[PATCH 0/2] powerpc/watchpoint: Disable watchpoint hit by larx/stcx instruction

[PATCH 0/2] powerpc/watchpoint: Disable watchpoint hit by larx/stcx instruction

[Ravi Bangoria]

I've prepared my patch on top of Christophe's patch as it's easy
to change stepping_handler() rather than hw_breakpoint_handler().
2nd patch is the actual fix.

Christophe Leroy (1):
  powerpc/hw_breakpoint: move instruction stepping out of
    hw_breakpoint_handler()

Ravi Bangoria (1):
  powerpc/watchpoint: Disable watchpoint hit by larx/stcx instructions

 arch/powerpc/kernel/hw_breakpoint.c | 77 +++++++++++++++++++----------
 1 file changed, 50 insertions(+), 27 deletions(-)

-- 
2.21.0

[PATCH 1/2] powerpc/hw_breakpoint: move instruction stepping out of hw_breakpoint_handler()

[Ravi Bangoria]

From: Christophe Leroy <christophe.leroy@c-s.fr>

On 8xx, breakpoints stop after executing the instruction, so
stepping/emulation is not needed. Move it into a sub-function and
remove the #ifdefs.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
---
 arch/powerpc/kernel/hw_breakpoint.c | 60 ++++++++++++++++-------------
 1 file changed, 33 insertions(+), 27 deletions(-)

diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
index c8d1fa2e9d53..28ad3171bb82 100644
--- a/arch/powerpc/kernel/hw_breakpoint.c
+++ b/arch/powerpc/kernel/hw_breakpoint.c
@@ -198,15 +198,43 @@ void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
 /*
  * Handle debug exception notifications.
  */
+static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,
+			     unsigned long addr)
+{
+	int stepped;
+	unsigned int instr;
+
+	/* Do not emulate user-space instructions, instead single-step them */
+	if (user_mode(regs)) {
+		current->thread.last_hit_ubp = bp;
+		regs->msr |= MSR_SE;
+		return false;
+	}
+
+	stepped = 0;
+	instr = 0;
+	if (!__get_user_inatomic(instr, (unsigned int *)regs->nip))
+		stepped = emulate_step(regs, instr);
+
+	/*
+	 * emulate_step() could not execute it. We've failed in reliably
+	 * handling the hw-breakpoint. Unregister it and throw a warning
+	 * message to let the user know about it.
+	 */
+	if (!stepped) {
+		WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
+			"0x%lx will be disabled.", addr);
+		perf_event_disable_inatomic(bp);
+		return false;
+	}
+	return true;
+}
+
 int hw_breakpoint_handler(struct die_args *args)
 {
 	int rc = NOTIFY_STOP;
 	struct perf_event *bp;
 	struct pt_regs *regs = args->regs;
-#ifndef CONFIG_PPC_8xx
-	int stepped = 1;
-	unsigned int instr;
-#endif
 	struct arch_hw_breakpoint *info;
 	unsigned long dar = regs->dar;
 
@@ -251,31 +279,9 @@ int hw_breakpoint_handler(struct die_args *args)
 	      (dar - bp->attr.bp_addr < bp->attr.bp_len)))
 		info->type |= HW_BRK_TYPE_EXTRANEOUS_IRQ;
 
-#ifndef CONFIG_PPC_8xx
-	/* Do not emulate user-space instructions, instead single-step them */
-	if (user_mode(regs)) {
-		current->thread.last_hit_ubp = bp;
-		regs->msr |= MSR_SE;
+	if (!IS_ENABLED(CONFIG_PPC_8xx) && !stepping_handler(regs, bp, info->address))
 		goto out;
-	}
-
-	stepped = 0;
-	instr = 0;
-	if (!__get_user_inatomic(instr, (unsigned int *) regs->nip))
-		stepped = emulate_step(regs, instr);
 
