[PATCH v2 2/3] arm/syscalls: Optimize address limit check

[Thomas Garnier]

On Mon, Aug 7, 2017 at 10:35 AM, Kees Cook <keescook at chromium.org> wrote:
> On Wed, Jul 26, 2017 at 10:00 AM, Thomas Garnier <thgarnie at google.com> wrote:
>> Disable the generic address limit check in favor of an architecture
>> specific optimized implementation. The generic implementation using
>> pending work flags did not work well with ARM and alignment faults.
>>
>> The address limit is checked on each syscall return path to user-mode
>> path as well as the irq user-mode return function. If the address limit
>> was changed, a function is called to stop the kernel with an explicit
>> message.
>>
>> The address limit check has to be done before any pending work because
>> they can reset the address limit. For example the lkdtm address limit
>> check does not work because the signal to kill the process will reset
>> the user-mode address limit.
>>
>> Signed-off-by: Thomas Garnier <thgarnie at google.com>
>> ---
>>  arch/arm/kernel/entry-common.S | 11 +++++++++++
>>  arch/arm/kernel/signal.c       |  5 +++++
>>  2 files changed, 16 insertions(+)
>>
>> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
>> index 0b60adf4a5d9..99c908226065 100644
>> --- a/arch/arm/kernel/entry-common.S
>> +++ b/arch/arm/kernel/entry-common.S
>> @@ -12,6 +12,7 @@
>>  #include <asm/unistd.h>
>>  #include <asm/ftrace.h>
>>  #include <asm/unwind.h>
>> +#include <asm/memory.h>
>>  #ifdef CONFIG_AEABI
>>  #include <asm/unistd-oabi.h>
>>  #endif
>> @@ -48,10 +49,14 @@ ret_fast_syscall:
>>   UNWIND(.fnstart       )
>>   UNWIND(.cantunwind    )
>>         disable_irq_notrace                     @ disable interrupts
>> +       ldr     r2, [tsk, #TI_ADDR_LIMIT]
>> +       cmp     r2, #TASK_SIZE
>> +       blne    addr_limit_check_failed
>>         ldr     r1, [tsk, #TI_FLAGS]            @ re-check for syscall tracing
>>         tst     r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
>>         bne     fast_work_pending
>>
>> +
>>         /* perform architecture specific actions before user return */
>>         arch_ret_to_user r1, lr
>>
>> @@ -74,6 +79,9 @@ ret_fast_syscall:
>>   UNWIND(.cantunwind    )
>>         str     r0, [sp, #S_R0 + S_OFF]!        @ save returned r0
>>         disable_irq_notrace                     @ disable interrupts
>> +       ldr     r2, [tsk, #TI_ADDR_LIMIT]
>> +       cmp     r2, #TASK_SIZE
>> +       blne    addr_limit_check_failed
>>         ldr     r1, [tsk, #TI_FLAGS]            @ re-check for syscall tracing
>>         tst     r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
>>         beq     no_work_pending
>> @@ -106,6 +114,9 @@ ENTRY(ret_to_user)
>>  ret_slow_syscall:
>>         disable_irq_notrace                     @ disable interrupts
>>  ENTRY(ret_to_user_from_irq)
>> +       ldr     r2, [tsk, #TI_ADDR_LIMIT]
>> +       cmp     r2, #TASK_SIZE
>> +       blne    addr_limit_check_failed
>>         ldr     r1, [tsk, #TI_FLAGS]
>>         tst     r1, #_TIF_WORK_MASK
>>         bne     slow_work_pending
>> diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
>> index 5814298ef0b7..5769c15cff89 100644
>> --- a/arch/arm/kernel/signal.c
>> +++ b/arch/arm/kernel/signal.c
>> @@ -673,3 +673,8 @@ struct page *get_signal_page(void)
>>
>>         return page;
>>  }
>> +
>> +asmlinkage void addr_limit_check_failed(void)
>> +{
>> +       panic("Incorrect address limit while returning to user-mode.");
>> +}
>
> Instead of taking the entire system down, how about a WARN/kill combo
> instead? If it's too late for "force_sig(SIGKILL, current)", then
> likely we should perform a "do_group_exit(SIGKILL)".

Sure, why not. I can also change the others architectures to move to a
do_group_exit(SIGKILL).

Before the next iteration, I want to know if Russel has any feedback
on this implementation, given the previous thread.

>
> -Kees
>
> --
> Kees Cook
> Pixel Security



-- 
Thomas