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    Subject[RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG
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    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 manual type annotations in
    assembly code, add error handling for the CFI ud2 traps, and allow
    CONFIG_CFI_CLANG to be selected on x86_64.

    Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
    ---
    arch/x86/Kconfig | 2 ++
    arch/x86/include/asm/linkage.h | 12 +++++++
    arch/x86/kernel/traps.c | 60 +++++++++++++++++++++++++++++++++-
    3 files changed, 73 insertions(+), 1 deletion(-)

    diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    index 4bed3abf444d..2e73d0792d48 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/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/traps.c b/arch/x86/kernel/traps.c
    index 1563fb995005..320e257eb4be 100644
    --- a/arch/x86/kernel/traps.c
    +++ b/arch/x86/kernel/traps.c
    @@ -40,6 +40,7 @@
    #include <linux/hardirq.h>
    #include <linux/atomic.h>
    #include <linux/ioasid.h>
    +#include <linux/cfi.h>

    #include <asm/stacktrace.h>
    #include <asm/processor.h>
    @@ -295,6 +296,62 @@ static inline void handle_invalid_op(struct pt_regs *regs)
    ILL_ILLOPN, error_get_trap_addr(regs));
    }

    +#ifdef CONFIG_CFI_CLANG
    +static void decode_cfi_insn(struct pt_regs *regs, unsigned long *target,
    + unsigned long *type)
    +{
    + char buffer[MAX_INSN_SIZE];
    + struct insn insn;
    + int offset;
    +
    + *target = *type = 0;
    +
    + /*
    + * The compiler generates the following instruction sequence
    + * for indirect call checks:
    + *
    + *   cmpl    <id>, -6(%reg) ; 7 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 - 9, MAX_INSN_SIZE))
    + return;
    + if (insn_decode_kernel(&insn, buffer))
    + return;
    + if (insn.opcode.value != 0x81 || X86_MODRM_REG(insn.modrm.value) != 7)
    + return;
    +
    + *type = insn.immediate.value;
    +
    + offset = insn_get_modrm_rm_off(&insn, regs);
    + if (offset < 0)
    + return;
    +
    + *target = *(unsigned long *)((void *)regs + offset);
    +}
    +
    +static enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
    +{
    + if (is_cfi_trap(regs->ip)) {
    + unsigned long target, type;
    +
    + decode_cfi_insn(regs, &target, &type);
    + return report_cfi_failure(regs, regs->ip, target, type);
    + }
    +
    + return BUG_TRAP_TYPE_NONE;
    +}
    +#else
    +static inline enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
    +{
    + return BUG_TRAP_TYPE_NONE;
    +}
    +#endif /* CONFIG_CFI_CLANG */
    +
    static noinstr bool handle_bug(struct pt_regs *regs)
    {
    bool handled = false;
    @@ -312,7 +369,8 @@ 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;
    }
    --
    2.36.0.550.gb090851708-goog
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