arm64: entry: Add fake CPU feature for unmapping the kernel at EL0
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>
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@ -41,7 +41,8 @@
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#define ARM64_WORKAROUND_CAVIUM_30115 20
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#define ARM64_HAS_DCPOP 21
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#define ARM64_SVE 22
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#define ARM64_UNMAP_KERNEL_AT_EL0 23
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#define ARM64_NCAPS 23
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#define ARM64_NCAPS 24
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#endif /* __ASM_CPUCAPS_H */
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@ -36,7 +36,8 @@ typedef struct {
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static inline bool arm64_kernel_unmapped_at_el0(void)
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{
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return IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0);
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return IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0) &&
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cpus_have_const_cap(ARM64_UNMAP_KERNEL_AT_EL0);
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}
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extern void paging_init(void);
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@ -845,6 +845,40 @@ static bool has_no_fpsimd(const struct arm64_cpu_capabilities *entry, int __unus
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ID_AA64PFR0_FP_SHIFT) < 0;
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}
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#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
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static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
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static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
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int __unused)
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{
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/* Forced on command line? */
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if (__kpti_forced) {
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pr_info_once("kernel page table isolation forced %s by command line option\n",
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__kpti_forced > 0 ? "ON" : "OFF");
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return __kpti_forced > 0;
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}
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/* Useful for KASLR robustness */
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if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
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return true;
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return false;
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}
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static int __init parse_kpti(char *str)
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{
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bool enabled;
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int ret = strtobool(str, &enabled);
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if (ret)
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return ret;
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__kpti_forced = enabled ? 1 : -1;
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return 0;
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}
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__setup("kpti=", parse_kpti);
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#endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
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static const struct arm64_cpu_capabilities arm64_features[] = {
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{
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.desc = "GIC system register CPU interface",
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@ -931,6 +965,13 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
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.def_scope = SCOPE_SYSTEM,
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.matches = hyp_offset_low,
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},
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#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
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{
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.capability = ARM64_UNMAP_KERNEL_AT_EL0,
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.def_scope = SCOPE_SYSTEM,
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.matches = unmap_kernel_at_el0,
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},
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#endif
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{
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/* FP/SIMD is not implemented */
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.capability = ARM64_HAS_NO_FPSIMD,
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@ -74,6 +74,7 @@
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.macro kernel_ventry, el, label, regsize = 64
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.align 7
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#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
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alternative_if ARM64_UNMAP_KERNEL_AT_EL0
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.if \el == 0
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.if \regsize == 64
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mrs x30, tpidrro_el0
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@ -82,6 +83,7 @@
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mov x30, xzr
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.endif
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.endif
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alternative_else_nop_endif
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#endif
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sub sp, sp, #S_FRAME_SIZE
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@ -323,10 +325,9 @@ alternative_else_nop_endif
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ldr lr, [sp, #S_LR]
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add sp, sp, #S_FRAME_SIZE // restore sp
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#ifndef CONFIG_UNMAP_KERNEL_AT_EL0
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eret
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#else
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.if \el == 0
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alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
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#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
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bne 4f
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msr far_el1, x30
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tramp_alias x30, tramp_exit_native
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@ -334,10 +335,10 @@ alternative_else_nop_endif
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4:
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tramp_alias x30, tramp_exit_compat
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br x30
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#endif
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.else
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eret
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.endif
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#endif
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.endm
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.macro irq_stack_entry
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