[RFC PATCH v3 20/20] x86: Add support for CONFIG_CFI_CLANG

Sami Tolvanen
2022-06-10T23:36:05Z

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
__cfi_function:
int3
int3
mov <id>, %eax
int3
int3
function:
...
; indirect call check
cmpl    <id>, -6(%r11)
je .Ltmp1
ud2
.Ltmp1:
call __x86_indirect_thunk_r11

Define the __CFI_TYPE helper macro for manually annotating indirectly
called assembly function with the identical premable, add error handling
code for the ud2 traps emitted for indirect call checks, and allow
CONFIG_CFI_CLANG to be selected on x86_64.

This produces the following oops on CFI failure (generated using lkdtm):

[ 15.896503] CFI failure at lkdtm_indirect_call+0x14/0x20 [lkdtm]
(target: lkdtm_increment_int+0x0/0x7 [lkdtm]; expected type: 0x7e0c52a5)
[ 15.898565] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
[ 15.898798] CPU: 2 PID: 133 Comm: sh Not tainted
5.19.0-rc1-00020-g524d4b861d15 #1
[ 15.898967] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 15.899004] RIP: 0010:lkdtm_indirect_call+0x14/0x20 [lkdtm]
[ 15.899004] Code: c7 c2 76 02 40 c0 48 c7 c1 95 00 40 c0 e9 3b cb 3e
e8 0f 1f 40 00 49 89 fb 48 c7 c7 70 64 40 c0 41 81 7b fa a5 52 0c 7f
[ 15.899004] RSP: 0018:ffff9d928029fdc0 EFLAGS: 00000283
[ 15.899004] RAX: 0000000000000027 RBX: ffffffffc0406320 RCX: 024cef129f458500
[ 15.899004] RDX: ffffffffa9251580 RSI: ffffffffa90736c8 RDI: ffffffffc0406470
[ 15.899004] RBP: 0000000000000006 R08: ffffffffa9251670 R09: 65686374616d7369
[ 15.899004] R10: 000000002e2e2e20 R11: ffffffffc03fdc69 R12: 0000000000000000
[ 15.899004] R13: ffff8b2c022ee000 R14: 0000000000000000 R15: 0000000000000002
[ 15.899004] FS: 00007f0a6e7e86a0(0000) GS:ffff8b2c1f500000(0000)
knlGS:0000000000000000
[ 15.899004] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 15.899004] CR2: 00000000010ddfd0 CR3: 0000000001bc8000 CR4: 00000000000006e0
[ 15.899004] Call Trace:
[ 15.899004] <TASK>
[ 15.899004] lkdtm_CFI_FORWARD_PROTO+0x30/0x57 [lkdtm]
[ 15.899004] direct_entry+0x129/0x137 [lkdtm]
[ 15.899004] full_proxy_write+0x5b/0xa7
[ 15.899004] vfs_write+0x142/0x457
[ 15.899004] ? __x64_sys_wait4+0x5a/0xb7
[ 15.899004] ksys_write+0x69/0xd7
[ 15.899004] do_syscall_64+0x4f/0x97
[ 15.899004] entry_SYSCALL_64_after_hwframe+0x46/0xb0
[ 15.899004] RIP: 0033:0x7f0a6e76dfe1
[ 15.899004] Code: be 07 00 00 00 41 89 c0 e8 7e ff ff ff 44 89 c7 89
04 24 e8 91 c6 02 00 8b 04 24 48 83 c4 68 c3 48 63 ff b8 01 00 00 03
[ 15.899004] RSP: 002b:00007ffe32e133c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 15.899004] RAX: ffffffffffffffda RBX: 00007f0a6e7e8690 RCX: 00007f0a6e76dfe1
[ 15.899004] RDX: 0000000000000012 RSI: 0000000001ad5910 RDI: 0000000000000001
[ 15.899004] RBP: 0000000000000001 R08: fefefefefefefeff R09: fefefeff00abff4e
[ 15.899004] R10: 00007f0a6e7e92b0 R11: 0000000000000246 R12: 0000000001ad5910
[ 15.899004] R13: 0000000000000012 R14: 00007ffe32e13501 R15: 0000000001ad2450
[ 15.899004] </TASK>
[ 15.899004] Modules linked in: lkdtm
[ 15.899004] Dumping ftrace buffer:
[ 15.899004] (ftrace buffer empty)
[ 15.925612] ---[ end trace 0000000000000000 ]---
[ 15.925661] RIP: 0010:lkdtm_indirect_call+0x14/0x20 [lkdtm]
[ 15.925689] Code: c7 c2 76 02 40 c0 48 c7 c1 95 00 40 c0 e9 3b cb 3e
e8 0f 1f 40 00 49 89 fb 48 c7 c7 70 64 40 c0 41 81 7b fa a5 52 0c 7f
[ 15.925697] RSP: 0018:ffff9d928029fdc0 EFLAGS: 00000283
[ 15.925709] RAX: 0000000000000027 RBX: ffffffffc0406320 RCX: 024cef129f458500
[ 15.925716] RDX: ffffffffa9251580 RSI: ffffffffa90736c8 RDI: ffffffffc0406470
[ 15.925722] RBP: 0000000000000006 R08: ffffffffa9251670 R09: 65686374616d7369
[ 15.925731] R10: 000000002e2e2e20 R11: ffffffffc03fdc69 R12: 0000000000000000
[ 15.925739] R13: ffff8b2c022ee000 R14: 0000000000000000 R15: 0000000000000002
[ 15.925746] FS: 00007f0a6e7e86a0(0000) GS:ffff8b2c1f500000(0000)
knlGS:0000000000000000
[ 15.925755] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 15.925762] CR2: 00000000010ddfd0 CR3: 0000000001bc8000 CR4: 00000000000006e0
[ 15.925889] Kernel panic - not syncing: Fatal exception

