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    Subject[PATCH 4.14 039/195] [Variant 3/Meltdown] arm64: entry: Add fake CPU feature for unmapping the kernel at EL0
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

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

    From: Will Deacon <will.deacon@arm.com>


    Commit ea1e3de85e94 upstream.

    Allow explicit disabling of the entry trampoline on the kernel command
    line (kpti=off) by adding a fake CPU feature (ARM64_UNMAP_KERNEL_AT_EL0)
    that can be used to toggle the alternative sequences in our entry code and
    avoid use of the trampoline altogether if desired. This also allows us to
    make use of a static key in arm64_kernel_unmapped_at_el0().

    Reviewed-by: Mark Rutland <mark.rutland@arm.com>
    Tested-by: Laura Abbott <labbott@redhat.com>
    Tested-by: Shanker Donthineni <shankerd@codeaurora.org>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/arm64/include/asm/cpucaps.h | 3 +-
    arch/arm64/include/asm/mmu.h | 3 +-
    arch/arm64/kernel/cpufeature.c | 41 +++++++++++++++++++++++++++++++++++++++
    arch/arm64/kernel/entry.S | 9 ++++----
    4 files changed, 50 insertions(+), 6 deletions(-)

    --- a/arch/arm64/include/asm/cpucaps.h
    +++ b/arch/arm64/include/asm/cpucaps.h
    @@ -40,7 +40,8 @@
    #define ARM64_WORKAROUND_858921 19
    #define ARM64_WORKAROUND_CAVIUM_30115 20
    #define ARM64_HAS_DCPOP 21
    +#define ARM64_UNMAP_KERNEL_AT_EL0 23

    -#define ARM64_NCAPS 22
    +#define ARM64_NCAPS 24

    #endif /* __ASM_CPUCAPS_H */
    --- a/arch/arm64/include/asm/mmu.h
    +++ b/arch/arm64/include/asm/mmu.h
    @@ -36,7 +36,8 @@ typedef struct {

    static inline bool arm64_kernel_unmapped_at_el0(void)
    {
    - return IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0);
    + return IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0) &&
    + cpus_have_const_cap(ARM64_UNMAP_KERNEL_AT_EL0);
    }

    extern void paging_init(void);
    --- a/arch/arm64/kernel/cpufeature.c
    +++ b/arch/arm64/kernel/cpufeature.c
    @@ -796,6 +796,40 @@ static bool has_no_fpsimd(const struct a
    ID_AA64PFR0_FP_SHIFT) < 0;
    }

    +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
    +static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
    +
    +static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
    + int __unused)
    +{
    + /* Forced on command line? */
    + if (__kpti_forced) {
    + pr_info_once("kernel page table isolation forced %s by command line option\n",
    + __kpti_forced > 0 ? "ON" : "OFF");
    + return __kpti_forced > 0;
    + }
    +
    + /* Useful for KASLR robustness */
    + if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
    + return true;
    +
    + return false;
    +}
    +
    +static int __init parse_kpti(char *str)
    +{
    + bool enabled;
    + int ret = strtobool(str, &enabled);
    +
    + if (ret)
    + return ret;
    +
    + __kpti_forced = enabled ? 1 : -1;
    + return 0;
    +}
    +__setup("kpti=", parse_kpti);
    +#endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
    +
    static const struct arm64_cpu_capabilities arm64_features[] = {
    {
    .desc = "GIC system register CPU interface",
    @@ -882,6 +916,13 @@ static const struct arm64_cpu_capabiliti
    .def_scope = SCOPE_SYSTEM,
    .matches = hyp_offset_low,
    },
    +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
    + {
    + .capability = ARM64_UNMAP_KERNEL_AT_EL0,
    + .def_scope = SCOPE_SYSTEM,
    + .matches = unmap_kernel_at_el0,
    + },
    +#endif
    {
    /* FP/SIMD is not implemented */
    .capability = ARM64_HAS_NO_FPSIMD,
    --- a/arch/arm64/kernel/entry.S
    +++ b/arch/arm64/kernel/entry.S
    @@ -74,6 +74,7 @@
    .macro kernel_ventry, el, label, regsize = 64
    .align 7
    #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
    +alternative_if ARM64_UNMAP_KERNEL_AT_EL0
    .if \el == 0
    .if \regsize == 64
    mrs x30, tpidrro_el0
    @@ -82,6 +83,7 @@
    mov x30, xzr
    .endif
    .endif
    +alternative_else_nop_endif
    #endif

    sub sp, sp, #S_FRAME_SIZE
    @@ -321,10 +323,9 @@ alternative_else_nop_endif
    ldr lr, [sp, #S_LR]
    add sp, sp, #S_FRAME_SIZE // restore sp

    -#ifndef CONFIG_UNMAP_KERNEL_AT_EL0
    - eret
    -#else
    .if \el == 0
    +alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
    +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
    bne 4f
    msr far_el1, x30
    tramp_alias x30, tramp_exit_native
    @@ -332,10 +333,10 @@ alternative_else_nop_endif
    4:
    tramp_alias x30, tramp_exit_compat
    br x30
    +#endif
    .else
    eret
    .endif
    -#endif
    .endm

    .macro irq_stack_entry

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