Re: [PATCH] ARM: mm: add testcases for RODATA

Kees Cook
2017-01-18T21:20:32Z

On Wed, Jan 18, 2017 at 11:20 AM, Laura Abbott <labbott@redhat.com> wrote:
> On 01/18/2017 05:53 AM, Jinbum Park wrote:
>> This patch adds testcases for the CONFIG_DEBUG_RODATA option.
>> It's similar to x86's testcases.
>> It tests read-only mapped data and page-size aligned rodata section.
>>
>> Signed-off-by: Jinbum Park <jinb.park7@gmail.com>
>> ---
>> arch/arm/Kconfig.debug | 5 +++
>> arch/arm/include/asm/cacheflush.h | 10 +++++
>> arch/arm/mm/Makefile | 1 +
>> arch/arm/mm/init.c | 6 +++
>> arch/arm/mm/test_rodata.c | 80 +++++++++++++++++++++++++++++++++++++++
>> 5 files changed, 102 insertions(+)
>> create mode 100644 arch/arm/mm/test_rodata.c
>>
>> diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
>> index d83f7c3..511e5e1 100644
>> --- a/arch/arm/Kconfig.debug
>> +++ b/arch/arm/Kconfig.debug
>> @@ -1749,6 +1749,11 @@ config DEBUG_SET_MODULE_RONX
>> against certain classes of kernel exploits.
>> If in doubt, say "N".
>>
>> +config DEBUG_RODATA_TEST
>> + bool "Testcase for the marking rodata read-only"
>> + ---help---
>> + This option enables a testcase for the setting rodata read-only.
>> +
>> source "drivers/hwtracing/coresight/Kconfig"
>>
>> endmenu
>> diff --git a/arch/arm/include/asm/cacheflush.h b/arch/arm/include/asm/cacheflush.h
>> index bdd283b..741e2e8 100644
>> --- a/arch/arm/include/asm/cacheflush.h
>> +++ b/arch/arm/include/asm/cacheflush.h
>> @@ -498,6 +498,16 @@ static inline void set_kernel_text_rw(void) { }
>> static inline void set_kernel_text_ro(void) { }
>> #endif
>>
>> +#ifdef CONFIG_DEBUG_RODATA_TEST
>> +extern const int rodata_test_data;
>> +int rodata_test(void);
>> +#else
>> +static inline int rodata_test(void)
>> +{
>> + return 0;
>> +}
>> +#endif
>> +
>> void flush_uprobe_xol_access(struct page *page, unsigned long uaddr,
>> void *kaddr, unsigned long len);
>>
>> diff --git a/arch/arm/mm/Makefile b/arch/arm/mm/Makefile
>> index b3dea80..dbb76ff 100644
>> --- a/arch/arm/mm/Makefile
>> +++ b/arch/arm/mm/Makefile
>> @@ -15,6 +15,7 @@ endif
>> obj-$(CONFIG_ARM_PTDUMP) += dump.o
>> obj-$(CONFIG_MODULES) += proc-syms.o
>> obj-$(CONFIG_DEBUG_VIRTUAL) += physaddr.o
>> +obj-$(CONFIG_DEBUG_RODATA_TEST) += test_rodata.o
>>
>> obj-$(CONFIG_ALIGNMENT_TRAP) += alignment.o
>> obj-$(CONFIG_HIGHMEM) += highmem.o
>> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
>> index 4127f57..3c15f3b 100644
>> --- a/arch/arm/mm/init.c
>> +++ b/arch/arm/mm/init.c
>> @@ -716,6 +716,7 @@ void fix_kernmem_perms(void)
>> int __mark_rodata_ro(void *unused)
>> {
>> update_sections_early(ro_perms, ARRAY_SIZE(ro_perms));
>> + rodata_test();
>
> We don't do anything with this return value, should we at least
> spit out a warning?
>
>> return 0;
>> }
>>
>> @@ -740,6 +741,11 @@ void set_kernel_text_ro(void)
>> static inline void fix_kernmem_perms(void) { }
>> #endif /* CONFIG_DEBUG_RODATA */
>>
>> +#ifdef CONFIG_DEBUG_RODATA_TEST
>> +const int rodata_test_data = 0xC3;
>
> This isn't accessed outside of test_rodata.c, it can just
> be moved there.
>
>> +EXPORT_SYMBOL_GPL(rodata_test_data);
>> +#endif
>> +
>> void free_tcmmem(void)
>> {
