diff mbox series

[v2,16/18] arm64: entry: Add fake CPU feature for unmapping the kernel at EL0

Message ID 1512059986-21325-17-git-send-email-will.deacon@arm.com
State Superseded
Headers show
Series arm64: Unmap the kernel whilst running in userspace (KAISER) | expand

Commit Message

Will Deacon Nov. 30, 2017, 4:39 p.m. UTC 
Allow explicit disabling of the entry trampoline on the kernel command
line (kaiser=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().

Signed-off-by: Will Deacon <will.deacon@arm.com>

---
 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        | 11 +++++++----
 4 files changed, 52 insertions(+), 6 deletions(-)

-- 
2.1.4

Comments

Mark Rutland Dec. 1, 2017, 1:55 p.m. UTC | #1 
On Thu, Nov 30, 2017 at 04:39:44PM +0000, Will Deacon wrote:
> -#ifndef CONFIG_UNMAP_KERNEL_AT_EL0

> -	eret

> -#else

>  	.if	\el == 0

> +alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0


Since we patch this eret ...

> +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0

> +alternative_if ARM64_UNMAP_KERNEL_AT_EL0

>  	bne	4f

>  	msr	far_el1, x30

>  	tramp_alias	x30, tramp_exit_native

> @@ -334,10 +336,11 @@ alternative_else_nop_endif

>  4:

>  	tramp_alias	x30, tramp_exit_compat

>  	br	x30

> +alternative_else_nop_endif

> +#endif


... we don't need the alternative here. This code won't be executed when
not needed, and the alternative just bloats the kernel.

We can/should keep the ifdef, though.

Thanks,
Mark.
diff mbox series

Patch

diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index 2ff7c5e8efab..b4537ffd1018 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -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 */
diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index c07954638658..da6f12e40714 100644
--- 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);
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index c5ba0097887f..72fc55d22ddb 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -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 __kaiser_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 (__kaiser_forced) {
+		pr_info("KAISER forced %s by command line option\n",
+			__kaiser_forced > 0 ? "ON" : "OFF");
+		return __kaiser_forced > 0;
+	}
+
+	/* Useful for KASLR robustness */
+	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
+		return true;
+
+	return false;
+}
+
+static int __init parse_kaiser(char *str)
+{
+	bool enabled;
+	int ret = strtobool(str, &enabled);
+
+	if (ret)
+		return ret;
+
+	__kaiser_forced = enabled ? 1 : -1;
+	return 0;
+}
+__setup("kaiser=", parse_kaiser);
+#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,
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index a5ec6ab5c711..d8775f55e930 100644
--- 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
@@ -323,10 +325,10 @@  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
+alternative_if ARM64_UNMAP_KERNEL_AT_EL0
 	bne	4f
 	msr	far_el1, x30
 	tramp_alias	x30, tramp_exit_native
@@ -334,10 +336,11 @@  alternative_else_nop_endif
 4:
 	tramp_alias	x30, tramp_exit_compat
 	br	x30
+alternative_else_nop_endif
+#endif
 	.else
 	eret
 	.endif
-#endif
 	.endm
 
 	.macro	irq_stack_entry