arm64: entry: Add fake CPU feature for unmapping the kernel at EL0

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>
[Alex: use first free cpucap number, use cpus_have_cap]
Signed-off-by: Alex Shi <alex.shi@linaro.org> [v4.9 backport]
Signed-off-by: Mark Rutland <mark.rutland@arm.com> [v4.9 backport]
Tested-by: Will Deacon <will.deacon@arm.com>
Tested-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index 87b44653518551d39797a67c1cd367e44ea27b55..7ddf233f05bd681c4b914c54e100e784b5b9270d 100644 (file)
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -34,7 +34,8 @@
 #define ARM64_HAS_32BIT_EL0                    13
 #define ARM64_HYP_OFFSET_LOW                   14
 #define ARM64_MISMATCHED_CACHE_LINE_SIZE       15
+#define ARM64_UNMAP_KERNEL_AT_EL0              16
 
-#define ARM64_NCAPS                            16
+#define ARM64_NCAPS                            17
 
 #endif /* __ASM_CPUCAPS_H */

diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index 279e75b8a49e794c78f02ecd43b35a69bad2bf18..a813edf28737bc9f11c0e69da0f167fcfeba8459 100644 (file)
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -34,7 +34,8 @@ 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_cap(ARM64_UNMAP_KERNEL_AT_EL0);
 }
 
 extern void paging_init(void);

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 3a129d48674e0edac3897e3f8bdf4679857ca55d..74b168c51abddaa936829d9b656d97c61acaa89e 100644 (file)
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -746,6 +746,40 @@ static bool hyp_offset_low(const struct arm64_cpu_capabilities *entry,
        return idmap_addr > GENMASK(VA_BITS - 2, 0) && !is_kernel_in_hyp_mode();
 }
 
+#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",
@@ -829,6 +863,13 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
                .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
        {},
 };
 

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 623c160bf68e9c391e307db8280da8498bbdf34e..b16d0534cda366c4de057777777aaef6ed8349f0 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -73,6 +73,7 @@
        .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
@@ -81,6 +82,7 @@
        mov     x30, xzr
        .endif
        .endif
+alternative_else_nop_endif
 #endif
 
        sub     sp, sp, #S_FRAME_SIZE
@@ -208,10 +210,9 @@ 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
@@ -219,10 +220,10 @@ alternative_else_nop_endif
 4:
        tramp_alias     x30, tramp_exit_compat
        br      x30
+#endif
        .else
        eret
        .endif
-#endif
        .endm
 
        .macro  get_thread_info, rd