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

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Date:   Sat, 11 Jul 2020 14:49:54 -0700
From:   hpa@...or.com
To:     Brendan Shanks <bshanks@...eweavers.com>,
        linux-kernel@...r.kernel.org
CC:     ricardo.neri-calderon@...ux.intel.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        ebiederm@...ssion.com, andi@...much.email, Babu.Moger@....com
Subject: Re: [PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions

On July 10, 2020 3:45:25 PM PDT, Brendan Shanks <bshanks@...eweavers.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@...much.email>
>Originally-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
>Signed-off-by: Brendan Shanks <bshanks@...eweavers.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.

-- 
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