdef CONFIG_MODIFY_LDT_SYSCALL
> down_read(&current->mm->context.ldt_usr_sem);
> if (current->mm->context.ldt)
> dummy_value = GDT_ENTRY_LDT * 8;
> else
> dummy_value = 0;
> up_read(&current->mm->context.ldt_usr_sem);
> #else
> dummy_value = 0;
> #endif
>

It looks fine to me. Perhaps Andy prefers a simpler, always-non-zero
implementation?

Thanks and BR,
Ricardo

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