Messages in this thread | | | Subject | Re: [RFC PATCH v2 1/2] uaccess: Check no rescheduling function is called in unsafe region | From | Julien Thierry <> | Date | Tue, 15 Jan 2019 11:48:41 +0000 |
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Hi,
On 14/01/2019 12:03, Valentin Schneider wrote: > 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@arm.com> >> --- >> 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? >
Yes, I was wondering whether to add something like that, I'll add a DEBUG_UACCESS_SLEEP to my next version.
>> 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. >
I think the wording is alright, I'll include your suggestion for the next version.
>> #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. >
Yes, I can't really convince myself to remove either, unless there is a magic place that covers both cases.
Thanks for the suggestions.
Cheers,
-- Julien Thierry
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