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SubjectRe: [PATCH v2 2/3] arm/syscalls: Optimize address limit check
On Mon, Aug 7, 2017 at 10:35 AM, Kees Cook <keescook@chromium.org> wrote:
> On Wed, Jul 26, 2017 at 10:00 AM, Thomas Garnier <thgarnie@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@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

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