Re: [PATCH] arm64: Clear OS lock in enable_debug_monitors

[Marc Zyngier <maz@kernel.org>]

On Thu, 09 Jun 2022 04:33:18 +0100,
Mark-PK Tsai <mark-pk.tsai@mediatek.com> wrote:
> 
> Always clear OS lock before enable debug event.
> 
> The OS lock is clear in cpuhp ops in recent kernel,
> but when the debug exception happened before it
> kernel might crash because debug event enable didn't
> take effect when OS lock is hold.
> 
> Below is the use case that having this problem:
> 
> Register kprobe in console_unlock and kernel will
> panic at secondary_start_kernel on secondary core.

Feels a bit extreme to do that, but hey...

> 
> CPU: 1 PID: 0 Comm: swapper/1 Tainted: P
> ...
> pstate: 004001c5 (nzcv dAIF +PAN -UAO)
> pc : do_undefinstr+0x5c/0x60
> lr : do_undefinstr+0x2c/0x60
> sp : ffffffc01338bc50
> pmr_save: 000000f0
> x29: ffffffc01338bc50 x28: ffffff8115e95a00 T
> x27: ffffffc01258e000 x26: ffffff8115e95a00
> x25: 00000000ffffffff x24: 0000000000000000
> x23: 00000000604001c5 x22: ffffffc014015008
> x21: 000000002232f000 x20: 00000000000000f0 j
> x19: ffffffc01338bc70 x18: ffffffc0132ed040
> x17: ffffffc01258eb48 x16: 0000000000000403 L&
> x15: 0000000000016480 x14: ffffffc01258e000 i/
> x13: 0000000000000006 x12: 0000000000006985
> x11: 00000000d5300000 x10: 0000000000000000
> x9 : 9f6c79217a8a0400 x8 : 00000000000000c5
> x7 : 0000000000000000 x6 : ffffffc01338bc08 2T
> x5 : ffffffc01338bc08 x4 : 0000000000000002
> x3 : 0000000000000000 x2 : 0000000000000004
> x1 : 0000000000000000 x0 : 0000000000000001 *q
> Call trace:
>  do_undefinstr+0x5c/0x60
>  el1_undef+0x10/0xb4
>  0xffffffc014015008
>  vprintk_func+0x210/0x290
>  printk+0x64/0x90
>  cpuinfo_detect_icache_policy+0x80/0xe0
>  __cpuinfo_store_cpu+0x150/0x160
>  secondary_start_kernel+0x154/0x440
> 
> The root cause is that OS_LSR_EL1.OSLK is reset
> to 1 on a cold reset[1] and the firmware didn't
> unlock it by default.
> So the core didn't go to el1_dbg as expected after
> kernel_enable_single_step and eret.
> 
> [1] https://developer.arm.com/documentation/ddi0595/2021-06/AArch64-Registers/OSLSR-EL1--OS-Lock-Status-Register?lang=en
> Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
> ---
>  arch/arm64/kernel/debug-monitors.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index bf9fe71589bc..186f2846d652 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -70,6 +70,17 @@ static int __init early_debug_disable(char *buf)
>  
>  early_param("nodebugmon", early_debug_disable);
>  
> +/*
> + * OS lock clearing.
> + */
> +static int clear_os_lock(unsigned int cpu)
> +{
> +	write_sysreg(0, osdlr_el1);
> +	write_sysreg(0, oslar_el1);
> +	isb();
> +	return 0;
> +}
> +
>  /*
>   * Keep track of debug users on each core.
>   * The ref counts are per-cpu so we use a local_t type.
> @@ -91,6 +102,7 @@ void enable_debug_monitors(enum dbg_active_el el)
>  		enable |= DBG_MDSCR_KDE;
>  
>  	if (enable && debug_enabled) {
> +		clear_os_lock(0);
>  		mdscr = mdscr_read();
>  		mdscr |= enable;
>  		mdscr_write(mdscr);
> @@ -119,17 +131,6 @@ void disable_debug_monitors(enum dbg_active_el el)
>  }
>  NOKPROBE_SYMBOL(disable_debug_monitors);
>  
> -/*
> - * OS lock clearing.
> - */
> -static int clear_os_lock(unsigned int cpu)
> -{
> -	write_sysreg(0, osdlr_el1);
> -	write_sysreg(0, oslar_el1);
> -	isb();
> -	return 0;
> -}
> -
>  static int __init debug_monitors_init(void)
>  {
>  	return cpuhp_setup_state(CPUHP_AP_ARM64_DEBUG_MONITORS_STARTING,

If the OS Lock is cleared every time we enabled debug, what is the
point of the notifier then?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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