arm64: kretprobes: acquire the regs via a BRK exception

On arm64, kprobes always take an exception and so create a struct
pt_regs through the usual exception entry logic. Similarly kretprobes
taskes and exception for function entry, but for function returns it
uses a trampoline which attempts to create a struct pt_regs without
taking an exception.

This is problematic for a few reasons, including:

1) The kretprobes trampoline neither saves nor restores all of the
   portions of PSTATE. Before invoking the handler it saves a number of
   portions of PSTATE, and after returning from the handler it restores
   NZCV before returning to the original return address provided by the
   handler.

2) The kretprobe trampoline constructs the PSTATE value piecemeal from
   special purpose registers as it cannot read all of PSTATE atomically
   without taking an exception. This is somewhat fragile, and it's not
   possible to reliably recover PSTATE information which only exists on
   some physical CPUs (e.g. when SSBS support is mismatched).

   Today the kretprobes trampoline does not record:

   - BTYPE
   - SSBS
   - ALLINT
   - SS
   - PAN
   - UAO
   - DIT
   - TCO

   ... and this will only get worse with future architecture extensions
   which add more PSTATE bits.

3) The kretprobes trampoline doesn't store portions of struct pt_regs
   (e.g. the PMR value when using pseudo-NMIs). Due to this, helpers
   which operate on a struct pt_regs, such as interrupts_enabled(), may
   not work correctly.

4) The function entry and function exit handlers run in different
   contexts. The entry handler will always be run in a debug exception
   context (which is currently treated as an NMI), but the return will
   be treated as whatever context the instrumented function was executed
   in. The differences between these contexts are liable to cause
   problems (e.g. as the two can be differently interruptible or
   preemptible, adversely affecting synchronization between the
   handlers).

5) As the kretprobes trampoline runs in the same context as the code
   being probed, it is subject to the same single-stepping context,
   which may not be desirable if this is being driven by the kprobes
   handlers.

Overall, this is fragile, painful to maintain, and gets in the way of
supporting other things (e.g. RELIABLE_STACKTRACE, FEAT_NMI).

This patch addresses these issues by replacing the kretprobes trampoline
with a `BRK` instruction, and using an exception boundary to acquire and
restore the regs, in the same way as the regular kprobes trampoline.

Ive tested this atop v6.8-rc3:

| KTAP version 1
| 1..1
|     KTAP version 1
|     # Subtest: kprobes_test
|     # module: test_kprobes
|     1..7
|     ok 1 test_kprobe
|     ok 2 test_kprobes
|     ok 3 test_kprobe_missed
|     ok 4 test_kretprobe
|     ok 5 test_kretprobes
|     ok 6 test_stacktrace_on_kretprobe
|     ok 7 test_stacktrace_on_nested_kretprobe
| # kprobes_test: pass:7 fail:0 skip:0 total:7
| # Totals: pass:7 fail:0 skip:0 total:7
| ok 1 kprobes_test

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Florent Revest <revest@chromium.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20240208145916.2004154-1-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: mrutland-arm

arch/arm64/include/asm/brk-imm.h

@@ -11,6 +11,7 @@
  * 0x004: for installing kprobes
  * 0x005: for installing uprobes
  * 0x006: for kprobe software single-step
+ * 0x007: for kretprobe return
  * Allowed values for kgdb are 0x400 - 0x7ff
  * 0x100: for triggering a fault on purpose (reserved)
  * 0x400: for dynamic BRK instruction
@@ -23,6 +24,7 @@
 #define KPROBES_BRK_IMM			0x004
 #define UPROBES_BRK_IMM			0x005
 #define KPROBES_BRK_SS_IMM		0x006
+#define KRETPROBES_BRK_IMM		0x007
 #define FAULT_BRK_IMM			0x100
 #define KGDB_DYN_DBG_BRK_IMM		0x400
 #define KGDB_COMPILED_DBG_BRK_IMM	0x401

arch/arm64/kernel/probes/kprobes.c

@@ -371,6 +371,21 @@ static struct break_hook kprobes_break_ss_hook = {
 	.fn = kprobe_breakpoint_ss_handler,
 };
 
+static int __kprobes
+kretprobe_breakpoint_handler(struct pt_regs *regs, unsigned long esr)
+{
+	if (regs->pc != (unsigned long)__kretprobe_trampoline)
+		return DBG_HOOK_ERROR;
+
+	regs->pc = kretprobe_trampoline_handler(regs, (void *)regs->regs[29]);
+	return DBG_HOOK_HANDLED;
+}
+
+static struct break_hook kretprobes_break_hook = {
+	.imm = KRETPROBES_BRK_IMM,
+	.fn = kretprobe_breakpoint_handler,
+};
+
 /*
  * Provide a blacklist of symbols identifying ranges which cannot be kprobed.
  * This blacklist is exposed to userspace via debugfs (kprobes/blacklist).
@@ -396,11 +411,6 @@ int __init arch_populate_kprobe_blacklist(void)
 	return ret;
 }
 
