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Subject: Re: [PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions
To:     Brendan Shanks <bshanks@codeweavers.com>,
        linux-kernel@vger.kernel.org
CC:     ricardo.neri-calderon@linux.intel.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        ebiederm@xmission.com, andi@notmuch.email, Babu.Moger@amd.com
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On July 10, 2020 3:45:25 PM PDT, Brendan Shanks <bshanks@codeweavers=2Ecom>=
 wrote:
>Add emulation/spoofing of SLDT and STR for both 32- and 64-bit
>processes=2E
>
>Wine users have found a small number of Windows apps using SLDT that
>were crashing when run on UMIP-enabled systems=2E
>
>Reported-by: Andreas Rammhold <andi@notmuch=2Eemail>
>Originally-by: Ricardo Neri <ricardo=2Eneri-calderon@linux=2Eintel=2Ecom>
>Signed-off-by: Brendan Shanks <bshanks@codeweavers=2Ecom>
>---
>
>v5: Capitalize instruction names in comments=2E
>
> arch/x86/kernel/umip=2Ec | 40 +++++++++++++++++++++++++++-------------
> 1 file changed, 27 insertions(+), 13 deletions(-)
>
>diff --git a/arch/x86/kernel/umip=2Ec b/arch/x86/kernel/umip=2Ec
>index 8d5cbe1bbb3b=2E=2E2c304fd0bb1a 100644
>--- a/arch/x86/kernel/umip=2Ec
>+++ b/arch/x86/kernel/umip=2Ec
>@@ -45,11 +45,12 @@
>* value that, lies close to the top of the kernel memory=2E The limit for
>the GDT
>  * and the IDT are set to zero=2E
>  *
>- * Given that SLDT and STR are not commonly used in programs that run
>on WineHQ
>- * or DOSEMU2, they are not emulated=2E
>- *
>- * 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=2E
>+ * 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=2E
>+ * - STR returns (GDT_ENTRY_TSS * 8)=2E
>  *
>  * Emulation is provided for both 32-bit and 64-bit processes=2E
>  *
>@@ -244,16 +245,34 @@ static int emulate_umip_insn(struct insn *insn,
>int umip_inst,
> 		*data_size +=3D UMIP_GDT_IDT_LIMIT_SIZE;
> 		memcpy(data, &dummy_limit, UMIP_GDT_IDT_LIMIT_SIZE);
>=20
>-	} else if (umip_inst =3D=3D UMIP_INST_SMSW) {
>-		unsigned long dummy_value =3D CR0_STATE;
>+	} else if (umip_inst =3D=3D UMIP_INST_SMSW || umip_inst =3D=3D UMIP_INS=
T_SLDT
>||
>+		   umip_inst =3D=3D UMIP_INST_STR) {
>+		unsigned long dummy_value;
>+
>+		if (umip_inst =3D=3D UMIP_INST_SMSW) {
>+			dummy_value =3D CR0_STATE;
>+		} else if (umip_inst =3D=3D UMIP_INST_STR) {
>+			dummy_value =3D GDT_ENTRY_TSS * 8;
>+		} else if (umip_inst =3D=3D UMIP_INST_SLDT) {
>+#ifdef CONFIG_MODIFY_LDT_SYSCALL
>+			down_read(&current->mm->context=2Eldt_usr_sem);
>+			if (current->mm->context=2Eldt)
>+				dummy_value =3D GDT_ENTRY_LDT * 8;
>+			else
>+				dummy_value =3D 0;
>+			up_read(&current->mm->context=2Eldt_usr_sem);
>+#else
>+			dummy_value =3D 0;
>+#endif
>+		}
>=20
> 		/*
>-		 * 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=2E
> 		 * If operand is a register, return as many bytes as the operand
> 		 * size=2E If operand is memory, return only the two least
>-		 * siginificant bytes of CR0=2E
>+		 * siginificant bytes=2E
> 		 */
> 		if (X86_MODRM_MOD(insn->modrm=2Evalue) =3D=3D 3)
> 			*data_size =3D insn->opnd_bytes;
>@@ -261,7 +280,6 @@ static int emulate_umip_insn(struct insn *insn, int
>umip_inst,
> 			*data_size =3D 2;
>=20
> 		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=2E\n",
> 			umip_insns[umip_inst]);
>=20
>-	/* Do not emulate (spoof) SLDT or STR=2E */
>-	if (umip_inst =3D=3D UMIP_INST_STR || umip_inst =3D=3D UMIP_INST_SLDT)
>-		return false;
>-
>	umip_pr_warn(regs, "For now, expensive software emulation returns the
>result=2E\n");
>=20
> 	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 L=
DT has been assigned" is formally a kernel internal property, separate from=
 the property of whenever there are user space enteies in the LDT=2E

--=20
Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E