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

Ingo Molnar
2019-02-11T13:45:35Z

* Julien Thierry <julien.thierry@arm.com> 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 <julien.thierry@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
>
> ---
> 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