Re: [RFC PATCH v2 1/2] uaccess: Check no rescheduling function is called in unsafe region

[Valentin Schneider]

Hi,

On 03/12/2018 13:55, Julien Thierry 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.
> 
> Also, add a comment clarifying the behaviour of user_access regions.
> 
> Signed-off-by: Julien Thierry <julien.thierry@xxxxxxx>
> ---
>  include/linux/kernel.h  |  6 ++++--
>  include/linux/uaccess.h | 11 +++++++++++
>  kernel/sched/core.c     | 19 +++++++++++++++++++
>  3 files changed, 34 insertions(+), 2 deletions(-)
> 
> I'm not sure these are the best locations to check this but I was hoping
> this patch could start the discussion.
> 
> Should I move the check? Should I add a config option to conditionally
> build those checks?
> 

I was going to say it's already under DEBUG_ATOMIC_SLEEP, but that's only
true for the __might_sleep() bit actually.

I think it'd make sense to blanket that under a config, but using
DEBUG_ATOMIC_SLEEP for that is a bit too much. What about a
DEBUG_UACCESS_SLEEP?

> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index d6aac75..fe0e984 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -237,11 +237,13 @@
>  struct pt_regs;
>  struct user;
> 
> +extern void __might_resched(const char *file, int line);
>  #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 efe79c1..50adb84 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -266,6 +266,13 @@ 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.
> + * During execution of this region, no sleeping functions should be called.
> + * Exceptions and interrupt shall exit the user_access region and re-enter it
> + * when returning to the interrupted context.
> + */

I would first have the bit about exceptions, then mention sleeping and add
something along the lines of

"[...] no sleeping functions should be called - we rely on exception
handling to take care of the user_access status for us, but that doesn't
happen when directly calling schedule()."

My wording's not the best but I just want something to point out *why*
sleeping ain't okay.

>  #ifndef user_access_begin
>  #define user_access_begin() do { } while (0)
>  #define user_access_end() do { } while (0)
> @@ -273,6 +280,10 @@ 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 6fedf3a..03f53c8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3289,6 +3289,13 @@ static inline void schedule_debug(struct task_struct *prev)
>  		__schedule_bug(prev);
>  		preempt_count_set(PREEMPT_DISABLED);
>  	}
> +
> +	if (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 +6158,18 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
>  EXPORT_SYMBOL(___might_sleep);
>  #endif
> 
> +void __might_resched(const char *file, int line)
> +{
> +	if (!unsafe_user_region_active())
> +		return;
> +
> +	printk(KERN_ERR
> +		"BUG: rescheduling function called from user access context at %s:%d\n",
> +			file, line);
> +	dump_stack();
> +}

So this check is "careful, things might go bad" and the schedule_debug()
one is "things went bad". IIUC we'll always get this warning when we hit
the schedule_debug() one. I was going to suggest only keeping one of them,
but I think both hold value.

> +EXPORT_SYMBOL(__might_resched);
> +
>  #ifdef CONFIG_MAGIC_SYSRQ
>  void normalize_rt_tasks(void)
>  {
> --
> 1.9.1
>