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
arch/x86/Kconfig | 2 +
arch/x86/include/asm/cfi.h | 22 +++++++++
arch/x86/include/asm/linkage.h | 12 +++++
arch/x86/kernel/Makefile | 2 +
arch/x86/kernel/cfi.c | 83 ++++++++++++++++++++++++++++++++++
arch/x86/kernel/traps.c | 4 +-
6 files changed, 124 insertions(+), 1 deletion(-)
create mode 100644 arch/x86/include/asm/cfi.h
create mode 100644 arch/x86/kernel/cfi.c

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index be0b95e51df6..8b8c12b223ae 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -108,6 +108,8 @@ config X86
select ARCH_SUPPORTS_PAGE_TABLE_CHECK if X86_64
select ARCH_SUPPORTS_NUMA_BALANCING if X86_64
select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP if NR_CPUS <= 4096
+ select ARCH_SUPPORTS_CFI_CLANG if X86_64
+ select ARCH_USES_CFI_TRAPS if X86_64 && CFI_CLANG
select ARCH_SUPPORTS_LTO_CLANG
select ARCH_SUPPORTS_LTO_CLANG_THIN
select ARCH_USE_BUILTIN_BSWAP
diff --git a/arch/x86/include/asm/cfi.h b/arch/x86/include/asm/cfi.h
new file mode 100644
index 000000000000..86a7b727ed89
--- /dev/null
+++ b/arch/x86/include/asm/cfi.h
@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_CFI_H
+#define _ASM_X86_CFI_H
+
+/*
+ * Clang Control Flow Integrity (CFI) support.
+ *
+ * Copyright (C) 2022 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_X86_CFI_H */
diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h
index 85865f1645bd..0ee4a0af3974 100644
--- a/arch/x86/include/asm/linkage.h
+++ b/arch/x86/include/asm/linkage.h
@@ -25,6 +25,18 @@
#define RET ret
#endif

+#ifdef CONFIG_CFI_CLANG
+#define __CFI_TYPE(name) \
+ .fill 7, 1, 0xCC ASM_NL \
+ SYM_START(__cfi_##name, SYM_L_LOCAL, SYM_A_NONE) \
+ int3 ASM_NL \
+ int3 ASM_NL \
+ mov __kcfi_typeid_##name, %eax ASM_NL \
+ int3 ASM_NL \
+ int3 ASM_NL \
+ SYM_FUNC_END(__cfi_##name)
+#endif
+
#else /* __ASSEMBLY__ */

