[PATCH v3 3/4] arm/syscalls: Optimize address limit check

[Thomas Garnier]

On Tue, Aug 22, 2017 at 9:42 AM, Thomas Garnier <thgarnie at google.com> wrote:
> On Mon, Aug 14, 2017 at 2:37 PM, 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 report data corruption (stopping
>> the kernel or process based on configuration).
>>
>> The address limit check has to be done before any pending work because
>> they can reset the address limit and the process is killed using a
>> SIGKILL signal. 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>
>
> Any feedback?

CCing LW at karo-electronics.de who experienced the same issue this patch
proposal fix.

Russel: Any feedback?

>
>> ---
>>  arch/arm/kernel/entry-common.S | 11 +++++++++++
>>  arch/arm/kernel/signal.c       |  7 +++++++
>>  2 files changed, 18 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..b67ae12503f3 100644
>> --- a/arch/arm/kernel/signal.c
>> +++ b/arch/arm/kernel/signal.c
>> @@ -14,6 +14,7 @@
>>  #include <linux/uaccess.h>
>>  #include <linux/tracehook.h>
>>  #include <linux/uprobes.h>
>> +#include <linux/syscalls.h>
>>
>>  #include <asm/elf.h>
>>  #include <asm/cacheflush.h>
>> @@ -673,3 +674,9 @@ struct page *get_signal_page(void)
>>
>>         return page;
>>  }
>> +
>> +/* Defer to generic check */
>> +asmlinkage void addr_limit_check_failed(void)
>> +{
>> +       addr_limit_user_check();
>> +}
>> --
>> 2.14.1.480.gb18f417b89-goog
>>
>
>
>
> --
> Thomas



-- 
Thomas