Re: [PATCH v10 2/3] arm/syscalls: Check address limit on user-mode return

[Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>]

On Wed, Jun 14, 2017 at 6:12 PM, Thomas Garnier <thgarnie-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
> Ensure the address limit is a user-mode segment before returning to
> user-mode. Otherwise a process can corrupt kernel-mode memory and
> elevate privileges [1].
>
> The set_fs function sets the TIF_SETFS flag to force a slow path on
> return. In the slow path, the address limit is checked to be USER_DS if
> needed.
>
> The TIF_SETFS flag is added to _TIF_WORK_MASK shifting _TIF_SYSCALL_WORK
> for arm instruction immediate support. The global work mask is too big
> to used on a single instruction so adapt ret_fast_syscall.
>
> [1] https://bugs.chromium.org/p/project-zero/issues/detail?id=990
>
> Signed-off-by: Thomas Garnier <thgarnie-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> ---
> v10 redesigns the change to use work flags on set_fs as recommended by
> Linus and agreed by others.
>
> Based on next-20170609
> ---
>  arch/arm/include/asm/thread_info.h | 15 +++++++++------
>  arch/arm/include/asm/uaccess.h     |  2 ++
>  arch/arm/kernel/entry-common.S     |  9 +++++++--
>  arch/arm/kernel/signal.c           |  5 +++++
>  4 files changed, 23 insertions(+), 8 deletions(-)
>
> diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
> index 776757d1604a..1d468b527b7b 100644
> --- a/arch/arm/include/asm/thread_info.h
> +++ b/arch/arm/include/asm/thread_info.h
> @@ -139,10 +139,11 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
>  #define TIF_NEED_RESCHED       1       /* rescheduling necessary */
>  #define TIF_NOTIFY_RESUME      2       /* callback before returning to user */
>  #define TIF_UPROBE             3       /* breakpointed or singlestepping */
> -#define TIF_SYSCALL_TRACE      4       /* syscall trace active */
> -#define TIF_SYSCALL_AUDIT      5       /* syscall auditing active */
> -#define TIF_SYSCALL_TRACEPOINT 6       /* syscall tracepoint instrumentation */
> -#define TIF_SECCOMP            7       /* seccomp syscall filtering active */
> +#define TIF_FSCHECK            4       /* Check FS is USER_DS on return */
> +#define TIF_SYSCALL_TRACE      5       /* syscall trace active */
> +#define TIF_SYSCALL_AUDIT      6       /* syscall auditing active */
> +#define TIF_SYSCALL_TRACEPOINT 7       /* syscall tracepoint instrumentation */
> +#define TIF_SECCOMP            8       /* seccomp syscall filtering active */
>
>  #define TIF_NOHZ               12      /* in adaptive nohz mode */
>  #define TIF_USING_IWMMXT       17
> @@ -153,6 +154,7 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
>  #define _TIF_NEED_RESCHED      (1 << TIF_NEED_RESCHED)
>  #define _TIF_NOTIFY_RESUME     (1 << TIF_NOTIFY_RESUME)
>  #define _TIF_UPROBE            (1 << TIF_UPROBE)
> +#define _TIF_FSCHECK           (1 << TIF_FSCHECK)
>  #define _TIF_SYSCALL_TRACE     (1 << TIF_SYSCALL_TRACE)
>  #define _TIF_SYSCALL_AUDIT     (1 << TIF_SYSCALL_AUDIT)
>  #define _TIF_SYSCALL_TRACEPOINT        (1 << TIF_SYSCALL_TRACEPOINT)
> @@ -166,8 +168,9 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
>  /*
>   * Change these and you break ASM code in entry-common.S
>   */
> -#define _TIF_WORK_MASK         (_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
> -                                _TIF_NOTIFY_RESUME | _TIF_UPROBE)
> +#define _TIF_WORK_MASK         (_TIF_NEED_RESCHED | _TIF_SIGPENDING |  \
> +                                _TIF_NOTIFY_RESUME | _TIF_UPROBE |     \
> +                                _TIF_FSCHECK)
>
>  #endif /* __KERNEL__ */
>  #endif /* __ASM_ARM_THREAD_INFO_H */
> diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
> index 2577405d082d..6cc882223e34 100644
> --- a/arch/arm/include/asm/uaccess.h
> +++ b/arch/arm/include/asm/uaccess.h
> @@ -77,6 +77,8 @@ static inline void set_fs(mm_segment_t fs)
>  {
>         current_thread_info()->addr_limit = fs;
>         modify_domain(DOMAIN_KERNEL, fs ? DOMAIN_CLIENT : DOMAIN_MANAGER);
> +       /* On user-mode return, check fs is correct */
> +       set_thread_flag(TIF_FSCHECK);
>  }
>
>  #define segment_eq(a, b)       ((a) == (b))
> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index eb5cd77bf1d8..e33c32d56193 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -41,7 +41,9 @@ ret_fast_syscall:
>   UNWIND(.cantunwind    )
>         disable_irq_notrace                     @ disable interrupts
>         ldr     r1, [tsk, #TI_FLAGS]            @ re-check for syscall tracing
> -       tst     r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
> +       tst     r1, #_TIF_SYSCALL_WORK
> +       bne     fast_work_pending
> +       tst     r1, #_TIF_WORK_MASK

(IIUC) MOV32 is 2 cycles (MOVW, MOVT), and each TST above is 1 cycle
and each BNE is 1 cycle (when not taken). So:

mov32 r2, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
tst r1, r2
bne fast_work_pending

is 4 cycles and tst, bne, tst, bne is also 4 cycles. Would mov32 be
more readable (since it keeps the flags together)?

-Kees

-- 
Kees Cook
Pixel Security