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

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.
+ */
#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();
+}
+EXPORT_SYMBOL(__might_resched);
+
#ifdef CONFIG_MAGIC_SYSRQ
void normalize_rt_tasks(void)
{
--
1.9.1
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