[kernel-hardening] Re: [PATCH] ARM: mm: flip priority of CONFIG_DEBUG_RODATA

[Laura Abbott <labbott@redhat.com>]

On 11/30/2015 03:38 PM, Kees Cook wrote:
> Given the choice between making things NX or making things RO, we want
> RO first. As such, redefine CONFIG_DEBUG_RODATA to actually do the bulk

Can you give a citation for why? The thread that inspired it might be
a good link.

> of the ROing (and NXing). In the place of the old CONFIG_DEBUG_RODATA,
> introduce CONFIG_DEBUG_RODATA_ALIGN (after arm64's config that does the
> same thing) to add the additional section alignment for making rodata
> explicitly NX. Also adds human readable names to the sections so I
> could more easily debug my typos, and makes CONFIG_DEBUG_RODATA default
> "y" for CPU_V7.
>
> Results in /sys/kernel/debug/kernel_page_tables for each config state:
>
>   # CONFIG_DEBUG_RODATA is not set
>   # CONFIG_DEBUG_ALIGN_RODATA is not set
>
> ---[ Kernel Mapping ]---
> 0x80000000-0x80900000           9M     RW x  SHD
> 0x80900000-0xa0000000         503M     RW NX SHD
>
>   CONFIG_DEBUG_RODATA=y
>   CONFIG_DEBUG_ALIGN_RODATA=y
>
> ---[ Kernel Mapping ]---
> 0x80000000-0x80100000           1M     RW NX SHD
> 0x80100000-0x80700000           6M     ro x  SHD
> 0x80700000-0x80a00000           3M     ro NX SHD
> 0x80a00000-0xa0000000         502M     RW NX SHD
>
>   CONFIG_DEBUG_RODATA=y
>   # CONFIG_DEBUG_ALIGN_RODATA is not set
>
> ---[ Kernel Mapping ]---
> 0x80000000-0x80100000           1M     RW NX SHD
> 0x80100000-0x80a00000           9M     ro x  SHD
> 0x80a00000-0xa0000000         502M     RW NX SHD
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>   arch/arm/kernel/vmlinux.lds.S | 10 +++++-----
>   arch/arm/mm/Kconfig           | 34 ++++++++++++++++++----------------
>   arch/arm/mm/init.c            | 18 ++++++++++--------
>   3 files changed, 33 insertions(+), 29 deletions(-)
>
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index 8b60fde5ce48..a6e395c53a48 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -8,7 +8,7 @@
>   #include <asm/thread_info.h>
>   #include <asm/memory.h>
>   #include <asm/page.h>
> -#ifdef CONFIG_ARM_KERNMEM_PERMS
> +#ifdef CONFIG_DEBUG_RODATA
>   #include <asm/pgtable.h>
>   #endif
>
> @@ -94,7 +94,7 @@ SECTIONS
>   		HEAD_TEXT
>   	}
>
> -#ifdef CONFIG_ARM_KERNMEM_PERMS
> +#ifdef CONFIG_DEBUG_RODATA
>   	. = ALIGN(1<<SECTION_SHIFT);
>   #endif
>
> @@ -117,7 +117,7 @@ SECTIONS
>   			ARM_CPU_KEEP(PROC_INFO)
>   	}
>
> -#ifdef CONFIG_DEBUG_RODATA
> +#ifdef CONFIG_DEBUG_ALIGN_RODATA
>   	. = ALIGN(1<<SECTION_SHIFT);
>   #endif
>   	RO_DATA(PAGE_SIZE)
> @@ -153,7 +153,7 @@ SECTIONS
>   	_etext = .;			/* End of text and rodata section */
>
>   #ifndef CONFIG_XIP_KERNEL
> -# ifdef CONFIG_ARM_KERNMEM_PERMS
> +# ifdef CONFIG_DEBUG_RODATA
>   	. = ALIGN(1<<SECTION_SHIFT);
>   # else
>   	. = ALIGN(PAGE_SIZE);
> @@ -231,7 +231,7 @@ SECTIONS
>   	__data_loc = ALIGN(4);		/* location in binary */
>   	. = PAGE_OFFSET + TEXT_OFFSET;
>   #else
> -#ifdef CONFIG_ARM_KERNMEM_PERMS
> +#ifdef CONFIG_DEBUG_RODATA
>   	. = ALIGN(1<<SECTION_SHIFT);
>   #else
>   	. = ALIGN(THREAD_SIZE);
> diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
> index 41218867a9a6..b617084e9520 100644
> --- a/arch/arm/mm/Kconfig
> +++ b/arch/arm/mm/Kconfig
> @@ -1039,24 +1039,26 @@ config ARCH_SUPPORTS_BIG_ENDIAN
>   	  This option specifies the architecture can support big endian
>   	  operation.
>
> -config ARM_KERNMEM_PERMS
> -	bool "Restrict kernel memory permissions"
> +config DEBUG_RODATA
> +	bool "Make kernel text and rodata read-only"
>   	depends on MMU
> +	default y if CPU_V7
>   	help
> -	  If this is set, kernel memory other than kernel text (and rodata)
> -	  will be made non-executable. The tradeoff is that each region is
> -	  padded to section-size (1MiB) boundaries (because their permissions
> -	  are different and splitting the 1M pages into 4K ones causes TLB
> -	  performance problems), wasting memory.
> +	  If this is set, kernel memory (text, rodata, etc) will be made
> +	  read-only, and non-text kernel memory will be made non-executable.
> +	  The tradeoff is that each region is padded to section-size (1MiB)
> +	  boundaries (because their permissions are different and splitting
> +	  the 1M pages into 4K ones causes TLB performance problems), which
> +	  can waste memory.
>
> -config DEBUG_RODATA
> -	bool "Make kernel text and rodata read-only"
> -	depends on ARM_KERNMEM_PERMS
> +config DEBUG_ALIGN_RODATA
> +	bool "Make rodata strictly non-executable"
> +	depends on DEBUG_RODATA
>   	default y
>   	help
> -	  If this is set, kernel text and rodata will be made read-only. This
> -	  is to help catch accidental or malicious attempts to change the
> -	  kernel's executable code. Additionally splits rodata from kernel
> -	  text so it can be made explicitly non-executable. This creates
> -	  another section-size padded region, so it can waste more memory
> -	  space while gaining the read-only protections.
> +	  If this is set, rodata will be made explicitly non-executable. This
> +	  provides protection on the rare chance that attackers might find and
> +	  use ROP gadgets that exist in the rodata section. This adds an
> +	  additional section-aligned split of rodata from kernel text so it
> +	  can be made explicitly non-executable. This padding may waste memory
> +	  space to gain this additional protection.

