[PATCH v20 05/25] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states

Yu-cheng Yu
2021-02-10T18:13:30Z

Control-flow Enforcement Technology (CET) introduces these MSRs:

MSR_IA32_U_CET (user-mode CET settings),
MSR_IA32_PL3_SSP (user-mode shadow stack pointer),

MSR_IA32_PL0_SSP (kernel-mode shadow stack pointer),
MSR_IA32_PL1_SSP (Privilege Level 1 shadow stack pointer),
MSR_IA32_PL2_SSP (Privilege Level 2 shadow stack pointer),
MSR_IA32_S_CET (kernel-mode CET settings),
MSR_IA32_INT_SSP_TAB (exception shadow stack table).

The two user-mode MSRs belong to XFEATURE_CET_USER. The first three of
kernel-mode MSRs belong to XFEATURE_CET_KERNEL. Both XSAVES states are
supervisor states. This means that there is no direct, unprivileged access
to these states, making it harder for an attacker to subvert CET.

For sigreturn and future ptrace() support, shadow stack address and MSR
reserved bits are checked before written to the supervisor states.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
arch/x86/include/asm/fpu/types.h | 23 +++++++++++++++++++++--
arch/x86/include/asm/fpu/xstate.h | 6 ++++--
arch/x86/include/asm/msr-index.h | 19 +++++++++++++++++++
arch/x86/kernel/fpu/xstate.c | 10 +++++++++-
4 files changed, 53 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index f5a38a5f3ae1..035eb0ec665e 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -115,8 +115,8 @@ enum xfeature {
XFEATURE_PT_UNIMPLEMENTED_SO_FAR,
XFEATURE_PKRU,
XFEATURE_PASID,
- XFEATURE_RSRVD_COMP_11,
- XFEATURE_RSRVD_COMP_12,
+ XFEATURE_CET_USER,
+ XFEATURE_CET_KERNEL,
XFEATURE_RSRVD_COMP_13,
XFEATURE_RSRVD_COMP_14,
XFEATURE_LBR,
@@ -135,6 +135,8 @@ enum xfeature {
#define XFEATURE_MASK_PT (1 << XFEATURE_PT_UNIMPLEMENTED_SO_FAR)
#define XFEATURE_MASK_PKRU (1 << XFEATURE_PKRU)
#define XFEATURE_MASK_PASID (1 << XFEATURE_PASID)
+#define XFEATURE_MASK_CET_USER (1 << XFEATURE_CET_USER)
+#define XFEATURE_MASK_CET_KERNEL (1 << XFEATURE_CET_KERNEL)
#define XFEATURE_MASK_LBR (1 << XFEATURE_LBR)

#define XFEATURE_MASK_FPSSE (XFEATURE_MASK_FP | XFEATURE_MASK_SSE)
@@ -237,6 +239,23 @@ struct pkru_state {
u32 pad;
} __packed;

+/*
+ * State component 11 is Control-flow Enforcement user states
+ */
+struct cet_user_state {
+ u64 user_cet; /* user control-flow settings */
+ u64 user_ssp; /* user shadow stack pointer */
+};
+
+/*
+ * State component 12 is Control-flow Enforcement kernel states
+ */
+struct cet_kernel_state {
+ u64 kernel_ssp; /* kernel shadow stack */
+ u64 pl1_ssp; /* privilege level 1 shadow stack */
+ u64 pl2_ssp; /* privilege level 2 shadow stack */
+};
+
/*
* State component 15: Architectural LBR configuration state.
* The size of Arch LBR state depends on the number of LBRs (lbr_depth).
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index 47a92232d595..582f3575e0bd 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -35,7 +35,8 @@
XFEATURE_MASK_BNDCSR)

/* All currently supported supervisor features */
-#define XFEATURE_MASK_SUPERVISOR_SUPPORTED (XFEATURE_MASK_PASID)
+#define XFEATURE_MASK_SUPERVISOR_SUPPORTED (XFEATURE_MASK_PASID | \
+ XFEATURE_MASK_CET_USER)

/*
* A supervisor state component may not always contain valuable information,
@@ -62,7 +63,8 @@
* Unsupported supervisor features. When a supervisor feature in this mask is
* supported in the future, move it to the supported supervisor feature mask.
*/
-#define XFEATURE_MASK_SUPERVISOR_UNSUPPORTED (XFEATURE_MASK_PT)
+#define XFEATURE_MASK_SUPERVISOR_UNSUPPORTED (XFEATURE_MASK_PT | \
+ XFEATURE_MASK_CET_KERNEL)

