Re: [PATCH 2/4] mm: security: Allow default HARDENED_USERCOPY to be set at compile time

[Kees Cook]

On Wed, Jan 22, 2025 at 05:19:23PM +0000, Mel Gorman wrote:
> HARDENED_USERCOPY defaults to on if enabled at compile time. Allow
> hardened_usercopy= default to be set at compile time similar to
> init_on_alloc= and init_on_free=. The intent is that hardening
> options that can be disabled at runtime can set their default at
> build time.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 4 +++-
>  mm/usercopy.c                                   | 3 ++-
>  security/Kconfig.hardening                      | 8 ++++++++
>  3 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 3872bc6ec49d..5d759b20540a 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1773,7 +1773,9 @@
>  			allocation boundaries as a proactive defense
>  			against bounds-checking flaws in the kernel's
>  			copy_to_user()/copy_from_user() interface.
> -		on	Perform hardened usercopy checks (default).
> +			The default is determined by
> +			CONFIG_HARDENED_USERCOPY_DEFAULT_ON.
> +		on	Perform hardened usercopy checks.
>  		off	Disable hardened usercopy checks.
>  
>  	hardlockup_all_cpu_backtrace=
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index 83c164aba6e0..4cf33305347a 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -255,7 +255,8 @@ void __check_object_size(const void *ptr, unsigned long n, bool to_user)
>  }
>  EXPORT_SYMBOL(__check_object_size);
>  
> -static bool enable_checks __initdata = true;
> +static bool enable_checks __initdata =
> +		IS_ENABLED(CONFIG_HARDENED_USERCOPY_DEFAULT_ON);
>  
>  static int __init parse_hardened_usercopy(char *str)
>  {
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index 9088d613d519..adcc260839c7 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -293,6 +293,14 @@ config HARDENED_USERCOPY
>  	  or are part of the kernel text. This prevents entire classes
>  	  of heap overflow exploits and similar kernel memory exposures.
>  
> +config HARDENED_USERCOPY_DEFAULT_ON
> +	bool "Harden memory copies by default"
> +	depends on HARDENED_USERCOPY
> +	default n

This must be "default HARDENED_USERCOPY" or existing distro builds will
break. All major distros enable this by default, and I don't want to
risk HARDENED_USERCOPY_DEFAULT_ON getting missed and getting globally
disabled.

> +	help
> +	  This has the effect of setting "hardened_usercopy=on" on the kernel
> +	  command line. This can be disabled with "hardened_usercopy=off".
> +
>  endmenu
>  
>  menu "Hardening of kernel data structures"

-Kees

-- 
Kees Cook