[PATCH v5 05/11] tracing/user_events: Add ioctl for disabling addresses

Beau Belgrave
2022-12-05T21:00:44Z

Enablements are now tracked by the lifetime of the task/mm. User
processes need to be able to disable their addresses if tracing is
requested to be turned off. Before unmapping the page would suffice.
However, we now need a stronger contract. Add an ioctl to enable this.

A new flag bit is added, freeing, to user_event_enabler to ensure that
if the event is attempted to be removed while a fault is being handled
that the remove is delayed until after the fault is reattempted.

Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
---
include/uapi/linux/user_events.h | 24 +++++++++
kernel/trace/trace_events_user.c | 93 +++++++++++++++++++++++++++++++-
2 files changed, 115 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/user_events.h b/include/uapi/linux/user_events.h
index 5bee4201dad0..833f1f8a8a4a 100644
--- a/include/uapi/linux/user_events.h
+++ b/include/uapi/linux/user_events.h
@@ -46,6 +46,27 @@ struct user_reg {
__u32 write_index;
} __attribute__((__packed__));

+/*
+ * Describes an event unregister, callers must set the size, address and bit.
+ * This structure is passed to the DIAG_IOCSUNREG ioctl to disable bit updates.
+ */
+struct user_unreg {
+ /* Input: Size of the user_unreg structure being used */
+ __u32 size;
+
+ /* Input: Bit to unregister */
+ __u8 disable_bit;
+
+ /* Input: Reserved, set to 0 */
+ __u8 __reserved;
+
+ /* Input: Reserved, set to 0 */
+ __u16 __reserved2;
+
+ /* Input: Address to unregister */
+ __u64 disable_addr;
+} __attribute__((__packed__));
+
#define DIAG_IOC_MAGIC '*'

/* Requests to register a user_event */
@@ -54,4 +75,7 @@ struct user_reg {
/* Requests to delete a user_event */
#define DIAG_IOCSDEL _IOW(DIAG_IOC_MAGIC, 1, char*)

+/* Requests to unregister a user_event */
+#define DIAG_IOCSUNREG _IOW(DIAG_IOC_MAGIC, 2, struct user_unreg*)
+
#endif /* _UAPI_LINUX_USER_EVENTS_H */
diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 5c061a54086f..ebbde72e1f85 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -102,6 +102,9 @@ struct user_event_enabler {
/* Bit 6 is for faulting status of enablement */
#define ENABLE_VAL_FAULTING_BIT 6

+/* Bit 7 is for freeing status of enablement */
+#define ENABLE_VAL_FREEING_BIT 7
+
/* Only duplicate the bit value */
#define ENABLE_VAL_DUP_MASK ENABLE_VAL_BIT_MASK

@@ -301,6 +304,12 @@ static void user_event_enabler_fault_fixup(struct work_struct *work)
/* Prevent state changes from racing */
mutex_lock(&event_mutex);

+ /* User asked for enabler to be removed during fault */
+ if (test_bit(ENABLE_VAL_FREEING_BIT, ENABLE_BITOPS(enabler))) {
+ user_event_enabler_destroy(enabler);
+ goto out;
+ }
+
/*
* If we managed to get the page, re-issue the write. We do not
* want to get into a possible infinite loop, which is why we only
@@ -315,7 +324,7 @@ static void user_event_enabler_fault_fixup(struct work_struct *work)
user_event_enabler_write(mm, enabler, true);
mmap_read_unlock(mm->mm);
}
-
+out:
mutex_unlock(&event_mutex);

/* In all cases we no longer need the mm or fault */
@@ -370,7 +379,8 @@ static int user_event_enabler_write(struct user_event_mm *mm,
if (refcount_read(&mm->tasks) == 0)
return -ENOENT;

- if (unlikely(test_bit(ENABLE_VAL_FAULTING_BIT, ENABLE_BITOPS(enabler))))
+ if (unlikely(test_bit(ENABLE_VAL_FAULTING_BIT, ENABLE_BITOPS(enabler)) ||
+ test_bit(ENABLE_VAL_FREEING_BIT, ENABLE_BITOPS(enabler))))
return -EBUSY;

ret = pin_user_pages_remote(mm->mm, uaddr, 1, FOLL_WRITE | FOLL_NOFAULT,
@@ -428,6 +438,10 @@ static bool user_event_enabler_dup(struct user_event_enabler *orig,
{
struct user_event_enabler *enabler;

+ /* Skip pending frees */
+ if (unlikely(test_bit(ENABLE_VAL_FREEING_BIT, ENABLE_BITOPS(orig))))
+ return true;
+
enabler = kzalloc(sizeof(*enabler), GFP_NOWAIT);

if (!enabler)
@@ -2069,6 +2083,75 @@ static long user_events_ioctl_del(struct user_event_file_info *info,
return ret;
}

+static long user_unreg_get(struct user_unreg __user *ureg,
+ struct user_unreg *kreg)
+{
+ u32 size;
+ long ret;
+
+ ret = get_user(size, &ureg->size);
+
+ if (ret)
+ return ret;
+
+ if (size > PAGE_SIZE)
+ return -E2BIG;
+
+ if (size < offsetofend(struct user_unreg, disable_addr))
+ return -EINVAL;
+
+ ret = copy_struct_from_user(kreg, sizeof(*kreg), ureg, size);
+
+ return ret;
+}
+
+/*
+ * Unregisters an enablement address/bit within a task/user mm.
+ */
+static long user_events_ioctl_unreg(unsigned long uarg)
+{
+ struct user_unreg __user *ureg = (struct user_unreg __user *)uarg;
+ struct user_event_mm *mm = current->user_event_mm;
+ struct user_event_enabler *enabler, *next;
+ struct user_unreg reg;
+ long ret;
+
+ ret = user_unreg_get(ureg, &reg);
+
+ if (ret)
+ return ret;
+
+ if (!mm)
+ return -ENOENT;
+
+ ret = -ENOENT;
+
+ /*
+ * Flags freeing and faulting are used to indicate if the enabler is in
+ * use at all. When faulting is set a page-fault is occurring asyncly.
+ * During async fault if freeing is set, the enabler will be destroyed.
+ * If no async fault is happening, we can destroy it now since we hold
+ * the event_mutex during these checks.
+ */
+ mutex_lock(&event_mutex);
+
+ list_for_each_entry_safe(enabler, next, &mm->enablers, link)
+ if (enabler->addr == reg.disable_addr &&
+ (enabler->values & ENABLE_VAL_BIT_MASK) == reg.disable_bit) {
+ set_bit(ENABLE_VAL_FREEING_BIT, ENABLE_BITOPS(enabler));
+
+ if (!test_bit(ENABLE_VAL_FAULTING_BIT, ENABLE_BITOPS(enabler)))
+ user_event_enabler_destroy(enabler);
+
+ /* Removed at least one */
+ ret = 0;
+ }
+
+ mutex_unlock(&event_mutex);
+
+ return ret;
+}
+
/*
* Handles the ioctl from user mode to register or alter operations.
*/
@@ -2091,6 +2174,12 @@ static long user_events_ioctl(struct file *file, unsigned int cmd,
ret = user_events_ioctl_del(info, uarg);
mutex_unlock(&group->reg_mutex);
break;
+
+ case DIAG_IOCSUNREG:
+ mutex_lock(&group->reg_mutex);
+ ret = user_events_ioctl_unreg(uarg);
+ mutex_unlock(&group->reg_mutex);
+ break;
}

return ret;
--
2.25.1