I get that you want to make this match arm64 but it's really not intuitive that
something with ALIGN_RODATA in the name is actually for setting NX. The purpose
of ALIGN_RODATA was also slightly different on arm64 since the RO/NX will still
be there, the difference is if the sections are present versus broken down into
pages.

> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 8a63b4cdc0f2..e99f65fbcf2b 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -568,8 +568,9 @@ void __init mem_init(void)
>   	}
>   }
>
> -#ifdef CONFIG_ARM_KERNMEM_PERMS
> +#ifdef CONFIG_DEBUG_RODATA
>   struct section_perm {
> +	const char *name;
>   	unsigned long start;
>   	unsigned long end;
>   	pmdval_t mask;
> @@ -580,6 +581,7 @@ struct section_perm {
>   static struct section_perm nx_perms[] = {
>   	/* Make pages tables, etc before _stext RW (set NX). */
>   	{
> +		.name	= "pre-text NX",
>   		.start	= PAGE_OFFSET,
>   		.end	= (unsigned long)_stext,
>   		.mask	= ~PMD_SECT_XN,
> @@ -587,14 +589,16 @@ static struct section_perm nx_perms[] = {
>   	},
>   	/* Make init RW (set NX). */
>   	{
> +		.name	= "init NX",
>   		.start	= (unsigned long)__init_begin,
>   		.end	= (unsigned long)_sdata,
>   		.mask	= ~PMD_SECT_XN,
>   		.prot	= PMD_SECT_XN,
>   	},
> -#ifdef CONFIG_DEBUG_RODATA
> +#ifdef CONFIG_DEBUG_ALIGN_RODATA
>   	/* Make rodata NX (set RO in ro_perms below). */
>   	{
> +		.name	= "rodata NX",
>   		.start  = (unsigned long)__start_rodata,
>   		.end    = (unsigned long)__init_begin,
>   		.mask   = ~PMD_SECT_XN,
> @@ -603,10 +607,10 @@ static struct section_perm nx_perms[] = {
>   #endif
>   };
>
> -#ifdef CONFIG_DEBUG_RODATA
>   static struct section_perm ro_perms[] = {
>   	/* Make kernel code and rodata RX (set RO). */
>   	{
> +		.name	= "text/rodata RO",
>   		.start  = (unsigned long)_stext,
>   		.end    = (unsigned long)__init_begin,
>   #ifdef CONFIG_ARM_LPAE
> @@ -619,7 +623,6 @@ static struct section_perm ro_perms[] = {
>   #endif
>   	},
>   };
> -#endif
>
>   /*
>    * Updates section permissions only for the current mm (sections are
> @@ -666,7 +669,8 @@ static inline bool arch_has_strict_perms(void)
>   	for (i = 0; i < ARRAY_SIZE(perms); i++) {			\
>   		if (!IS_ALIGNED(perms[i].start, SECTION_SIZE) ||	\
>   		    !IS_ALIGNED(perms[i].end, SECTION_SIZE)) {		\
> -			pr_err("BUG: section %lx-%lx not aligned to %lx\n", \
> +			pr_err("BUG: %s section %lx-%lx not aligned to %lx\n", \
> +				perms[i].name,				\
>   				perms[i].start, perms[i].end,		\
>   				SECTION_SIZE);				\
>   			continue;					\
> @@ -685,7 +689,6 @@ static inline void fix_kernmem_perms(void)
>   	set_section_perms(nx_perms, prot);
>   }
>
> -#ifdef CONFIG_DEBUG_RODATA
>   void mark_rodata_ro(void)
>   {
>   	set_section_perms(ro_perms, prot);
> @@ -700,11 +703,10 @@ void set_kernel_text_ro(void)
>   {
>   	set_section_perms(ro_perms, prot);
>   }
> -#endif /* CONFIG_DEBUG_RODATA */
>
>   #else
>   static inline void fix_kernmem_perms(void) { }
> -#endif /* CONFIG_ARM_KERNMEM_PERMS */
> +#endif /* CONFIG_DEBUG_RODATA */
>
>   void free_tcmmem(void)
>   {
>

Thanks,
Laura