[PATCH 01/10] arm64: debug: Don't propagate UNKNOWN FAR into si_code for debug signals

[PATCH 01/10] arm64: debug: Don't propagate UNKNOWN FAR into si_code for debug signals

[Will Deacon]

FAR_EL1 is UNKNOWN for all debug exceptions other than those caused by
taking a hardware watchpoint. Unfortunately, if a debug handler returns
a non-zero value, then we will propagate the UNKNOWN FAR value to
userspace via the si_addr field of the SIGTRAP siginfo_t.

Instead, let's set si_addr to take on the PC of the faulting instruction,
which we have available in the current pt_regs.

Cc: <stable@vger.kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 arch/arm64/mm/fault.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index efb7b2cbead5..ef46925096f0 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -824,11 +824,12 @@ void __init hook_debug_fault_code(int nr,
 	debug_fault_info[nr].name	= name;
 }
 
-asmlinkage int __exception do_debug_exception(unsigned long addr,
+asmlinkage int __exception do_debug_exception(unsigned long addr_if_watchpoint,
 					      unsigned int esr,
 					      struct pt_regs *regs)
 {
 	const struct fault_info *inf = esr_to_debug_fault_info(esr);
+	unsigned long pc = instruction_pointer(regs);
 	int rv;
 
 	/*
@@ -838,14 +839,14 @@ asmlinkage int __exception do_debug_exception(unsigned long addr,
 	if (interrupts_enabled(regs))
 		trace_hardirqs_off();
 
-	if (user_mode(regs) && !is_ttbr0_addr(instruction_pointer(regs)))
+	if (user_mode(regs) && !is_ttbr0_addr(pc))
 		arm64_apply_bp_hardening();
 
-	if (!inf->fn(addr, esr, regs)) {
+	if (!inf->fn(addr_if_watchpoint, esr, regs)) {
 		rv = 1;
 	} else {
 		arm64_notify_die(inf->name, regs,
-				 inf->sig, inf->code, (void __user *)addr, esr);
+				 inf->sig, inf->code, (void __user *)pc, esr);
 		rv = 0;
 	}
 
-- 
2.11.0

[PATCH 02/10] arm64: debug: Ensure debug handlers check triggering exception level

[Will Deacon]

Debug exception handlers may be called for exceptions generated both by
user and kernel code. In many cases, this is checked explicitly, but
in other cases things either happen to work by happy accident or they
go slightly wrong. For example, executing 'brk #4' from userspace will
enter the kprobes code and be ignored, but the instruction will be
retried forever in userspace instead of delivering a SIGTRAP.

Fix this issue in the most stable-friendly fashion by simply adding
explicit checks of the triggering exception level to all of our debug
exception handlers.

Cc: <stable@vger.kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 arch/arm64/kernel/kgdb.c           | 14 ++++++++++----
 arch/arm64/kernel/probes/kprobes.c |  6 ++++++
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
index ce46c4cdf368..691854b77c7f 100644
--- a/arch/arm64/kernel/kgdb.c
+++ b/arch/arm64/kernel/kgdb.c
@@ -244,27 +244,33 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
 
 static int kgdb_brk_fn(struct pt_regs *regs, unsigned int esr)
 {
+	if (user_mode(regs))
+		return DBG_HOOK_ERROR;
+
 	kgdb_handle_exception(1, SIGTRAP, 0, regs);
-	return 0;
+	return DBG_HOOK_HANDLED;
 }
 NOKPROBE_SYMBOL(kgdb_brk_fn)
 
 static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned int esr)
 {
+	if (user_mode(regs))
+		return DBG_HOOK_ERROR;
+
 	compiled_break = 1;
 	kgdb_handle_exception(1, SIGTRAP, 0, regs);
 
-	return 0;
+	return DBG_HOOK_HANDLED;
 }
 NOKPROBE_SYMBOL(kgdb_compiled_brk_fn);
 
 static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr)
 {
-	if (!kgdb_single_step)
+	if (user_mode(regs) || !kgdb_single_step)
 		return DBG_HOOK_ERROR;
 
 	kgdb_handle_exception(1, SIGTRAP, 0, regs);
-	return 0;
+	return DBG_HOOK_HANDLED;
 }
 NOKPROBE_SYMBOL(kgdb_step_brk_fn);
 
diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index f17afb99890c..7fb6f3aa5ceb 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -450,6 +450,9 @@ kprobe_single_step_handler(struct pt_regs *regs, unsigned int esr)
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 	int retval;
 
+	if (user_mode(regs))
+		return DBG_HOOK_ERROR;
+
 	/* return error if this is not our step */
 	retval = kprobe_ss_hit(kcb, instruction_pointer(regs));
 
@@ -466,6 +469,9 @@ kprobe_single_step_handler(struct pt_regs *regs, unsigned int esr)
 int __kprobes
 kprobe_breakpoint_handler(struct pt_regs *regs, unsigned int esr)
 {
+	if (user_mode(regs))
+		return DBG_HOOK_ERROR;
+
 	kprobe_handler(regs);
 	return DBG_HOOK_HANDLED;
 }
-- 
2.11.0

Re: [PATCH 01/10] arm64: debug: Don't propagate UNKNOWN FAR into si_code for debug signals

[Mark Rutland]

