Subject: [PATCH 2/2] arm64: Fix watchpoint recursion when single-step is wrongly triggered in irq

From: [email-masked] (Pratyush Anand)
Date: Fri, 8 Apr 2016 10:44:08 +0530

Hi Li,

On 07/04/2016:07:34:37 PM, Li Bin wrote:
> Hi Pratyush,
> 
> on 2016/4/4 13:17, Pratyush Anand wrote:
> > Hi Li,
> > 
> > On 31/03/2016:08:45:05 PM, Li Bin wrote:
> >> Hi Pratyush,
> >>
> >> on 2016/3/21 18:24, Pratyush Anand wrote:
> >>> On 21/03/2016:08:37:50 AM, He Kuang wrote:
> >>>> On arm64, watchpoint handler enables single-step to bypass the next
> >>>> instruction for not recursive enter. If an irq is triggered right
> >>>> after the watchpoint, a single-step will be wrongly triggered in irq
> >>>> handler, which causes the watchpoint address not stepped over and
> >>>> system hang.
> >>>
> >>> Does patch [1] resolves this issue as well? I hope it should. Patch[1] has still
> >>> not been sent for review. Your test result will be helpful.
> >>>
> >>> ~Pratyush
> >>>
> >>> [1] https://github.com/pratyushanand/linux/commit/7623c8099ac22eaa00e7e0f52430f7a4bd154652
> >>
> >> This patch did not consider that, when excetpion return, the singlestep flag
> >> should be restored, otherwise the right singlestep will not triggered.
> >> Right?
> > 
> > Yes, you are right, and there are other problems as well. Will Deacon pointed
> > out [1] that kernel debugging is per-cpu rather than per-task. So, I did thought
> > of a per-cpu implementation by introducing a new element "flags" in struct
> > pt_regs. But even with that I see issues. For example:
> > - While executing single step instruction, we get a data abort
> > - In the kernel_entry of data abort we disable single stepping based on "flags"
> >   bit field
> > - While handling data abort we receive anther interrupt, so we are again in
> >   kernel_entry (for el1_irq). Single stepping will be disabled again (although
> >   it does not matter).
> > 
> > Now the issue is that, what condition should be verified in kernel_exit for
> > enabling single step again? In the above scenario, kernel_exit for el1_irq
> > should not enable single stepping, but how to prevent that elegantly?
> 
> The condition for kernel_entry to disable the single step is that MDSCR_EL1.SS
> has been set. And only when the corresponding kernel_entry has disabled the single
> step, the kernel_exit should enable it, but the kernel_exit of single-step exception
> should be handled specially, that when disable single step in single-step exception
> handler, flag of pt_regs stored in stack should be cleard to prevent to be re-enabled
> by kernel_exit.

Nice, :-)
I had latter on almost similar patch [1], but it did fail when I merged two of
the tests.
-- I inserted kprobe to an instruction in function __copy_to_user() which could
generate data abort.
-- In parallel I also run test case which is defined here [2]
-- As soon as I did `cat /proc/version`, kernel crashed.

Although, just by comparing visually I do not see any functional difference with
the patch you inlined below, still can you please try both of the above test
case in parallel and see how does it behave? To insert kprobe within
__copy_to_user() you need to revert "arm64: add copy_to/from_user to kprobes
blacklist".

