Re: [PATCH] Arm: mm: ftrace: Only set text back to ro after kernel has been marked ro

[Matthias Reichl]

On Wed, Aug 23, 2017 at 01:58:36PM -0400, Steven Rostedt wrote:
> 
> ftrace needs to modify the kernel text in order to enable function tracing.
> For security reasons, the kernel text is marked to read-only (ro) at the end
> of system bootup. When enabling function tracing after that, ftrace calls
> arch specific code that needs to enable the modification of kernel text
> while ftrace does the update, and reset it back again when finished.
> 
> The issue arises when function tracing is enabled during system bootup. The
> text hasn't been marked as read-only yet, but the same code to modify the
> kernel is executed, and when it is finished, it will cause the kernel to
> become read-only. This causes issues for other init code that requires
> modification of kernel text during system bootup. This appears to cause
> issue with Raspberry Pi 2.
> 
> By implementing the feature that is used in x86 to deal with this issue, it
> fixes the problem. The solution is simple. Have a variable
> (kernel_set_to_readonly) get set when the system finished boot and marks the
> kernel to readonly. If that variable is not set, both
> kernel_set_to_readonly() and kernel_set_to_rw() return without doing any
> modifications. Those functions are used by ftrace to change the permissions
> of the kernel text. By not doing anything, ftrace will not mess with the
> permissions when it is enabled at system bootup.
> 
> Link: http://lkml.kernel.org/r/20170821153402.7so2u364htvt6tnf@xxxxxxxxxx
> Link: https://github.com/raspberrypi/linux/issues/2166#issuecomment-323355145
> Reported-by: Matthias Reichl <hias@xxxxxxxxx>
> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Eric Anholt <eric@xxxxxxxxxx>
> Cc: Stefan Wahren <stefan.wahren@xxxxxxxx>
> Cc: Phil Elwell <phil@xxxxxxxxxxxxxxx>
> Cc: linux-rpi-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 80d6b0c2ee ("ARM: mm: allow text and rodata sections to be read-only")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

Tested-by: Matthias Reichl <hias@xxxxxxxxx>

Thanks, again, for resolving the issue so quickly!

so long,

Hias

> ---
>  arch/arm/mm/init.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index ad80548..fd75f38 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -745,19 +745,29 @@ static int __mark_rodata_ro(void *unused)
>  	return 0;
>  }
>  
> +static int kernel_set_to_readonly;
> +
>  void mark_rodata_ro(void)
>  {
> +	kernel_set_to_readonly = 1;
> +
>  	stop_machine(__mark_rodata_ro, NULL, NULL);
>  }
>  
>  void set_kernel_text_rw(void)
>  {
> +	if (!kernel_set_to_readonly)
> +		return;
> +
>  	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), false,
>  				current->active_mm);
>  }
>  
>  void set_kernel_text_ro(void)
>  {
> +	if (!kernel_set_to_readonly)
> +		return;
> +
>  	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,
>  				current->active_mm);
>  }
> -- 
> 2.9.5
>