/* All supervisor states including supported and unsupported states. */
#define XFEATURE_MASK_SUPERVISOR_ALL (XFEATURE_MASK_SUPERVISOR_SUPPORTED | \
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 546d6ecf0a35..fae6b3ea1f6d 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -933,4 +933,23 @@
#define MSR_VM_IGNNE 0xc0010115
#define MSR_VM_HSAVE_PA 0xc0010117

+/* Control-flow Enforcement Technology MSRs */
+#define MSR_IA32_U_CET 0x6a0 /* user mode cet setting */
+#define MSR_IA32_S_CET 0x6a2 /* kernel mode cet setting */
+#define CET_SHSTK_EN BIT_ULL(0)
+#define CET_WRSS_EN BIT_ULL(1)
+#define CET_ENDBR_EN BIT_ULL(2)
+#define CET_LEG_IW_EN BIT_ULL(3)
+#define CET_NO_TRACK_EN BIT_ULL(4)
+#define CET_SUPPRESS_DISABLE BIT_ULL(5)
+#define CET_RESERVED (BIT_ULL(6) | BIT_ULL(7) | BIT_ULL(8) | BIT_ULL(9))
+#define CET_SUPPRESS BIT_ULL(10)
+#define CET_WAIT_ENDBR BIT_ULL(11)
+
+#define MSR_IA32_PL0_SSP 0x6a4 /* kernel shadow stack pointer */
+#define MSR_IA32_PL1_SSP 0x6a5 /* ring-1 shadow stack pointer */
+#define MSR_IA32_PL2_SSP 0x6a6 /* ring-2 shadow stack pointer */
+#define MSR_IA32_PL3_SSP 0x6a7 /* user shadow stack pointer */
+#define MSR_IA32_INT_SSP_TAB 0x6a8 /* exception shadow stack table */
+
#endif /* _ASM_X86_MSR_INDEX_H */
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 5d8047441a0a..22eedf8066bf 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -38,6 +38,8 @@ static const char *xfeature_names[] =
"Processor Trace (unused)" ,
"Protection Keys User registers",
"PASID state",
+ "Control-flow User registers" ,
+ "Control-flow Kernel registers" ,
"unknown xstate feature" ,
};

@@ -53,6 +55,8 @@ static short xsave_cpuid_features[] __initdata = {
X86_FEATURE_INTEL_PT,
X86_FEATURE_PKU,
X86_FEATURE_ENQCMD,
+ X86_FEATURE_CET, /* XFEATURE_CET_USER */
+ X86_FEATURE_CET, /* XFEATURE_CET_KERNEL */
};

/*
@@ -321,6 +325,8 @@ static void __init print_xstate_features(void)
print_xstate_feature(XFEATURE_MASK_Hi16_ZMM);
print_xstate_feature(XFEATURE_MASK_PKRU);
print_xstate_feature(XFEATURE_MASK_PASID);
+ print_xstate_feature(XFEATURE_MASK_CET_USER);
+ print_xstate_feature(XFEATURE_MASK_CET_KERNEL);
}

/*
@@ -596,6 +602,8 @@ static void check_xstate_against_struct(int nr)
XCHECK_SZ(sz, nr, XFEATURE_Hi16_ZMM, struct avx_512_hi16_state);
XCHECK_SZ(sz, nr, XFEATURE_PKRU, struct pkru_state);
XCHECK_SZ(sz, nr, XFEATURE_PASID, struct ia32_pasid_state);
+ XCHECK_SZ(sz, nr, XFEATURE_CET_USER, struct cet_user_state);
+ XCHECK_SZ(sz, nr, XFEATURE_CET_KERNEL, struct cet_kernel_state);

/*
* Make *SURE* to add any feature numbers in below if
@@ -605,7 +613,7 @@ static void check_xstate_against_struct(int nr)
if ((nr < XFEATURE_YMM) ||
(nr >= XFEATURE_MAX) ||
(nr == XFEATURE_PT_UNIMPLEMENTED_SO_FAR) ||
- ((nr >= XFEATURE_RSRVD_COMP_11) && (nr <= XFEATURE_LBR))) {
+ ((nr >= XFEATURE_RSRVD_COMP_13) && (nr <= XFEATURE_LBR))) {
WARN_ONCE(1, "no structure for xstate: %d\n", nr);
XSTATE_WARN_ON(1);
}
--
2.21.0