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[PATCH v3 10/20] arm64: entry: Add exception trampoline page for exceptions from EL0




To allow unmapping of the kernel whilst running at EL0, we need to
point the exception vectors at an entry trampoline that can map/unmap
the kernel on entry/exit respectively.

This patch adds the trampoline page, although it is not yet plugged
into the vector table and is therefore unused.

Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
 arch/arm64/kernel/entry.S       | 86 +++++++++++++++++++++++++++++++++++++++++
 arch/arm64/kernel/vmlinux.lds.S | 17 ++++++++
 2 files changed, 103 insertions(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index d454d8ed45e4..716b5ef42e29 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -28,6 +28,8 @@
 #include <asm/errno.h>
 #include <asm/esr.h>
 #include <asm/irq.h>
+#include <asm/memory.h>
+#include <asm/mmu.h>
 #include <asm/processor.h>
 #include <asm/ptrace.h>
 #include <asm/thread_info.h>
@@ -943,6 +945,90 @@ __ni_sys_trace:
 
 	.popsection				// .entry.text
 
+#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+/*
+ * Exception vectors trampoline.
+ */
+	.pushsection ".entry.tramp.text", "ax"
+
+	.macro tramp_map_kernel, tmp
+	mrs	\tmp, ttbr1_el1
+	sub	\tmp, \tmp, #(SWAPPER_DIR_SIZE + RESERVED_TTBR0_SIZE)
+	bic	\tmp, \tmp, #USER_ASID_FLAG
+	msr	ttbr1_el1, \tmp
+	.endm
+
+	.macro tramp_unmap_kernel, tmp
+	mrs	\tmp, ttbr1_el1
+	add	\tmp, \tmp, #(SWAPPER_DIR_SIZE + RESERVED_TTBR0_SIZE)
+	orr	\tmp, \tmp, #USER_ASID_FLAG
+	msr	ttbr1_el1, \tmp
+	/*
+	 * We avoid running the post_ttbr_update_workaround here because the
+	 * user and kernel ASIDs don't have conflicting mappings, so any
+	 * "blessing" as described in:
+	 *
+	 *   http://lkml.kernel.org/r/56BB848A.6060603@xxxxxxxxxxxxxxxxxx
+	 *
+	 * will not hurt correctness. Whilst this may partially defeat the
+	 * point of using split ASIDs in the first place, it avoids
+	 * the hit of invalidating the entire I-cache on every return to
+	 * userspace.
+	 */
+	.endm
+
+	.macro tramp_ventry, regsize = 64
+	.align	7
+1:
+	.if	\regsize == 64
+	msr	tpidrro_el0, x30	// Restored in kernel_ventry
+	.endif
+	tramp_map_kernel	x30
+	ldr	x30, =vectors
+	prfm	plil1strm, [x30, #(1b - tramp_vectors)]
+	msr	vbar_el1, x30
+	add	x30, x30, #(1b - tramp_vectors)
+	isb
+	br	x30
+	.endm
+
+	.macro tramp_exit, regsize = 64
+	adr	x30, tramp_vectors
+	msr	vbar_el1, x30
+	tramp_unmap_kernel	x30
+	.if	\regsize == 64
+	mrs	x30, far_el1
+	.endif
+	eret
+	.endm
+
+	.align	11
+ENTRY(tramp_vectors)
+	.space	0x400
+
+	tramp_ventry
+	tramp_ventry
+	tramp_ventry
+	tramp_ventry
+
+	tramp_ventry	32
+	tramp_ventry	32
+	tramp_ventry	32
+	tramp_ventry	32
+END(tramp_vectors)
+
+ENTRY(tramp_exit_native)
+	tramp_exit
+END(tramp_exit_native)
+
+ENTRY(tramp_exit_compat)
+	tramp_exit	32
+END(tramp_exit_compat)
+
+	.ltorg
+	.popsection				// .entry.tramp.text
+#endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
+
 /*
  * Special system call wrappers.
  */
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 7da3e5c366a0..6b4260f22aab 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -57,6 +57,17 @@ jiffies = jiffies_64;
 #define HIBERNATE_TEXT
 #endif
 
+#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+#define TRAMP_TEXT					\
+	. = ALIGN(PAGE_SIZE);				\
+	VMLINUX_SYMBOL(__entry_tramp_text_start) = .;	\
+	*(.entry.tramp.text)				\
+	. = ALIGN(PAGE_SIZE);				\
+	VMLINUX_SYMBOL(__entry_tramp_text_end) = .;
+#else
+#define TRAMP_TEXT
+#endif
+
 /*
  * The size of the PE/COFF section that covers the kernel image, which
  * runs from stext to _edata, must be a round multiple of the PE/COFF
@@ -113,6 +124,7 @@ SECTIONS
 			HYPERVISOR_TEXT
 			IDMAP_TEXT
 			HIBERNATE_TEXT
+			TRAMP_TEXT
 			*(.fixup)
 			*(.gnu.warning)
 		. = ALIGN(16);
@@ -214,6 +226,11 @@ SECTIONS
 	. += RESERVED_TTBR0_SIZE;
 #endif
 
+#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+	tramp_pg_dir = .;
+	. += PAGE_SIZE;
+#endif
+
 	__pecoff_data_size = ABSOLUTE(. - __initdata_begin);
 	_end = .;
 
-- 
2.1.4