-	/*
-	 * emulate_step() could not execute it. We've failed in reliably
-	 * handling the hw-breakpoint. Unregister it and throw a warning
-	 * message to let the user know about it.
-	 */
-	if (!stepped) {
-		WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
-			"0x%lx will be disabled.", info->address);
-		perf_event_disable_inatomic(bp);
-		goto out;
-	}
-#endif
 	/*
 	 * As a policy, the callback is invoked in a 'trigger-after-execute'
 	 * fashion
-- 
2.21.0

[PATCH 2/2] powerpc/watchpoint: Disable watchpoint hit by larx/stcx instructions

[Ravi Bangoria]

If watchpoint exception is generated by larx/stcx instructions, the
reservation created by larx gets lost while handling exception, and
thus stcx instruction always fails. Generally these instructions are
used in a while(1) loop, for example spinlocks. And because stcx
never succeeds, it loops forever and ultimately hangs the system.

Note that ptrace anyway works in one-shot mode and thus for ptrace
we don't change the behaviour. It's up to ptrace user to take care
of this.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
---
 arch/powerpc/kernel/hw_breakpoint.c | 49 +++++++++++++++++++----------
 1 file changed, 33 insertions(+), 16 deletions(-)

diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
index 28ad3171bb82..9fa496a598ce 100644
--- a/arch/powerpc/kernel/hw_breakpoint.c
+++ b/arch/powerpc/kernel/hw_breakpoint.c
@@ -195,14 +195,32 @@ void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
 	tsk->thread.last_hit_ubp = NULL;
 }
 
+static bool is_larx_stcx_instr(struct pt_regs *regs, unsigned int instr)
+{
+	int ret, type;
+	struct instruction_op op;
+
+	ret = analyse_instr(&op, regs, instr);
+	type = GETTYPE(op.type);
+	return (!ret && (type == LARX || type == STCX));
+}
+
 /*
  * Handle debug exception notifications.
  */
 static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,
 			     unsigned long addr)
 {
-	int stepped;
-	unsigned int instr;
+	unsigned int instr = 0;
+
+	if (__get_user_inatomic(instr, (unsigned int *)regs->nip))
+		goto fail;
+
+	if (is_larx_stcx_instr(regs, instr)) {
+		printk_ratelimited("Watchpoint: Can't emulate/single-step larx/"
+				   "stcx instructions. Disabling watchpoint.\n");
+		goto disable;
+	}
 
 	/* Do not emulate user-space instructions, instead single-step them */
 	if (user_mode(regs)) {
@@ -211,23 +229,22 @@ static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,
 		return false;
 	}
 
-	stepped = 0;
-	instr = 0;
-	if (!__get_user_inatomic(instr, (unsigned int *)regs->nip))
-		stepped = emulate_step(regs, instr);
+	if (!emulate_step(regs, instr))
+		goto fail;
 
+	return true;
+
+fail:
 	/*
-	 * emulate_step() could not execute it. We've failed in reliably
-	 * handling the hw-breakpoint. Unregister it and throw a warning
-	 * message to let the user know about it.
+	 * We've failed in reliably handling the hw-breakpoint. Unregister
+	 * it and throw a warning message to let the user know about it.
 	 */
-	if (!stepped) {
-		WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
-			"0x%lx will be disabled.", addr);
-		perf_event_disable_inatomic(bp);
-		return false;
-	}
-	return true;
+	WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
+		"0x%lx will be disabled.", addr);
+
+disable:
+	perf_event_disable_inatomic(bp);
+	return false;
 }
 
 int hw_breakpoint_handler(struct die_args *args)
-- 
2.21.0

Re: [PATCH 0/2] powerpc/watchpoint: Disable watchpoint hit by larx/stcx instruction

[Christophe Leroy]

Le 10/09/2019 à 12:24, Ravi Bangoria a écrit :
> I've prepared my patch on top of Christophe's patch as it's easy
> to change stepping_handler() rather than hw_breakpoint_handler().
> 2nd patch is the actual fix.

