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

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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>
This commit is contained in:
Will Deacon 2017-11-14 14:38:19 +00:00
parent 18011eac28
commit ea1e3de85e
4 changed files with 50 additions and 6 deletions
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3
arch/arm64/include/asm/cpucaps.h
View File

@ -41,7 +41,8 @@
#define ARM64_WORKAROUND_CAVIUM_30115 20
#define ARM64_HAS_DCPOP 21
#define ARM64_SVE 22
#define ARM64_UNMAP_KERNEL_AT_EL0 23
#define ARM64_NCAPS 23
#define ARM64_NCAPS 24
#endif /* __ASM_CPUCAPS_H */

3
arch/arm64/include/asm/mmu.h
View File

@ -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);

41
arch/arm64/kernel/cpufeature.c
View File

@ -845,6 +845,40 @@ static bool has_no_fpsimd(const struct arm64_cpu_capabilities *entry, int __unus
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",
@ -931,6 +965,13 @@ 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
{
/* FP/SIMD is not implemented */
.capability = ARM64_HAS_NO_FPSIMD,

9
arch/arm64/kernel/entry.S
View File

@ -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
@ -323,10 +325,9 @@ 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
@ -334,10 +335,10 @@ alternative_else_nop_endif
4:
tramp_alias x30, tramp_exit_compat
br x30
#endif
.else
eret
.endif
#endif
.endm
.macro irq_stack_entry