[PATCH 2/4] arm64: implement support for static call trampolines

[Frederic Weisbecker <frederic@kernel.org>]

From: Ard Biesheuvel <ardb@kernel.org>

[fweisbec: rebased against 5.15-rc2. There has been quite some changes
 on arm64 since then, especially with insn/patching, so some naming may
 not be relevant anymore]

Implement arm64 support for the 'unoptimized' static call variety, which
routes all calls through a single trampoline that is patched to perform a
tail call to the selected function.

Since static call targets may be located in modules loaded out of direct
branching range, we need to use a ADRP/ADD pair to load the branch target
into R16 and use a branch-to-register (BR) instruction to perform an
indirect call. Unlike on x86, there is no pressing need on arm64 to avoid
indirect calls at all cost, but hiding it from the compiler as is done
here does have some benefits:
- the literal is located in .rodata, which gives us the same robustness
  advantage that code patching does;
- no performance hit on CFI enabled Clang builds that decorate compiler
  emitted indirect calls with branch target validity checks.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Quentin Perret <qperret@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 arch/arm64/Kconfig                   |  1 +
 arch/arm64/include/asm/insn.h        |  2 ++
 arch/arm64/include/asm/static_call.h | 28 ++++++++++++++++++++++++++++
 arch/arm64/kernel/Makefile           |  4 ++--
 arch/arm64/kernel/patching.c         | 14 +++++++++++---
 arch/arm64/kernel/vmlinux.lds.S      |  1 +
 6 files changed, 45 insertions(+), 5 deletions(-)
 create mode 100644 arch/arm64/include/asm/static_call.h

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 5c7ae4c3954b..5b51b359ccda 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -192,6 +192,7 @@ config ARM64
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_REGS_AND_STACK_ACCESS_API
+	select HAVE_STATIC_CALL
 	select HAVE_FUNCTION_ARG_ACCESS_API
 	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select MMU_GATHER_RCU_TABLE_FREE
diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
index 6b776c8667b2..681c08b170df 100644
--- a/arch/arm64/include/asm/insn.h
+++ b/arch/arm64/include/asm/insn.h
@@ -547,6 +547,8 @@ u32 aarch64_set_branch_offset(u32 insn, s32 offset);
 s32 aarch64_insn_adrp_get_offset(u32 insn);
 u32 aarch64_insn_adrp_set_offset(u32 insn, s32 offset);
 
+int aarch64_literal_write(void *addr, u64 literal);
+
 bool aarch32_insn_is_wide(u32 insn);
 
 #define A32_RN_OFFSET	16
diff --git a/arch/arm64/include/asm/static_call.h b/arch/arm64/include/asm/static_call.h
new file mode 100644
index 000000000000..665ec2a7cdb2
--- /dev/null
+++ b/arch/arm64/include/asm/static_call.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_STATIC_CALL_H
+#define _ASM_STATIC_CALL_H
+
+#define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, target)			    \
+	asm("	.pushsection	.static_call.text, \"ax\"		\n" \
+	    "	.align		3					\n" \
+	    "	.globl		" STATIC_CALL_TRAMP_STR(name) "		\n" \
+	    STATIC_CALL_TRAMP_STR(name) ":				\n" \
+	    "	hint 	34	/* BTI C */				\n" \
+	    "	adrp	x16, 1f						\n" \
+	    "	ldr	x16, [x16, :lo12:1f]				\n" \
+	    "	cbz	x16, 0f						\n" \
+	    "	br	x16						\n" \
+	    "0:	ret							\n" \
+	    "	.popsection						\n" \
+	    "	.pushsection	.rodata, \"a\"				\n" \
+	    "	.align		3					\n" \
+	    "1:	.quad		" target "				\n" \
+	    "	.popsection						\n")
+
+#define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func)			\
+	__ARCH_DEFINE_STATIC_CALL_TRAMP(name, #func)
+
+#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)			\
+	__ARCH_DEFINE_STATIC_CALL_TRAMP(name, "0x0")
+
+#endif /* _ASM_STATIC_CALL_H */
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 3f1490bfb938..83f03fc1e402 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -28,8 +28,8 @@ obj-y			:= debug-monitors.o entry.o irq.o fpsimd.o		\
 			   return_address.o cpuinfo.o cpu_errata.o		\
 			   cpufeature.o alternative.o cacheinfo.o		\
 			   smp.o smp_spin_table.o topology.o smccc-call.o	\
-			   syscall.o proton-pack.o idreg-override.o idle.o	\
-			   patching.o
+			   syscall.o proton-pack.o static_call.o		\
+			   idreg-override.o idle.o patching.o
 
 targets			+= efi-entry.o
 
diff --git a/arch/arm64/kernel/patching.c b/arch/arm64/kernel/patching.c
index 771f543464e0..841c0499eca5 100644
--- a/arch/arm64/kernel/patching.c
+++ b/arch/arm64/kernel/patching.c
@@ -66,7 +66,7 @@ int __kprobes aarch64_insn_read(void *addr, u32 *insnp)
 	return ret;
 }
 
-static int __kprobes __aarch64_insn_write(void *addr, __le32 insn)
+static int __kprobes __aarch64_insn_write(void *addr, void *insn, int size)
 {
 	void *waddr = addr;
 	unsigned long flags = 0;
@@ -75,7 +75,7 @@ static int __kprobes __aarch64_insn_write(void *addr, __le32 insn)
 	raw_spin_lock_irqsave(&patch_lock, flags);
 	waddr = patch_map(addr, FIX_TEXT_POKE0);
 
-	ret = copy_to_kernel_nofault(waddr, &insn, AARCH64_INSN_SIZE);
+	ret = copy_to_kernel_nofault(waddr, insn, size);
 
 	patch_unmap(FIX_TEXT_POKE0);
 	raw_spin_unlock_irqrestore(&patch_lock, flags);
@@ -85,7 +85,15 @@ static int __kprobes __aarch64_insn_write(void *addr, __le32 insn)
 
 int __kprobes aarch64_insn_write(void *addr, u32 insn)
 {
-	return __aarch64_insn_write(addr, cpu_to_le32(insn));
+	__le32 i = cpu_to_le32(insn);
+
+	return __aarch64_insn_write(addr, &i, AARCH64_INSN_SIZE);
+}
+
+int aarch64_literal_write(void *addr, u64 literal)
+{
+	BUG_ON(!IS_ALIGNED((u64)addr, sizeof(u64)));
+	return __aarch64_insn_write(addr, &literal, sizeof(u64));
 }
 
 int __kprobes aarch64_insn_patch_text_nosync(void *addr, u32 insn)
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index f6b1a88245db..ceb35c35192c 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -161,6 +161,7 @@ SECTIONS
 			IDMAP_TEXT
 			HIBERNATE_TEXT
 			TRAMP_TEXT
+			STATIC_CALL_TEXT
 			*(.fixup)
 			*(.gnu.warning)
 		. = ALIGN(16);
-- 
2.25.1