[PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions

[PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions

[Brendan Shanks]

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>
---

v5: Capitalize instruction names in comments.

 arch/x86/kernel/umip.c | 40 +++++++++++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
index 8d5cbe1bbb3b..2c304fd0bb1a 100644
--- a/arch/x86/kernel/umip.c
+++ b/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.
  *
@@ -244,16 +245,34 @@ 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;
@@ -261,7 +280,6 @@ static int emulate_umip_insn(struct insn *insn, int umip_inst,
 			*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,
-- 
2.26.2

Re: [PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions

[Andy Lutomirski]

On Fri, Jul 10, 2020 at 3:45 PM Brendan Shanks <bshanks@codeweavers.com> 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.
>

Acked-by: Andy Lutomirski <luto@kernel.org>

I tested this under KVM-emulated UMIP.  I don't have a real UMIP
system handle to test STR.

--Andy

Re: [PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions

[hpa]

On July 10, 2020 3:45:25 PM PDT, Brendan Shanks <bshanks@codeweavers.com> 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>
>---
>
>v5: Capitalize instruction names in comments.
>
> arch/x86/kernel/umip.c | 40 +++++++++++++++++++++++++++-------------
> 1 file changed, 27 insertions(+), 13 deletions(-)
>
>diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
>index 8d5cbe1bbb3b..2c304fd0bb1a 100644
>--- a/arch/x86/kernel/umip.c
>+++ b/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.
>  *
>@@ -244,16 +245,34 @@ 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;
>@@ -261,7 +280,6 @@ static int emulate_umip_insn(struct insn *insn, int
>umip_inst,
> 			*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,

It's there any reason for SLDT to not *always* return a fixed value? "An LDT has been assigned" is formally a kernel internal property, separate from the property of whenever there are user space enteies in the LDT.

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

Re: [PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions

[Ricardo Neri]

On Sat, Jul 11, 2020 at 02:49:54PM -0700, hpa@zytor.com wrote:
> On July 10, 2020 3:45:25 PM PDT, Brendan Shanks <bshanks@codeweavers.com> 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>
> >---
> >
> >v5: Capitalize instruction names in comments.
> >
> > arch/x86/kernel/umip.c | 40 +++++++++++++++++++++++++++-------------
> > 1 file changed, 27 insertions(+), 13 deletions(-)
> >
> >diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
> >index 8d5cbe1bbb3b..2c304fd0bb1a 100644
> >--- a/arch/x86/kernel/umip.c
> >+++ b/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.
> >  *
> >@@ -244,16 +245,34 @@ 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;
> >@@ -261,7 +280,6 @@ static int emulate_umip_insn(struct insn *insn, int
> >umip_inst,
> > 			*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,
> 
> It's there any reason for SLDT to not *always* return a fixed value? "An LDT has been assigned" is formally a kernel internal property, separate from the property of whenever there are user space enteies in the LDT.

But isn't it true that sldt returns 0 if the application has not set an
LDT and non-zero otherwise?

In native_set_ldt() I see that the the LDT register is set to 0 if the
table has no entries and to GDT_ENTRY_LDT*8 otherwise.

Please correct me if I understand this wrong.

Thanks and BR,
Ricardo

Re: [PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions

[Andy Lutomirski]

On Mon, Jul 13, 2020 at 4:45 PM Ricardo Neri
<ricardo.neri-calderon@linux.intel.com> wrote:
>
> On Sat, Jul 11, 2020 at 02:49:54PM -0700, hpa@zytor.com wrote:
> > On July 10, 2020 3:45:25 PM PDT, Brendan Shanks <bshanks@codeweavers.com> 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>
> > >---
> > >
> > >v5: Capitalize instruction names in comments.
> > >
> > > arch/x86/kernel/umip.c | 40 +++++++++++++++++++++++++++-------------
> > > 1 file changed, 27 insertions(+), 13 deletions(-)
> > >
> > >diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
> > >index 8d5cbe1bbb3b..2c304fd0bb1a 100644
> > >--- a/arch/x86/kernel/umip.c
> > >+++ b/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.
> > >  *
> > >@@ -244,16 +245,34 @@ 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;
> > >@@ -261,7 +280,6 @@ static int emulate_umip_insn(struct insn *insn, int
> > >umip_inst,
> > >                     *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,
> >
> > It's there any reason for SLDT to not *always* return a fixed value? "An LDT has been assigned" is formally a kernel internal property, separate from the property of whenever there are user space enteies in the LDT.
>
> But isn't it true that sldt returns 0 if the application has not set an
> LDT and non-zero otherwise?
>
> In native_set_ldt() I see that the the LDT register is set to 0 if the
> table has no entries and to GDT_ENTRY_LDT*8 otherwise.
>
> Please correct me if I understand this wrong.

You're understanding the LDT right, but that's none of the user's
business.  hpa may well be right -- unless it actively causes
problems, we might as well return a constant value.

Re: [PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions

[Ricardo Neri]

On Fri, Jul 10, 2020 at 03:45:25PM -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>

FWIW, tested on hardware with UMIP.

Reviewed-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Tested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>

Thanks and BR,
Ricardo

[tip: x86/cpu] x86/umip: Add emulation/spoofing for SLDT and STR instructions

[tip-bot2 for Brendan Shanks]

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     b91e7089ae70d2f7c81a4456e5b78fef498663d9
Gitweb:        https://git.kernel.org/tip/b91e7089ae70d2f7c81a4456e5b78fef498663d9
Author:        Brendan Shanks <bshanks@codeweavers.com>
AuthorDate:    Fri, 10 Jul 2020 15:45:25 -07:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Thu, 20 Aug 2020 19:10:26 +02:00

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 | 40 +++++++++++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
index 8d5cbe1..2c304fd 100644
--- a/arch/x86/kernel/umip.c
+++ b/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.
  *
@@ -244,16 +245,34 @@ 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;
@@ -261,7 +280,6 @@ static int emulate_umip_insn(struct insn *insn, int umip_inst,
 			*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,