Re: [RFC][PATCH] x86: Verify access_ok() context

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Nov 22, 2016 at 1:57 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> I recently encountered wreckage because access_ok() was used where it
> should not be, add an explicit WARN when access_ok() is used wrongly.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/include/asm/uaccess.h |  7 +++++--
>  include/linux/preempt.h        | 21 +++++++++++++--------
>  2 files changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index faf3687f1035..b139c46ba122 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -88,8 +88,11 @@ static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, un
>   * checks that the pointer is in the user space range - after calling
>   * this function, memory access functions may still return -EFAULT.
>   */
> -#define access_ok(type, addr, size) \
> -       likely(!__range_not_ok(addr, size, user_addr_max()))
> +#define access_ok(type, addr, size)                                    \
> +({                                                                     \
> +       WARN_ON_ONCE(!in_task());                                       \

Should this be guarded by some debug option?  This may hurt
performance on production systems quite a bit.

> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index 75e4e30677f1..7eeceac52dea 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -65,19 +65,24 @@
>
>  /*
>   * Are we doing bottom half or hardware interrupt processing?
> - * Are we in a softirq context? Interrupt context?
> - * in_softirq - Are we currently processing softirq or have bh disabled?
> - * in_serving_softirq - Are we currently processing softirq?
> + *
> + * in_irq()       - We're in (hard) IRQ context
> + * in_softirq()   - We have BH disabled, or are processing softirqs
> + * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
> + * in_serving_softirq() - We're in softirq context
> + * in_nmi()       - We're in NMI context
> + * in_task()     - We're in task context
> + *
> + * Note: due to the BH disabled confusion: in_softirq(),in_interrupt() really
> + *       should not be used in new code.
>   */
>  #define in_irq()               (hardirq_count())
>  #define in_softirq()           (softirq_count())
>  #define in_interrupt()         (irq_count())
>  #define in_serving_softirq()   (softirq_count() & SOFTIRQ_OFFSET)
> -
> -/*
> - * Are we in NMI context?
> - */
> -#define in_nmi()       (preempt_count() & NMI_MASK)
> +#define in_nmi()               (preempt_count() & NMI_MASK)
> +#define in_task()              (!(preempt_count() & \
> +                                  (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))

LGTM.

For what it's worth, I think ARM recently started saving the address
limit and resetting it to USER_DS on NMI entry.

--Andy