#ifdef CONFIG_SLS
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 03364dc40d8d..2a3dc7e3e6ca 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -145,6 +145,8 @@ obj-$(CONFIG_UNWINDER_GUESS) += unwind_guess.o

obj-$(CONFIG_AMD_MEM_ENCRYPT) += sev.o

+obj-$(CONFIG_CFI_CLANG) += cfi.o
+
###
# 64 bit specific files
ifeq ($(CONFIG_X86_64),y)
diff --git a/arch/x86/kernel/cfi.c b/arch/x86/kernel/cfi.c
new file mode 100644
index 000000000000..7954342b2b3a
--- /dev/null
+++ b/arch/x86/kernel/cfi.c
@@ -0,0 +1,83 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Clang Control Flow Integrity (CFI) support.
+ *
+ * Copyright (C) 2022 Google LLC
+ */
+#include <asm/cfi.h>
+#include <asm/insn.h>
+#include <asm/insn-eval.h>
+
+/*
+ * Returns the target address and the expected type when regs->ip points
+ * to a compiler-generated CFI trap.
+ */
+static bool decode_cfi_insn(struct pt_regs *regs, unsigned long *target,
+ u32 *type)
+{
+ char buffer[MAX_INSN_SIZE];
+ struct insn insn;
+ int offset = 0;
+
+ *target = *type = 0;
+
+ /*
+ * The compiler generates the following instruction sequence
+ * for indirect call checks:
+ *
+ *   cmpl    <id>, -6(%reg) ; 7-8 bytes
+ * je .Ltmp1 ; 2 bytes
+ * ud2 ; <- addr
+ * .Ltmp1:
+ *
+ * Both the type and the target address can be decoded from the
+ * cmpl instruction.
+ */
+ if (copy_from_kernel_nofault(buffer, (void *)regs->ip - 10, MAX_INSN_SIZE))
+ return false;
+ /*
+ * The compiler may not use r8-r15 without retpolines. Skip the
+ * first byte if it's not the expected REX prefix.
+ */
+ if (buffer[0] != 0x41)
+ ++offset;
+ if (insn_decode_kernel(&insn, &buffer[offset]))
+ return false;
+ if (insn.opcode.value != 0x81 || X86_MODRM_REG(insn.modrm.value) != 7)
+ return false;
+
+ *type = insn.immediate.value;
+
+ /* Read the target address from the register. */
+ offset = insn_get_modrm_rm_off(&insn, regs);
+ if (offset < 0)
+ return false;
+
+ *target = *(unsigned long *)((void *)regs + offset);
+
+ return true;
+}
+
+/*
+ * Checks if a ud2 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->ip))
+ return BUG_TRAP_TYPE_NONE;
+
+ if (!decode_cfi_insn(regs, &target, &type))
+ return report_cfi_failure_noaddr(regs, regs->ip);
+
+ return report_cfi_failure(regs, regs->ip, &target, type);
+}
+
+/*
+ * Ensure that __kcfi_typeid_ symbols are emitted for functions that may
+ * not be indirectly called with all configurations.
+ */
+__ADDRESSABLE(memcpy)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index d62b2cb85cea..178015a820f0 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -63,6 +63,7 @@
#include <asm/insn-eval.h>
#include <asm/vdso.h>
#include <asm/tdx.h>
+#include <asm/cfi.h>

#ifdef CONFIG_X86_64
#include <asm/x86_init.h>
@@ -313,7 +314,8 @@ static noinstr bool handle_bug(struct pt_regs *regs)
*/
if (regs->flags & X86_EFLAGS_IF)
raw_local_irq_enable();
- if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+ if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
+ handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
regs->ip += LEN_UD2;
handled = true;
}
--
2.36.1.476.g0c4daa206d-goog