Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region

From: Ingo Molnar <[email-masked]>
Date: Mon, 11 Feb 2019 14:45:27 +0100

* Julien Thierry <[email-masked]> wrote:

> While running a user_access regions, it is not supported to reschedule.
> Add an overridable primitive to indicate whether a user_access region is
> active and check that this is not the case when calling rescheduling
> functions.
> 
> These checks are only performed when DEBUG_UACCESS_SLEEP is selected.
> 
> Also, add a comment clarifying the behaviour of user_access regions.
> 
> Signed-off-by: Julien Thierry <[email-masked]>
> Cc: Ingo Molnar <[email-masked]>
> Cc: Peter Zijlstra <[email-masked]>
> 
> ---
>  include/linux/kernel.h  | 11 +++++++++--
>  include/linux/uaccess.h | 13 +++++++++++++
>  kernel/sched/core.c     | 22 ++++++++++++++++++++++
>  lib/Kconfig.debug       |  8 ++++++++
>  4 files changed, 52 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 8f0e68e..73f1f82 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -237,11 +237,18 @@
>  struct pt_regs;
>  struct user;
> 
> +#ifdef CONFIG_DEBUG_UACCESS_SLEEP
> +extern void __might_resched(const char *file, int line);
> +#else
> +#define __might_resched(file, line) do { } while (0)
> +#endif
> +
>  #ifdef CONFIG_PREEMPT_VOLUNTARY
>  extern int _cond_resched(void);
> -# define might_resched() _cond_resched()
> +# define might_resched() \
> +	do { __might_resched(__FILE__, __LINE__); _cond_resched(); } while (0)
>  #else
> -# define might_resched() do { } while (0)
> +# define might_resched() __might_resched(__FILE__, __LINE__)
>  #endif
> 
>  #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 37b226e..2c0c39e 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -263,6 +263,15 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
>  #define probe_kernel_address(addr, retval)		\
>  	probe_kernel_read(&retval, addr, sizeof(retval))
> 
> +/*
> + * user_access_begin() and user_access_end() define a region where
> + * unsafe user accessors can be used. Exceptions and interrupt shall exit the
> + * user_access region and re-enter it when returning to the interrupted context.
> + *
> + * No sleeping function should get called during a user_access region - we rely
> + * on exception handling to take care of the user_access status for us, but that
> + * doesn't happen when directly calling schedule().
> + */
>  #ifndef user_access_begin
>  #define user_access_begin(ptr,len) access_ok(ptr, len)
>  #define user_access_end() do { } while (0)
> @@ -270,6 +279,10 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
>  #define unsafe_put_user(x, ptr, err) do { if (unlikely(__put_user(x, ptr))) goto err; } while (0)
>  #endif
> 
> +#ifndef unsafe_user_region_active
> +#define unsafe_user_region_active()	false
> +#endif
> +
>  #ifdef CONFIG_HARDENED_USERCOPY
>  void usercopy_warn(const char *name, const char *detail, bool to_user,
>  		   unsigned long offset, unsigned long len);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a674c7db..b1bb7e9 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3289,6 +3289,14 @@ static inline void schedule_debug(struct task_struct *prev)
>  		__schedule_bug(prev);
>  		preempt_count_set(PREEMPT_DISABLED);
>  	}
> +
> +	if (IS_ENABLED(CONFIG_DEBUG_UACCESS_SLEEP) &&
> +	    unlikely(unsafe_user_region_active())) {
> +		printk(KERN_ERR "BUG: scheduling while user_access enabled: %s/%d/0x%08x\n",
> +		       prev->comm, prev->pid, preempt_count());
> +		dump_stack();
> +	}
> +
>  	rcu_sleep_check();
> 
>  	profile_hit(SCHED_PROFILING, __builtin_return_address(0));
> @@ -6151,6 +6159,20 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
>  EXPORT_SYMBOL(___might_sleep);
>  #endif
> 
> +#ifdef CONFIG_DEBUG_UACCESS_SLEEP
> +void __might_resched(const char *file, int line)
> +{
> +	if (!unsafe_user_region_active())
> +		return;

Could you please more clearly explain why you want/need an exception from 
the __might_resched() debug warning?

Thanks,

	Ingo

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