Subject: [PATCH v15 04/10] arm64: Kprobes with single stepping support

From: [email-masked] (Marc Zyngier)
Date: Wed, 20 Jul 2016 10:36:08 +0100

On 08/07/16 17:35, David Long wrote:
> From: Sandeepa Prabhu <[email-masked]>
> 
> 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 <[email-masked]>
> Signed-off-by: David A. Long <[email-masked]>
> Signed-off-by: Pratyush Anand <[email-masked]>
> Acked-by: Masami Hiramatsu <[email-masked]>
> ---

[...]

> +void __kprobes jprobe_return(void)
> +{
> +	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> +
> +	/*
> +	 * Jprobe handler return by entering break exception,
> +	 * encoded same as kprobe, but with following conditions
> +	 * -a magic number in x0 to identify from rest of other kprobes.
> +	 * -restore stack addr to original saved pt_regs
> +	 */
> +	asm volatile ("ldr x0, [%0]\n\t"
> +		      "mov sp, x0\n\t"
> +		      ".globl jprobe_return_break\n\t"
> +		      "jprobe_return_break:\n\t"
> +		      "brk %1\n\t"
> +		      :
> +		      : "r"(&kcb->jprobe_saved_regs.sp),
> +		      "I"(BRK64_ESR_KPROBES)
> +		      : "memory");

A couple of remarks here:
- the comment seems wrong, as you load the stack pointer in X0, nothing
else, and seem to identify the jprobe by looking at the PC, not X0.
- using explicit registers is really ugly. How about something like this
instead:

diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index c89811d..823cf92 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -513,13 +513,12 @@ void __kprobes jprobe_return(void)
 	 * -a magic number in x0 to identify from rest of other kprobes.
 	 * -restore stack addr to original saved pt_regs
 	 */
-	asm volatile ("ldr x0, [%0]\n\t"
-		      "mov sp, x0\n\t"
+	asm volatile ("mov sp, %0\n\t"
 		      ".globl jprobe_return_break\n\t"
 		      "jprobe_return_break:\n\t"
 		      "brk %1\n\t"
 		      :
-		      : "r"(&kcb->jprobe_saved_regs.sp),
+		      : "r" (kcb->jprobe_saved_regs.sp),
 		      "I"(BRK64_ESR_KPROBES)
 		      : "memory");
 }

though hijacking SP in the middle of a C function still feels pretty fragile.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...