Re: [PATCH 2/2] ARM: mm: make text and rodata read-only

From: Kees Cook
Date: Fri Apr 04 2014 - 20:08:48 EST

On Fri, Apr 4, 2014 at 12:58 PM, Rabin Vincent <rabin@xxxxxx> wrote:
> On Thu, Apr 03, 2014 at 07:15:19PM -0700, Kees Cook wrote:
>> diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c
>> index 34e56647dcee..4ae343c1e2a3 100644
>> --- a/arch/arm/kernel/ftrace.c
>> +++ b/arch/arm/kernel/ftrace.c
>> @@ -14,6 +14,7 @@
>>
>> #include <linux/ftrace.h>
>> #include <linux/uaccess.h>
>> +#include <linux/stop_machine.h>
>>
>> #include <asm/cacheflush.h>
>> #include <asm/opcodes.h>
>> @@ -34,6 +35,22 @@
>>
>> #define OLD_NOP 0xe1a00000 /* mov r0, r0 */
>>
>> +static int __ftrace_modify_code(void *data)
>
> This is in the CONFIG_OLD_MCOUNT ifdef, but should be in the outer ifdef
> (CONFIG_DYNAMIC_FTRACE) instead, otherwise it will not get enabled for
> for example Thumb-2 kernels. This was wrong in my example patch too.

Ah! Yes, good point. I've moved this now.

>> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
>> index 8539eb2a01ad..3baac4ad165f 100644
>> --- a/arch/arm/mm/init.c
>> +++ b/arch/arm/mm/init.c
>> @@ -681,30 +716,52 @@ static inline bool arch_has_strict_perms(void)
>> return true;
>> }
>>
>> +#define set_section_perms(perms, field) { \
>> + size_t i; \
>> + unsigned long addr; \
>> + \
>> + if (!arch_has_strict_perms()) \
>> + return; \
>> + \
>> + 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", \
>> + perms[i].start, perms[i].end, \
>> + SECTION_SIZE); \
>> + continue; \
>> + } \
>> + \
>> + for (addr = perms[i].start; \
>> + addr < perms[i].end; \
>> + addr += SECTION_SIZE) \
>> + section_update(addr, perms[i].mask, \
>> + perms[i].field); \
>> + } \
>> +}
>> +
>> static inline void fix_kernmem_perms(void)
>> {
>> - unsigned long addr;
>> - unsigned int i;
>> + set_section_perms(nx_perms, prot);
>> +}
>>
>> - if (!arch_has_strict_perms())
>> - return;
>> +#ifdef CONFIG_DEBUG_RODATA
>> +void mark_rodata_ro(void)
>> +{
>> + set_section_perms(ro_perms, prot);
>> +}
>>
>> - for (i = 0; i < ARRAY_SIZE(section_perms); i++) {
>> - if (!IS_ALIGNED(section_perms[i].start, SECTION_SIZE) ||
>> - !IS_ALIGNED(section_perms[i].end, SECTION_SIZE)) {
>> - pr_err("BUG: section %lx-%lx not aligned to %lx\n",
>> - section_perms[i].start, section_perms[i].end,
>> - SECTION_SIZE);
>> - continue;
>> - }
>> +void set_kernel_text_rw(void)
>> +{
>> + set_section_perms(ro_perms, clear);
>> +}
>
> You need a TLB flush. I had a flush_tlb_all() in my example patch,
> http://lists.infradead.org/pipermail/linux-arm-kernel/2014-April/244335.html,
> but the following is probably nicer (on top of this patch):
>
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 9bea524..a92c45a 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -741,6 +741,8 @@ static inline bool arch_has_strict_perms(void)
> addr += SECTION_SIZE) \
> section_update(addr, perms[i].mask, \
> perms[i].field); \
> + \
> + flush_tlb_kernel_range(perms[i].start, perms[i].end); \
> } \
> }
>

When I do this, I hang the system, and get a WARN due to the tlb call
attempting to flush on all CPUs, I think:

[ 34.246034] WARNING: at
/mnt/host/source/src/third_party/kernel-next/kernel/smp.c:466
smp_call_function_many+0xac/0x26c()
...
[ 34.246617] Backtrace:
[ 34.246697] [<c010d3b8>] (unwind_backtrace+0x0/0x118) from
[<c060b9d8>] (dump_stack+0x28/0x30)
[ 34.246765] [<c060b9d8>] (dump_stack+0x28/0x30) from [<c0123044>]
(warn_slowpath_null+0x44/0x5c)
[ 34.246824] [<c0123044>] (warn_slowpath_null+0x44/0x5c) from
[<c017426c>] (smp_call_function_many+0xac/0x26c)
[ 34.246881] [<c017426c>] (smp_call_function_many+0xac/0x26c) from
[<c0174468>] (smp_call_function+0x3c/0x48)
[ 34.246937] [<c0174468>] (smp_call_function+0x3c/0x48) from
[<c010c0fc>] (broadcast_tlb_a15_erratum+0x40/0x4c)
[ 34.246994] [<c010c0fc>] (broadcast_tlb_a15_erratum+0x40/0x4c) from
[<c010c590>] (flush_tlb_kernel_range+0x74/0xa0)
[ 34.247046] [<c010c590>] (flush_tlb_kernel_range+0x74/0xa0) from
[<c011403c>] (set_kernel_text_rw+0xd8/0xec)
[ 34.247099] [<c011403c>] (set_kernel_text_rw+0xd8/0xec) from
[<c010c878>] (__ftrace_modify_code+0x14/0x28)
[ 34.247156] [<c010c878>] (__ftrace_modify_code+0x14/0x28) from
[<c0184318>] (stop_machine_cpu_stop+0xc0/0x114)
[ 34.247212] [<c0184318>] (stop_machine_cpu_stop+0xc0/0x114) from
[<c01841cc>] (cpu_stopper_thread+0xd8/0x164)
[ 34.247266] [<c01841cc>] (cpu_stopper_thread+0xd8/0x164) from
[<c0145c14>] (kthread+0xc8/0xd8)
[ 34.247323] [<c0145c14>] (kthread+0xc8/0xd8) from [<c0106118>]
(ret_from_fork+0x14/0x20)

Using local_flush_tlb_kernel_range() fixed it though. Thank you for
your help on this! :)

-Kees

--
Kees Cook
Chrome OS Security
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