x86/umip: Add emulation/spoofing for SLDT and STR instructions

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.

Originally-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Reported-by: Andreas Rammhold <andi@notmuch.email>
Signed-off-by: Brendan Shanks <bshanks@codeweavers.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Tested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Link: https://lkml.kernel.org/r/20200710224525.21966-1-bshanks@codeweavers.com

arch/x86/kernel/umip.c

@@ -45,11 +45,12 @@
  * value that, lies close to the top of the kernel memory. The limit for the GDT
  * and the IDT are set to zero.
  *
- * Given that SLDT and STR are not commonly used in programs that run on WineHQ
- * or DOSEMU2, they are not emulated.
- *
- * The instruction smsw is emulated to return the value that the register CR0
+ * The instruction SMSW is emulated to return the value that the register CR0
  * has at boot time as set in the head_32.
+ * SLDT and STR are emulated to return the values that the kernel programmatically
+ * assigns:
+ * - SLDT returns (GDT_ENTRY_LDT * 8) if an LDT has been set, 0 if not.
+ * - STR returns (GDT_ENTRY_TSS * 8).
  *
  * Emulation is provided for both 32-bit and 64-bit processes.
  *
@@ -244,24 +245,41 @@ 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 = GDT_ENTRY_TSS * 8;
+		} 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
+		}
 
 		/*
-		 * Even though the CR0 register has 4 bytes, the number
+		 * For these 3 instructions, the number
 		 * of bytes to be copied in the result buffer is determined
 		 * by whether the operand is a register or a memory location.
 		 * If operand is a register, return as many bytes as the operand
 		 * size. If operand is memory, return only the two least
-		 * siginificant bytes of CR0.
+		 * siginificant bytes.
 		 */
 		if (X86_MODRM_MOD(insn->modrm.value) == 3)
 			*data_size = insn->opnd_bytes;
 		else
 			*data_size = 2;
 
 		memcpy(data, &dummy_value, *data_size);
-	/* STR and SLDT  are not emulated */
 	} else {
 		return -EINVAL;
 	}
@@ -383,10 +401,6 @@ bool fixup_umip_exception(struct pt_regs *regs)
 	umip_pr_warn(regs, "%s instruction cannot be used by applications.\n",
 			umip_insns[umip_inst]);
 
-	/* Do not emulate (spoof) SLDT or STR. */
-	if (umip_inst == UMIP_INST_STR || umip_inst == UMIP_INST_SLDT)
-		return false;
-
 	umip_pr_warn(regs, "For now, expensive software emulation returns the result.\n");
 
 	if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size,