Anyway, my patch is already commited on powerpc/next

https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20190904&id=658d029df0bce6472c94b724ca54d74bc6659c2e

Christophe

> 
> Christophe Leroy (1):
>    powerpc/hw_breakpoint: move instruction stepping out of
>      hw_breakpoint_handler()
> 
> Ravi Bangoria (1):
>    powerpc/watchpoint: Disable watchpoint hit by larx/stcx instructions
> 
>   arch/powerpc/kernel/hw_breakpoint.c | 77 +++++++++++++++++++----------
>   1 file changed, 50 insertions(+), 27 deletions(-)
> 

Re: [PATCH 2/2] powerpc/watchpoint: Disable watchpoint hit by larx/stcx instructions

[Naveen N. Rao]

Ravi Bangoria wrote:
> If watchpoint exception is generated by larx/stcx instructions, the
> reservation created by larx gets lost while handling exception, and
> thus stcx instruction always fails. Generally these instructions are
> used in a while(1) loop, for example spinlocks. And because stcx
> never succeeds, it loops forever and ultimately hangs the system.
> 
> Note that ptrace anyway works in one-shot mode and thus for ptrace
> we don't change the behaviour. It's up to ptrace user to take care
> of this.
> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
> ---
>  arch/powerpc/kernel/hw_breakpoint.c | 49 +++++++++++++++++++----------
>  1 file changed, 33 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
> index 28ad3171bb82..9fa496a598ce 100644
> --- a/arch/powerpc/kernel/hw_breakpoint.c
> +++ b/arch/powerpc/kernel/hw_breakpoint.c
> @@ -195,14 +195,32 @@ void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
>  	tsk->thread.last_hit_ubp = NULL;
>  }
>  
> +static bool is_larx_stcx_instr(struct pt_regs *regs, unsigned int instr)
> +{
> +	int ret, type;
> +	struct instruction_op op;
> +
> +	ret = analyse_instr(&op, regs, instr);
> +	type = GETTYPE(op.type);
> +	return (!ret && (type == LARX || type == STCX));
> +}
> +
>  /*
>   * Handle debug exception notifications.
>   */
>  static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,
>  			     unsigned long addr)
>  {
> -	int stepped;
> -	unsigned int instr;
> +	unsigned int instr = 0;
> +
> +	if (__get_user_inatomic(instr, (unsigned int *)regs->nip))
> +		goto fail;
> +
> +	if (is_larx_stcx_instr(regs, instr)) {
> +		printk_ratelimited("Watchpoint: Can't emulate/single-step larx/"
> +				   "stcx instructions. Disabling watchpoint.\n");

The below WARN() uses the term 'breakpoint'. Better to use consistent 
terminology. I would rewrite the above as:
	printk_ratelimited("Breakpoint hit on instruction that can't be emulated. "
				"Breakpoint at 0x%lx will be disabled.\n", addr);

Otherwise:
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>

- Naveen

> +		goto disable;
> +	}
>  
>  	/* Do not emulate user-space instructions, instead single-step them */
>  	if (user_mode(regs)) {
> @@ -211,23 +229,22 @@ static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,
>  		return false;
>  	}
>  
> -	stepped = 0;
> -	instr = 0;
> -	if (!__get_user_inatomic(instr, (unsigned int *)regs->nip))
> -		stepped = emulate_step(regs, instr);
> +	if (!emulate_step(regs, instr))
> +		goto fail;
>  
> +	return true;
> +
> +fail:
>  	/*
> -	 * emulate_step() could not execute it. We've failed in reliably
> -	 * handling the hw-breakpoint. Unregister it and throw a warning
> -	 * message to let the user know about it.
> +	 * We've failed in reliably handling the hw-breakpoint. Unregister
> +	 * it and throw a warning message to let the user know about it.
>  	 */
> -	if (!stepped) {
> -		WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
> -			"0x%lx will be disabled.", addr);
> -		perf_event_disable_inatomic(bp);
> -		return false;
> -	}
> -	return true;
> +	WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
> +		"0x%lx will be disabled.", addr);
> +
> +disable:
> +	perf_event_disable_inatomic(bp);
> +	return false;
>  }
>  
>  int hw_breakpoint_handler(struct die_args *args)
> -- 
> 2.21.0
> 
>