[PATCH 4/6] riscv: Add CFI error handling

From: Sami Tolvanen
Date: Thu Jun 29 2023 - 19:43:10 EST

With CONFIG_CFI_CLANG, the compiler injects a type preamble immediately
before each function and a check to validate the target function type
before indirect calls:

; type preamble
.word <id>
function:
...
; indirect call check
lw t1, -4(a0)
lui t2, <hi20>
addiw t2, t2, <lo12>
beq t1, t2, .Ltmp0
ebreak
.Ltmp0:
jarl a0

Implement error handling code for the ebreak traps emitted for the
checks. This produces the following oops on a CFI failure (generated
using lkdtm):

[ 21.177245] CFI failure at lkdtm_indirect_call+0x22/0x32 [lkdtm]
(target: lkdtm_increment_int+0x0/0x18 [lkdtm]; expected type: 0x3ad55aca)
[ 21.178483] Kernel BUG [#1]
[ 21.178671] Modules linked in: lkdtm
[ 21.179037] CPU: 1 PID: 104 Comm: sh Not tainted
6.3.0-rc6-00037-g37d5ec6297ab #1
[ 21.179511] Hardware name: riscv-virtio,qemu (DT)
[ 21.179818] epc : lkdtm_indirect_call+0x22/0x32 [lkdtm]
[ 21.180106] ra : lkdtm_CFI_FORWARD_PROTO+0x48/0x7c [lkdtm]
[ 21.180426] epc : ffffffff01387092 ra : ffffffff01386f14 sp : ff20000000453cf0
[ 21.180792] gp : ffffffff81308c38 tp : ff6000000243f080 t0 : ff20000000453b78
[ 21.181157] t1 : 000000003ad55aca t2 : 000000007e0c52a5 s0 : ff20000000453d00
[ 21.181506] s1 : 0000000000000001 a0 : ffffffff0138d170 a1 : ffffffff013870bc
[ 21.181819] a2 : b5fea48dd89aa700 a3 : 0000000000000001 a4 : 0000000000000fff
[ 21.182169] a5 : 0000000000000004 a6 : 00000000000000b7 a7 : 0000000000000000
[ 21.182591] s2 : ff20000000453e78 s3 : ffffffffffffffea s4 : 0000000000000012
[ 21.183001] s5 : ff600000023c7000 s6 : 0000000000000006 s7 : ffffffff013882a0
[ 21.183653] s8 : 0000000000000008 s9 : 0000000000000002 s10: ffffffff0138d878
[ 21.184245] s11: ffffffff0138d878 t3 : 0000000000000003 t4 : 0000000000000000
[ 21.184591] t5 : ffffffff8133df08 t6 : ffffffff8133df07
[ 21.184858] status: 0000000000000120 badaddr: 0000000000000000
cause: 0000000000000003
[ 21.185415] [<ffffffff01387092>] lkdtm_indirect_call+0x22/0x32 [lkdtm]
[ 21.185772] [<ffffffff01386f14>] lkdtm_CFI_FORWARD_PROTO+0x48/0x7c [lkdtm]
[ 21.186093] [<ffffffff01383552>] lkdtm_do_action+0x22/0x34 [lkdtm]
[ 21.186445] [<ffffffff0138350c>] direct_entry+0x128/0x13a [lkdtm]
[ 21.186817] [<ffffffff8033ed8c>] full_proxy_write+0x58/0xb2
[ 21.187352] [<ffffffff801d4fe8>] vfs_write+0x14c/0x33a
[ 21.187644] [<ffffffff801d5328>] ksys_write+0x64/0xd4
[ 21.187832] [<ffffffff801d53a6>] sys_write+0xe/0x1a
[ 21.188171] [<ffffffff80003996>] ret_from_syscall+0x0/0x2
[ 21.188595] Code: 0513 0f65 a303 ffc5 53b7 7e0c 839b 2a53 0363 0073 (9002) 9582
[ 21.189178] ---[ end trace 0000000000000000 ]---
[ 21.189590] Kernel panic - not syncing: Fatal exception

Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
---
arch/riscv/Kconfig | 1 +
arch/riscv/include/asm/cfi.h | 22 ++++++++++
arch/riscv/include/asm/insn.h | 10 +++++
arch/riscv/kernel/Makefile | 2 +
arch/riscv/kernel/cfi.c | 77 +++++++++++++++++++++++++++++++++++
arch/riscv/kernel/traps.c | 4 +-
6 files changed, 115 insertions(+), 1 deletion(-)
create mode 100644 arch/riscv/include/asm/cfi.h
create mode 100644 arch/riscv/kernel/cfi.c

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index b54a830eb5c6..20a40927175e 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -44,6 +44,7 @@ config RISCV
select ARCH_SUPPORTS_DEBUG_PAGEALLOC if MMU
select ARCH_SUPPORTS_HUGETLBFS if MMU
select ARCH_SUPPORTS_PAGE_TABLE_CHECK if MMU
+ select ARCH_USES_CFI_TRAPS if CFI_CLANG
select ARCH_USE_MEMTEST
select ARCH_USE_QUEUED_RWLOCKS
select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
diff --git a/arch/riscv/include/asm/cfi.h b/arch/riscv/include/asm/cfi.h
new file mode 100644
index 000000000000..56bf9d69d5e3
--- /dev/null
+++ b/arch/riscv/include/asm/cfi.h
@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_RISCV_CFI_H
+#define _ASM_RISCV_CFI_H
+
+/*
+ * Clang Control Flow Integrity (CFI) support.
+ *
+ * Copyright (C) 2023 Google LLC
+ */
+
+#include <linux/cfi.h>
+
+#ifdef CONFIG_CFI_CLANG
+enum bug_trap_type handle_cfi_failure(struct pt_regs *regs);
+#else
+static inline enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
+{
+ return BUG_TRAP_TYPE_NONE;
+}
+#endif /* CONFIG_CFI_CLANG */
+
+#endif /* _ASM_RISCV_CFI_H */
diff --git a/arch/riscv/include/asm/insn.h b/arch/riscv/include/asm/insn.h
index 8d5c84f2d5ef..45bc485fcf3f 100644
--- a/arch/riscv/include/asm/insn.h
+++ b/arch/riscv/include/asm/insn.h
@@ -63,6 +63,7 @@
#define RVG_RS1_OPOFF 15
#define RVG_RS2_OPOFF 20
#define RVG_RD_OPOFF 7
+#define RVG_RS1_MASK GENMASK(4, 0)
#define RVG_RD_MASK GENMASK(4, 0)

/* The bit field of immediate value in RVC J instruction */
@@ -129,6 +130,7 @@
#define RVC_C2_RS1_OPOFF 7
#define RVC_C2_RS2_OPOFF 2
#define RVC_C2_RD_OPOFF 7
+#define RVC_C2_RS1_MASK GENMASK(4, 0)

/* parts of opcode for RVG*/
#define RVG_OPCODE_FENCE 0x0f
@@ -258,6 +260,10 @@ static __always_inline bool riscv_insn_is_branch(u32 code)
#define RV_X(X, s, mask) (((X) >> (s)) & (mask))
#define RVC_X(X, s, mask) RV_X(X, s, mask)

+#define RV_EXTRACT_RS1_REG(x) \
+ ({typeof(x) x_ = (x); \
+ (RV_X(x_, RVG_RS1_OPOFF, RVG_RS1_MASK)); })
+
#define RV_EXTRACT_RD_REG(x) \
({typeof(x) x_ = (x); \
(RV_X(x_, RVG_RD_OPOFF, RVG_RD_MASK)); })
@@ -285,6 +291,10 @@ static __always_inline bool riscv_insn_is_branch(u32 code)
(RV_X(x_, RV_B_IMM_11_OPOFF, RV_B_IMM_11_MASK) << RV_B_IMM_11_OFF) | \
(RV_IMM_SIGN(x_) << RV_B_IMM_SIGN_OFF); })

