arm64: Kprobes with single stepping support

Add support for basic kernel probes(kprobes) and jump probes
(jprobes) for ARM64.

Kprobes utilizes software breakpoint and single step debug
exceptions supported on ARM v8.

A software breakpoint is placed at the probe address to trap the
kernel execution into the kprobe handler.

ARM v8 supports enabling single stepping before the break exception
return (ERET), with next PC in exception return address (ELR_EL1). The
kprobe handler prepares an executable memory slot for out-of-line
execution with a copy of the original instruction being probed, and
enables single stepping. The PC is set to the out-of-line slot address
before the ERET. With this scheme, the instruction is executed with the
exact same register context except for the PC (and DAIF) registers.

Debug mask (PSTATE.D) is enabled only when single stepping a recursive
kprobe, e.g.: during kprobes reenter so that probed instruction can be
single stepped within the kprobe handler -exception- context.
The recursion depth of kprobe is always 2, i.e. upon probe re-entry,
any further re-entry is prevented by not calling handlers and the case
counted as a missed kprobe).

Single stepping from the x-o-l slot has a drawback for PC-relative accesses
like branching and symbolic literals access as the offset from the new PC
(slot address) may not be ensured to fit in the immediate value of
the opcode. Such instructions need simulation, so reject
probing them.

Instructions generating exceptions or cpu mode change are rejected
for probing.

Exclusive load/store instructions are rejected too.  Additionally, the
code is checked to see if it is inside an exclusive load/store sequence
(code from Pratyush).

System instructions are mostly enabled for stepping, except MSR/MRS
accesses to "DAIF" flags in PSTATE, which are not safe for
probing.

This also changes arch/arm64/include/asm/ptrace.h to use
include/asm-generic/ptrace.h.

Thanks to Steve Capper and Pratyush Anand for several suggested
Changes.

Signed-off-by: Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com>
Signed-off-by: David A. Long <dave.long@linaro.org>
Signed-off-by: Pratyush Anand <panand@redhat.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 14 insertions, 2 deletions

diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 4fbf3c54275c..395de61108d8 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -23,6 +23,7 @@
 #include <linux/hardirq.h>
 #include <linux/init.h>
 #include <linux/ptrace.h>
+#include <linux/kprobes.h>
 #include <linux/stat.h>
 #include <linux/uaccess.h>
 
@@ -266,6 +267,10 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
 		 */
 		user_rewind_single_step(current);
 	} else {
+#ifdef	CONFIG_KPROBES
+		if (kprobe_single_step_handler(regs, esr) == DBG_HOOK_HANDLED)
+			return 0;
+#endif
 		if (call_step_hook(regs, esr) == DBG_HOOK_HANDLED)
 			return 0;
 
@@ -322,8 +327,15 @@ static int brk_handler(unsigned long addr, unsigned int esr,
 {
 	if (user_mode(regs)) {
 		send_user_sigtrap(TRAP_BRKPT);
-	} else if (call_break_hook(regs, esr) != DBG_HOOK_HANDLED) {
-		pr_warning("Unexpected kernel BRK exception at EL1\n");
+	}
+#ifdef	CONFIG_KPROBES
+	else if ((esr & BRK64_ESR_MASK) == BRK64_ESR_KPROBES) {
+		if (kprobe_breakpoint_handler(regs, esr) != DBG_HOOK_HANDLED)
+			return -EFAULT;
+	}
+#endif
+	else if (call_break_hook(regs, esr) != DBG_HOOK_HANDLED) {
+		pr_warn("Unexpected kernel BRK exception at EL1\n");
 		return -EFAULT;
 	}