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@xxxxxxxxx wrote:
> On July 10, 2020 3:45:25 PM PDT, Brendan Shanks <bshanks@xxxxxxxxxxxxxxx> 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>
> >---
> >
> >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