Re: [PATCH] ARM: mm: flip priority of CONFIG_DEBUG_RODATA

From: Kees Cook
Date: Mon Nov 30 2015 - 20:08:29 EST

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On Mon, Nov 30, 2015 at 5:03 PM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> 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.

This was inspired by my examining the existing architecture's
implementations of CONFIG_DEBUG_RODATA after Ingo suggested it be made
a common feature not a build-time config (or at least renamed):
http://www.openwall.com/lists/kernel-hardening/2015/11/30/13

>> 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@xxxxxxxxxxxx>
>> ---
>> 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.

Well, it seems to have the same effect: without the alignment, a
portion of rodata may remain executable on arm64. Unless I
misunderstand?

-Kees

>
>> 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

--
Kees Cook
Chrome OS & Brillo Security
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Next message: Stephen Boyd: "[PATCH] ARM: defconfig: qcom: Enable SSBI drivers"
Previous message: Eric W. Biederman: "Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR."
In reply to: Laura Abbott: "Re: [PATCH] ARM: mm: flip priority of CONFIG_DEBUG_RODATA"
Next in thread: Laura Abbott: "Re: [PATCH] ARM: mm: flip priority of CONFIG_DEBUG_RODATA"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]