-void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
-{
-	return (void *)kretprobe_trampoline_handler(regs, (void *)regs->regs[29]);
-}
-
 void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
 				      struct pt_regs *regs)
 {
@@ -420,6 +430,7 @@ int __init arch_init_kprobes(void)
 {
 	register_kernel_break_hook(&kprobes_break_hook);
 	register_kernel_break_hook(&kprobes_break_ss_hook);
+	register_kernel_break_hook(&kretprobes_break_hook);
 
 	return 0;
 }

arch/arm64/kernel/probes/kprobes_trampoline.S

@@ -4,83 +4,17 @@
  */
 
 #include <linux/linkage.h>
-#include <asm/asm-offsets.h>
+#include <asm/asm-bug.h>
 #include <asm/assembler.h>
 
 	.text
 
-	.macro	save_all_base_regs
-	stp x0, x1, [sp, #S_X0]
-	stp x2, x3, [sp, #S_X2]
-	stp x4, x5, [sp, #S_X4]
-	stp x6, x7, [sp, #S_X6]
-	stp x8, x9, [sp, #S_X8]
-	stp x10, x11, [sp, #S_X10]
-	stp x12, x13, [sp, #S_X12]
-	stp x14, x15, [sp, #S_X14]
-	stp x16, x17, [sp, #S_X16]
-	stp x18, x19, [sp, #S_X18]
-	stp x20, x21, [sp, #S_X20]
-	stp x22, x23, [sp, #S_X22]
-	stp x24, x25, [sp, #S_X24]
-	stp x26, x27, [sp, #S_X26]
-	stp x28, x29, [sp, #S_X28]
-	add x0, sp, #PT_REGS_SIZE
-	stp lr, x0, [sp, #S_LR]
-	/*
-	 * Construct a useful saved PSTATE
-	 */
-	mrs x0, nzcv
-	mrs x1, daif
-	orr x0, x0, x1
-	mrs x1, CurrentEL
-	orr x0, x0, x1
-	mrs x1, SPSel
-	orr x0, x0, x1
-	stp xzr, x0, [sp, #S_PC]
-	.endm
-
-	.macro	restore_all_base_regs
-	ldr x0, [sp, #S_PSTATE]
-	and x0, x0, #(PSR_N_BIT | PSR_Z_BIT | PSR_C_BIT | PSR_V_BIT)
-	msr nzcv, x0
-	ldp x0, x1, [sp, #S_X0]
-	ldp x2, x3, [sp, #S_X2]
-	ldp x4, x5, [sp, #S_X4]
-	ldp x6, x7, [sp, #S_X6]
-	ldp x8, x9, [sp, #S_X8]
-	ldp x10, x11, [sp, #S_X10]
-	ldp x12, x13, [sp, #S_X12]
-	ldp x14, x15, [sp, #S_X14]
-	ldp x16, x17, [sp, #S_X16]
-	ldp x18, x19, [sp, #S_X18]
-	ldp x20, x21, [sp, #S_X20]
-	ldp x22, x23, [sp, #S_X22]
-	ldp x24, x25, [sp, #S_X24]
-	ldp x26, x27, [sp, #S_X26]
-	ldp x28, x29, [sp, #S_X28]
-	.endm
-
 SYM_CODE_START(__kretprobe_trampoline)
-	sub sp, sp, #PT_REGS_SIZE
-
-	save_all_base_regs
-
-	/* Setup a frame pointer. */
-	add x29, sp, #S_FP
-
-	mov x0, sp
-	bl trampoline_probe_handler
 	/*
-	 * Replace trampoline address in lr with actual orig_ret_addr return
-	 * address.
+	 * Trigger a breakpoint exception. The PC will be adjusted by
+	 * kretprobe_breakpoint_handler(), and no subsequent instructions will
+	 * be executed from the trampoline.
 	 */
-	mov lr, x0
-
-	/* The frame pointer (x29) is restored with other registers. */
-	restore_all_base_regs
-
-	add sp, sp, #PT_REGS_SIZE
-	ret
-
+	brk #KRETPROBES_BRK_IMM
+	ASM_BUG()
 SYM_CODE_END(__kretprobe_trampoline)