[PATCH v6 2/2] arm64: implement support for static call trampolines

From: Ard Biesheuvel
Date: Fri Nov 05 2021 - 10:59:33 EST

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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.

It is expected that the direct branch instruction will be able to cover
the common case. However, given that static call targets may be located
in modules loaded out of direct branching range, we need a fallback path
that loads the address into R16 and uses 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 .text, 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.

Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
arch/arm64/Kconfig | 2 +
arch/arm64/include/asm/static_call.h | 40 ++++++++++
arch/arm64/kernel/patching.c | 77 +++++++++++++++++++-
arch/arm64/kernel/vmlinux.lds.S | 1 +
4 files changed, 117 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 176d6fddc4f2..ccc33b85769c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -193,6 +193,8 @@ config ARM64
select HAVE_PERF_USER_STACK_DUMP
select HAVE_REGS_AND_STACK_ACCESS_API
select HAVE_POSIX_CPU_TIMERS_TASK_WORK
+ # https://github.com/ClangBuiltLinux/linux/issues/1354
+ select HAVE_STATIC_CALL if !LTO_CLANG_THIN || CLANG_VERSION >= 130000
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/static_call.h b/arch/arm64/include/asm/static_call.h
new file mode 100644
index 000000000000..6ee918991510
--- /dev/null
+++ b/arch/arm64/include/asm/static_call.h
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_STATIC_CALL_H
+#define _ASM_STATIC_CALL_H
+
+/*
+ * The sequence below is laid out in a way that guarantees that the literal and
+ * the instruction are always covered by the same cacheline, and can be updated
+ * using a single store-pair instruction (provided that we rewrite the BTI C
+ * instruction as well). This means the literal and the instruction are always
+ * in sync when observed via the D-side.
+ *
+ * However, this does not guarantee that the I-side will catch up immediately
+ * as well: until the I-cache maintenance completes, CPUs may branch to the old
+ * target, or execute a stale NOP or RET. We deal with this by writing the
+ * literal unconditionally, even if it is 0x0 or the branch is in range. That
+ * way, a stale NOP will fall through and call the new target via an indirect
+ * call. Stale RETs or Bs will be taken as before, and branch to the old
+ * target.
+ */
+#define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, insn) \
+ asm(" .pushsection .static_call.text, \"ax\" \n" \
+ " .align 4 \n" \
+ " .globl " STATIC_CALL_TRAMP_STR(name) " \n" \
+ "0: .quad 0x0 \n" \
+ STATIC_CALL_TRAMP_STR(name) ": \n" \
+ " hint 34 /* BTI C */ \n" \
+ insn " \n" \
+ " ldr x16, 0b \n" \
+ " cbz x16, 1f \n" \
+ " br x16 \n" \
+ "1: ret \n" \
+ " .popsection \n")
+
+#define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func) \
+ __ARCH_DEFINE_STATIC_CALL_TRAMP(name, "b " #func)
+
+#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \
+ __ARCH_DEFINE_STATIC_CALL_TRAMP(name, "ret")
+
+#endif /* _ASM_STATIC_CALL_H */
diff --git a/arch/arm64/kernel/patching.c b/arch/arm64/kernel/patching.c
index 771f543464e0..a265a87d4d9e 100644
--- a/arch/arm64/kernel/patching.c
+++ b/arch/arm64/kernel/patching.c
@@ -3,6 +3,7 @@
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/spinlock.h>
+#include <linux/static_call.h>
#include <linux/stop_machine.h>
#include <linux/uaccess.h>

@@ -66,7 +67,7 @@ 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 +76,7 @@ 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 +86,77 @@ 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);
+}
+
+static void *strip_cfi_jt(void *addr)
+{
+ if (IS_ENABLED(CONFIG_CFI_CLANG)) {
+ void *p = addr;
+ u32 insn;
+
+ /*
+ * Taking the address of a function produces the address of the
+ * jump table entry when Clang CFI is enabled. Such entries are
+ * ordinary jump instructions, preceded by a BTI C instruction
+ * if BTI is enabled for the kernel.
+ */
+ if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
+ p += 4;
+
+ insn = le32_to_cpup(p);
+ if (aarch64_insn_is_b(insn))
+ return p + aarch64_get_branch_offset(insn);
+
+ WARN_ON(1);
+ }
+ return addr;
+}
+
+void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
+{
+ /*
+ * -0x8 <literal>
+ * 0x0 bti c <--- trampoline entry point
+ * 0x4 <branch or nop>
+ * 0x8 ldr x16, <literal>
+ * 0xc cbz x16, 20
+ * 0x10 br x16
+ * 0x14 ret
+ */
+ struct {
+ u64 literal;
+ __le32 insn[2];
+ } insns;
+ u32 insn;
+ int ret;
+
+ insn = aarch64_insn_gen_hint(AARCH64_INSN_HINT_BTIC);
+ insns.literal = (u64)func;
+ insns.insn[0] = cpu_to_le32(insn);
+
+ if (!func) {
+ insn = aarch64_insn_gen_branch_reg(AARCH64_INSN_REG_LR,
+ AARCH64_INSN_BRANCH_RETURN);
+ } else {
+ insn = aarch64_insn_gen_branch_imm((u64)tramp + 4,
+ (u64)strip_cfi_jt(func),
+ AARCH64_INSN_BRANCH_NOLINK);
+
+ /*
+ * Use a NOP if the branch target is out of range, and rely on
+ * the indirect call instead.
+ */
+ if (insn == AARCH64_BREAK_FAULT)
+ insn = aarch64_insn_gen_hint(AARCH64_INSN_HINT_NOP);
+ }
+ insns.insn[1] = cpu_to_le32(insn);
+
+ ret = __aarch64_insn_write(tramp - 8, &insns, sizeof(insns));
+ if (!WARN_ON(ret))
+ caches_clean_inval_pou((u64)tramp - 8, sizeof(insns));
}

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 50bab186c49b..e16860a14eaf 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -173,6 +173,7 @@ SECTIONS
HIBERNATE_TEXT
KEXEC_TEXT
TRAMP_TEXT
+ STATIC_CALL_TEXT
*(.gnu.warning)
. = ALIGN(16);
*(.got) /* Global offset table */
--
2.30.2

Next message: Artem S. Tashkinov: "Re: [PATCH v1 (RFC)] docs: discourage users from using bugzilla.kernel.org"
Previous message: Ard Biesheuvel: "[PATCH v6 1/2] static_call: use non-function types to refer to the trampolines"
In reply to: Ard Biesheuvel: "[PATCH v6 1/2] static_call: use non-function types to refer to the trampolines"
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