+#define RVC_EXTRACT_C2_RS1_REG(x) \
+ ({typeof(x) x_ = (x); \
+ (RV_X(x_, RVC_C2_RS1_OPOFF, RVC_C2_RS1_MASK)); })
+
#define RVC_EXTRACT_JTYPE_IMM(x) \
({typeof(x) x_ = (x); \
(RVC_X(x_, RVC_J_IMM_3_1_OPOFF, RVC_J_IMM_3_1_MASK) << RVC_J_IMM_3_1_OFF) | \
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index 153864e4f399..c173a7cbf4e1 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -90,6 +90,8 @@ obj-$(CONFIG_CRASH_CORE) += crash_core.o

obj-$(CONFIG_JUMP_LABEL) += jump_label.o

+obj-$(CONFIG_CFI_CLANG) += cfi.o
+
obj-$(CONFIG_EFI) += efi.o
obj-$(CONFIG_COMPAT) += compat_syscall_table.o
obj-$(CONFIG_COMPAT) += compat_signal.o
diff --git a/arch/riscv/kernel/cfi.c b/arch/riscv/kernel/cfi.c
new file mode 100644
index 000000000000..820158d7a291
--- /dev/null
+++ b/arch/riscv/kernel/cfi.c
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Clang Control Flow Integrity (CFI) support.
+ *
+ * Copyright (C) 2023 Google LLC
+ */
+#include <asm/cfi.h>
+#include <asm/insn.h>
+
+/*
+ * Returns the target address and the expected type when regs->epc points
+ * to a compiler-generated CFI trap.
+ */
+static bool decode_cfi_insn(struct pt_regs *regs, unsigned long *target,
+ u32 *type)
+{
+ unsigned long *regs_ptr = (unsigned long *)regs;
+ int rs1_num;
+ u32 insn;
+
+ *target = *type = 0;
+
+ /*
+ * The compiler generates the following instruction sequence
+ * for indirect call checks:
+ *
+ *   lw t1, -4(<reg>)
+ * lui t2, <hi20>
+ * addiw t2, t2, <lo12>
+ * beq t1, t2, .Ltmp1
+ * ebreak ; <- regs->epc
+ * .Ltmp1:
+ * jalr <reg>
+ *
+ * We can read the expected type and the target address from the
+ * registers passed to the beq/jalr instructions.
+ */
+ if (get_kernel_nofault(insn, (void *)regs->epc - 4))
+ return false;
+ if (!riscv_insn_is_beq(insn))
+ return false;
+
+ *type = (u32)regs_ptr[RV_EXTRACT_RS1_REG(insn)];
+
+ if (get_kernel_nofault(insn, (void *)regs->epc) ||
+ get_kernel_nofault(insn, (void *)regs->epc + GET_INSN_LENGTH(insn)))
+ return false;
+
+ if (riscv_insn_is_jalr(insn))
+ rs1_num = RV_EXTRACT_RS1_REG(insn);
+ else if (riscv_insn_is_c_jalr(insn))
+ rs1_num = RVC_EXTRACT_C2_RS1_REG(insn);
+ else
+ return false;
+
+ *target = regs_ptr[rs1_num];
+
+ return true;
+}
+
+/*
+ * Checks if the ebreak trap is because of a CFI failure, and handles the trap
+ * if needed. Returns a bug_trap_type value similarly to report_bug.
+ */
+enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
+{
+ unsigned long target;
+ u32 type;
+
+ if (!is_cfi_trap(regs->epc))
+ return BUG_TRAP_TYPE_NONE;
+
+ if (!decode_cfi_insn(regs, &target, &type))
+ return report_cfi_failure_noaddr(regs, regs->epc);
+
+ return report_cfi_failure(regs, regs->epc, &target, type);
+}
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 8c258b78c925..39dce00c6ed7 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -21,6 +21,7 @@

#include <asm/asm-prototypes.h>
#include <asm/bug.h>
+#include <asm/cfi.h>
#include <asm/csr.h>
#include <asm/processor.h>
#include <asm/ptrace.h>
@@ -242,7 +243,8 @@ void handle_break(struct pt_regs *regs)
== NOTIFY_STOP)
return;
#endif
- else if (report_bug(regs->epc, regs) == BUG_TRAP_TYPE_WARN)
+ else if (report_bug(regs->epc, regs) == BUG_TRAP_TYPE_WARN ||
+ handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN)
regs->epc += get_break_insn_length(regs->epc);
else
die(regs, "Kernel BUG");
--
2.41.0.255.g8b1d071c50-goog