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    Subject[PATCH 4.19 031/139] x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support
    Date
    4.19-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Zhenzhong Duan zhenzhong.duan@oracle.com

    commit 4cd24de3a0980bf3100c9dcb08ef65ca7c31af48 upstream

    Since retpoline capable compilers are widely available, make
    CONFIG_RETPOLINE hard depend on the compiler capability.

    Break the build when CONFIG_RETPOLINE is enabled and the compiler does not
    support it. Emit an error message in that case:

    "arch/x86/Makefile:226: *** You are building kernel with non-retpoline
    compiler, please update your compiler.. Stop."

    [dwmw: Fail the build with non-retpoline compiler]

    Suggested-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: David Woodhouse <dwmw@amazon.co.uk>
    Cc: Borislav Petkov <bp@suse.de>
    Cc: Daniel Borkmann <daniel@iogearbox.net>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
    Cc: Michal Marek <michal.lkml@markovi.net>
    Cc: <srinivas.eeda@oracle.com>
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/cca0cb20-f9e2-4094-840b-fb0f8810cd34@default
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/Kconfig | 4 ----
    arch/x86/Makefile | 5 +++--
    arch/x86/include/asm/nospec-branch.h | 10 ++++++----
    arch/x86/kernel/cpu/bugs.c | 2 +-
    scripts/Makefile.build | 2 --
    5 files changed, 10 insertions(+), 13 deletions(-)

    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -441,10 +441,6 @@ config RETPOLINE
    branches. Requires a compiler with -mindirect-branch=thunk-extern
    support for full protection. The kernel may run slower.

    - Without compiler support, at least indirect branches in assembler
    - code are eliminated. Since this includes the syscall entry path,
    - it is not entirely pointless.
    -
    config INTEL_RDT
    bool "Intel Resource Director Technology support"
    default n
    --- a/arch/x86/Makefile
    +++ b/arch/x86/Makefile
    @@ -223,9 +223,10 @@ KBUILD_CFLAGS += -fno-asynchronous-unwin

    # Avoid indirect branches in kernel to deal with Spectre
    ifdef CONFIG_RETPOLINE
    -ifneq ($(RETPOLINE_CFLAGS),)
    - KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
    +ifeq ($(RETPOLINE_CFLAGS),)
    + $(error You are building kernel with non-retpoline compiler, please update your compiler.)
    endif
    + KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
    endif

    archscripts: scripts_basic
    --- a/arch/x86/include/asm/nospec-branch.h
    +++ b/arch/x86/include/asm/nospec-branch.h
    @@ -162,11 +162,12 @@
    _ASM_PTR " 999b\n\t" \
    ".popsection\n\t"

    -#if defined(CONFIG_X86_64) && defined(RETPOLINE)
    +#ifdef CONFIG_RETPOLINE
    +#ifdef CONFIG_X86_64

    /*
    - * Since the inline asm uses the %V modifier which is only in newer GCC,
    - * the 64-bit one is dependent on RETPOLINE not CONFIG_RETPOLINE.
    + * Inline asm uses the %V modifier which is only in newer GCC
    + * which is ensured when CONFIG_RETPOLINE is defined.
    */
    # define CALL_NOSPEC \
    ANNOTATE_NOSPEC_ALTERNATIVE \
    @@ -181,7 +182,7 @@
    X86_FEATURE_RETPOLINE_AMD)
    # define THUNK_TARGET(addr) [thunk_target] "r" (addr)

    -#elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
    +#else /* CONFIG_X86_32 */
    /*
    * For i386 we use the original ret-equivalent retpoline, because
    * otherwise we'll run out of registers. We don't care about CET
    @@ -211,6 +212,7 @@
    X86_FEATURE_RETPOLINE_AMD)

    # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
    +#endif
    #else /* No retpoline for C / inline asm */
    # define CALL_NOSPEC "call *%[thunk_target]\n"
    # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
    --- a/arch/x86/kernel/cpu/bugs.c
    +++ b/arch/x86/kernel/cpu/bugs.c
    @@ -252,7 +252,7 @@ static void __init spec2_print_if_secure

    static inline bool retp_compiler(void)
    {
    - return __is_defined(RETPOLINE);
    + return __is_defined(CONFIG_RETPOLINE);
    }

    static inline bool match_option(const char *arg, int arglen, const char *opt)
    --- a/scripts/Makefile.build
    +++ b/scripts/Makefile.build
    @@ -248,10 +248,8 @@ ifdef CONFIG_GCOV_KERNEL
    objtool_args += --no-unreachable
    endif
    ifdef CONFIG_RETPOLINE
    -ifneq ($(RETPOLINE_CFLAGS),)
    objtool_args += --retpoline
    endif
    -endif


    ifdef CONFIG_MODVERSIONS

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