>> #ifdef CONFIG_HAVE_TCM
>> diff --git a/arch/arm/mm/test_rodata.c b/arch/arm/mm/test_rodata.c
>> new file mode 100644
>> index 0000000..133d092
>> --- /dev/null
>> +++ b/arch/arm/mm/test_rodata.c
>> @@ -0,0 +1,79 @@
>> +/*
>> + * test_rodata.c: functional test for mark_rodata_ro function
>> + *
>> + * (C) Copyright 2017 Jinbum Park <jinb.park7@gmail.com>
>> + *
>> + * This program is free software; you can redistribute it and/or
>> + * modify it under the terms of the GNU General Public License
>> + * as published by the Free Software Foundation; version 2
>> + * of the License.
>> + */
>> +#include <asm/cacheflush.h>
>> +#include <asm/sections.h>
>> +
>> +int rodata_test(void)
>> +{
>> + unsigned long result;
>> + unsigned long start, end;
>> +
>> + /* test 1: read the value */
>> + /* If this test fails, some previous testrun has clobbered the state */
>> +
>> + if (!rodata_test_data) {
>> + pr_err("rodata_test: test 1 fails (start data)\n");
>> + return -ENODEV;
>> + }
>> +
>> + /* test 2: write to the variable; this should fault */
>> + /*
>> + * If this test fails, we managed to overwrite the data
>> + *
>> + * This is written in assembly to be able to catch the
>> + * exception that is supposed to happen in the correct
>> + * case
>> + */
>> +
>> + result = 1;
>> + asm volatile(
>> + "0: str %[zero], [%[rodata_test]]\n"
>> + " mov %[rslt], %[zero]\n"
>> + "1:\n"
>> + ".pushsection .text.fixup,\"ax\"\n"
>> + ".align 2\n"
>> + "2:\n"
>> + "b 1b\n"
>> + ".popsection\n"
>> + ".pushsection __ex_table,\"a\"\n"
>> + ".align 3\n"
>> + ".long 0b, 2b\n"
>> + ".popsection\n"
>> + : [rslt] "=r" (result)
>> + : [zero] "r" (0UL), [rodata_test] "r" (&rodata_test_data)
>> + );
>> +
>> + if (!result) {
>> + pr_err("rodata_test: test data was not read only\n");
>> + return -ENODEV;
>> + }
>> +
>> + /* test 3: check the value hasn't changed */
>> + /* If this test fails, we managed to overwrite the data */
>> + if (!rodata_test_data) {
>> + pr_err("rodata_test: Test 3 fails (end data)\n");
>> + return -ENODEV;
>> + }
>
> I'm confused why we are checking this again when we have the result
> check above. Is there a case where we would still have result = 1
> but rodata_test_data overwritten?
>
>> +
>> + /* test 4: check if the rodata section is 4Kb aligned */
>> + start = (unsigned long)__start_rodata;
>> + end = (unsigned long)__end_rodata;
>> + if (start & (PAGE_SIZE - 1)) {
>> + pr_err("rodata_test: .rodata is not 4k aligned\n");
>> + return -ENODEV;
>> + }
>> + if (end & (PAGE_SIZE - 1)) {
>> + pr_err("rodata_test: .rodata end is not 4k aligned\n");
>> + return -ENODEV;
>> + }
>
> Why does this need to be page aligned (yes, I know why but this
> test case does not make it clear)
>
> Better yet, this should be a build time assertion not a runtime
> one.
>
>> +
>> + return 0;
>> +}
>>
>
> As Mark mentioned, this is possibly redundant with LKDTM. It
> would be good to explain what benefit this is bringing in addition
> to LKDTM.

The only thing I could think of would be to make failures block the
boot. (Though that would mean changing x86 too.) That's kind of like
the W^X test, which spits out a BUG IIRC.

-Kees

--
Kees Cook
Nexus Security