On Fri, Mar 01, 2019 at 01:28:00PM +0000, Will Deacon wrote:
> FAR_EL1 is UNKNOWN for all debug exceptions other than those caused by
> taking a hardware watchpoint. Unfortunately, if a debug handler returns
> a non-zero value, then we will propagate the UNKNOWN FAR value to
> userspace via the si_addr field of the SIGTRAP siginfo_t.
> 
> Instead, let's set si_addr to take on the PC of the faulting instruction,
> which we have available in the current pt_regs.
> 
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Will Deacon <will.deacon@arm.com>

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  arch/arm64/mm/fault.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index efb7b2cbead5..ef46925096f0 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -824,11 +824,12 @@ void __init hook_debug_fault_code(int nr,
>  	debug_fault_info[nr].name	= name;
>  }
>  
> -asmlinkage int __exception do_debug_exception(unsigned long addr,
> +asmlinkage int __exception do_debug_exception(unsigned long addr_if_watchpoint,
>  					      unsigned int esr,
>  					      struct pt_regs *regs)
>  {
>  	const struct fault_info *inf = esr_to_debug_fault_info(esr);
> +	unsigned long pc = instruction_pointer(regs);
>  	int rv;
>  
>  	/*
> @@ -838,14 +839,14 @@ asmlinkage int __exception do_debug_exception(unsigned long addr,
>  	if (interrupts_enabled(regs))
>  		trace_hardirqs_off();
>  
> -	if (user_mode(regs) && !is_ttbr0_addr(instruction_pointer(regs)))
> +	if (user_mode(regs) && !is_ttbr0_addr(pc))
>  		arm64_apply_bp_hardening();
>  
> -	if (!inf->fn(addr, esr, regs)) {
> +	if (!inf->fn(addr_if_watchpoint, esr, regs)) {
>  		rv = 1;
>  	} else {
>  		arm64_notify_die(inf->name, regs,
> -				 inf->sig, inf->code, (void __user *)addr, esr);
> +				 inf->sig, inf->code, (void __user *)pc, esr);
>  		rv = 0;
>  	}
>  
> -- 
> 2.11.0
> 

Re: [PATCH 02/10] arm64: debug: Ensure debug handlers check triggering exception level

[Mark Rutland]

On Fri, Mar 01, 2019 at 01:28:01PM +0000, Will Deacon wrote:
> Debug exception handlers may be called for exceptions generated both by
> user and kernel code. In many cases, this is checked explicitly, but
> in other cases things either happen to work by happy accident or they
> go slightly wrong. For example, executing 'brk #4' from userspace will
> enter the kprobes code and be ignored, but the instruction will be
> retried forever in userspace instead of delivering a SIGTRAP.
> 
> Fix this issue in the most stable-friendly fashion by simply adding
> explicit checks of the triggering exception level to all of our debug
> exception handlers.
> 
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Will Deacon <will.deacon@arm.com>

It might be worth noting in the commit message that this also makes the
functions consistentluy use the DBG_HOOK_* mnemonics, but either way:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  arch/arm64/kernel/kgdb.c           | 14 ++++++++++----
>  arch/arm64/kernel/probes/kprobes.c |  6 ++++++
>  2 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> index ce46c4cdf368..691854b77c7f 100644
> --- a/arch/arm64/kernel/kgdb.c
> +++ b/arch/arm64/kernel/kgdb.c
> @@ -244,27 +244,33 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
>  
>  static int kgdb_brk_fn(struct pt_regs *regs, unsigned int esr)
>  {
> +	if (user_mode(regs))
> +		return DBG_HOOK_ERROR;
> +
>  	kgdb_handle_exception(1, SIGTRAP, 0, regs);
> -	return 0;
> +	return DBG_HOOK_HANDLED;
>  }
>  NOKPROBE_SYMBOL(kgdb_brk_fn)
>  
>  static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned int esr)
>  {
> +	if (user_mode(regs))
> +		return DBG_HOOK_ERROR;
> +
>  	compiled_break = 1;
>  	kgdb_handle_exception(1, SIGTRAP, 0, regs);
>  
> -	return 0;
> +	return DBG_HOOK_HANDLED;
>  }
>  NOKPROBE_SYMBOL(kgdb_compiled_brk_fn);
>  
>  static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr)
>  {
> -	if (!kgdb_single_step)
> +	if (user_mode(regs) || !kgdb_single_step)
>  		return DBG_HOOK_ERROR;
>  
>  	kgdb_handle_exception(1, SIGTRAP, 0, regs);
> -	return 0;
> +	return DBG_HOOK_HANDLED;
>  }
>  NOKPROBE_SYMBOL(kgdb_step_brk_fn);
>  
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index f17afb99890c..7fb6f3aa5ceb 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -450,6 +450,9 @@ kprobe_single_step_handler(struct pt_regs *regs, unsigned int esr)
>  	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
>  	int retval;
>  
> +	if (user_mode(regs))
> +		return DBG_HOOK_ERROR;
> +
>  	/* return error if this is not our step */
>  	retval = kprobe_ss_hit(kcb, instruction_pointer(regs));
>  
> @@ -466,6 +469,9 @@ kprobe_single_step_handler(struct pt_regs *regs, unsigned int esr)
>  int __kprobes
>  kprobe_breakpoint_handler(struct pt_regs *regs, unsigned int esr)
>  {
> +	if (user_mode(regs))
> +		return DBG_HOOK_ERROR;
> +
>  	kprobe_handler(regs);
>  	return DBG_HOOK_HANDLED;
>  }
> -- 
> 2.11.0
>