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Date: Mon, 13 Jul 2020 18:10:07 -0700
From: Andy Lutomirski <luto@...nel.org>
To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>,
Brendan Shanks <bshanks@...eweavers.com>,
LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
X86 ML <x86@...nel.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Andreas Rammhold <andi@...much.email>,
"Moger, Babu" <Babu.Moger@....com>
Subject: Re: [PATCH v5] x86/umip: Add emulation/spoofing for SLDT and STR instructions
On Mon, Jul 13, 2020 at 4:45 PM Ricardo Neri
<ricardo.neri-calderon@...ux.intel.com> wrote:
>
> On Sat, Jul 11, 2020 at 02:49:54PM -0700, hpa@...or.com wrote:
> > 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(¤t->mm->context.ldt_usr_sem);
> > >+ if (current->mm->context.ldt)
> > >+ dummy_value = GDT_ENTRY_LDT * 8;
> > >+ else
> > >+ dummy_value = 0;
> > >+ up_read(¤t->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.
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