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Date: Mon, 30 Nov 2015 17:03:33 -0800
From: Laura Abbott <labbott@...hat.com>
To: Kees Cook <keescook@...omium.org>, Russell King <linux@....linux.org.uk>
Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Will Deacon <will.deacon@....com>, Nicolas Pitre <nico@...aro.org>,
Arnd Bergmann <arnd@...db.de>,
Catalin Marinas <catalin.marinas@....com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] ARM: mm: flip priority of CONFIG_DEBUG_RODATA
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@...omium.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
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