On Thu, 29 Aug 2024 11:24:36 +0800
Chi Zhiling <[email protected]> wrote:
From: Chi Zhiling <[email protected]>
We found a null pointer accessing in tracefs[1], the reason is that
the variable 'ei_child' is set to LIST_POISON1, that means the list
was removed in eventfs_remove_rec. so when access the ei_child->is_freed,
the panic triggered.
the linked list is protected by eventfs_mutex in eventfs_remove_rec,
Only writes of the link list is protected by the mutex. Reads are not.
so when we access the list of ei_child in eventfs_iterate, we also need
a mutex_lock in there to avoid eventfs_remove_rec modify the list.
Yes you hit a bug, but no this is *not* the solution!
Signed-off-by: Chi Zhiling <[email protected]>
---
fs/tracefs/event_inode.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 01e99e98457d..4895ed07376b 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -642,6 +642,7 @@ static int eventfs_iterate(struct file *file, struct
dir_context *ctx)
/* Subtract the skipped entries above */
c -= min((unsigned int)c, (unsigned int)ei->nr_entries);
+ mutex_unlock(&eventfs_mutex);
This list is protected by SRCU (hence the name of the iterator), if you
need to add a mutex around it, something else is broken.
list_for_each_entry_srcu(ei_child, &ei->children, list,
srcu_read_lock_held(&eventfs_srcu)) {
@@ -659,9 +660,12 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
ino = eventfs_dir_ino(ei_child);
- if (!dir_emit(ctx, name, strlen(name), ino, DT_DIR))
+ if (!dir_emit(ctx, name, strlen(name), ino, DT_DIR)) {
+ mutex_unlock(&eventfs_mutex);
goto out_dec;
+ }
}
+ mutex_unlock(&eventfs_mutex);
ret = 1;
out:
srcu_read_unlock(&eventfs_srcu, idx);
The real fix is:
diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 01e99e98457d..8705c77a9e75 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -862,7 +862,7 @@ static void eventfs_remove_rec(struct eventfs_inode *ei,
int level)
list_for_each_entry(ei_child, &ei->children, list)
eventfs_remove_rec(ei_child, level + 1);
- list_del(&ei->list);
+ list_del_rcu(&ei->list);
free_ei(ei);
}
Can you test that and let me know if it fixes your issue. I'll just go
ahead and apply it as it is an obvious bug.