[1] https://github.com/pratyushanand/linux/commit/9182afbe640ea7caf52e209eb8e5f00bdf91b0c0
[2] http://thread.gmane.org/gmane.linux.kernel/2167918
> 
> Thanks,
> Li Bin
> 
> ---
>  arch/arm64/include/asm/assembler.h      |   11 +++++++++++
>  arch/arm64/include/asm/debug-monitors.h |    2 +-
>  arch/arm64/include/asm/ptrace.h         |    2 ++
>  arch/arm64/kernel/asm-offsets.c         |    1 +
>  arch/arm64/kernel/debug-monitors.c      |    3 ++-
>  arch/arm64/kernel/entry.S               |   18 ++++++++++++++++++
>  arch/arm64/kernel/hw_breakpoint.c       |    2 +-
>  arch/arm64/kernel/kgdb.c                |    2 +-
>  8 files changed, 37 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> index 70f7b9e..56a4335 100644
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -60,6 +60,17 @@
>  	msr	daifclr, #8
>  	.endm
> 
> +	.macro	disable_step, orig
> +	bic \orig, \orig, #1
> +	msr	mdscr_el1, \orig
> +	.endm
> +
> +	.macro	enable_step, orig
> +	disable_dbg
> +	orr \orig, \orig, #1
> +	msr	mdscr_el1, \orig
> +	.endm
> +
>  	.macro	disable_step_tsk, flgs, tmp
>  	tbz	\flgs, #TIF_SINGLESTEP, 9990f
>  	mrs	\tmp, mdscr_el1
> diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
> index 2fcb9b7..50f114c 100644
> --- a/arch/arm64/include/asm/debug-monitors.h
> +++ b/arch/arm64/include/asm/debug-monitors.h
> @@ -115,7 +115,7 @@ void user_rewind_single_step(struct task_struct *task);
>  void user_fastforward_single_step(struct task_struct *task);
> 
>  void kernel_enable_single_step(struct pt_regs *regs);
> -void kernel_disable_single_step(void);
> +void kernel_disable_single_step(struct pt_regs *regs);
>  int kernel_active_single_step(void);
> 
>  #ifdef CONFIG_HAVE_HW_BREAKPOINT
> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
> index a307eb6..da00cd8 100644
> --- a/arch/arm64/include/asm/ptrace.h
> +++ b/arch/arm64/include/asm/ptrace.h
> @@ -113,6 +113,8 @@ struct pt_regs {
>  			u64 sp;
>  			u64 pc;
>  			u64 pstate;
> +			u64 flags;
> +			u64 padding;
>  		};
>  	};
>  	u64 orig_x0;
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 3ae6b31..7936ba9 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -56,6 +56,7 @@ int main(void)
>    DEFINE(S_COMPAT_SP,		offsetof(struct pt_regs, compat_sp));
>  #endif
>    DEFINE(S_PSTATE,		offsetof(struct pt_regs, pstate));
> +  DEFINE(S_FLAGS,		offsetof(struct pt_regs, flags));
>    DEFINE(S_PC,			offsetof(struct pt_regs, pc));
>    DEFINE(S_ORIG_X0,		offsetof(struct pt_regs, orig_x0));
>    DEFINE(S_SYSCALLNO,		offsetof(struct pt_regs, syscallno));
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index c45f296..9d2bdbf 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -403,9 +403,10 @@ void kernel_enable_single_step(struct pt_regs *regs)
>  	enable_debug_monitors(DBG_ACTIVE_EL1);
>  }
> 
> -void kernel_disable_single_step(void)
> +void kernel_disable_single_step(struct pt_regs *regs)
>  {
>  	WARN_ON(!irqs_disabled());
> +	regs->flags = 0;
>  	mdscr_write(mdscr_read() & ~DBG_MDSCR_SS);
>  	disable_debug_monitors(DBG_ACTIVE_EL1);
>  }
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 12e8d2b..753bef2 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -87,6 +87,15 @@
>  	stp	x26, x27, [sp, #16 * 13]
>  	stp	x28, x29, [sp, #16 * 14]
> 
> +	.if \el == 1
> +	mrs     x19, mdscr_el1
> +	and     x20, x19, #1
> +	str     x20, [sp, #S_FLAGS]
> +	tbz     x20, #0, 1f
> +	disable_step x19
> +1:
> +	.endif
> +
>  	.if	\el == 0
>  	mrs	x21, sp_el0
>  	mov	tsk, sp
> @@ -154,6 +163,15 @@ alternative_endif
>  	.endif
>  	msr	elr_el1, x21			// set up the return data
>  	msr	spsr_el1, x22
> +
> +	.if	\el == 1
> +	ldr	x19, [sp, #S_FLAGS]
> +	tbz	x19, #0, 1f
> +	mrs	x19, mdscr_el1
> +	enable_step	x19
> +1:
> +	.endif
> +
>  	ldp	x0, x1, [sp, #16 * 0]
>  	ldp	x2, x3, [sp, #16 * 1]
>  	ldp	x4, x5, [sp, #16 * 2]
> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> index b45c95d..6afbb80 100644
> --- a/arch/arm64/kernel/hw_breakpoint.c
> +++ b/arch/arm64/kernel/hw_breakpoint.c
> @@ -802,7 +802,7 @@ int reinstall_suspended_bps(struct pt_regs *regs)
>  			toggle_bp_registers(AARCH64_DBG_REG_WCR, DBG_ACTIVE_EL0, 1);
> 
>  		if (*kernel_step != ARM_KERNEL_STEP_SUSPEND) {
> -			kernel_disable_single_step();
> +			kernel_disable_single_step(regs);
>  			handled_exception = 1;
>  		} else {
>  			handled_exception = 0;
> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> index b67531a..f92c5fb 100644
> --- a/arch/arm64/kernel/kgdb.c
> +++ b/arch/arm64/kernel/kgdb.c
> @@ -183,7 +183,7 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
>  		 * Received continue command, disable single step
>  		 */
>  		if (kernel_active_single_step())
> -			kernel_disable_single_step();
> +			kernel_disable_single_step(linux_regs);
> 
>  		err = 0;
>  		break;
> -- 
> 1.7.1
> 
> > 
> > [1] http://www.spinics.net/lists/arm-kernel/msg491844.html
> > 
> > .
> >