[PATCH v3 1/2] debugfs: fix use-after-free in debugfs_read_file_str()

[Aldo Ariel Panzardo <qwe.aldo@gmail.com>]

debugfs_write_file_str() publishes a new string pointer via
rcu_assign_pointer(), waits for a grace period with synchronize_rcu(),
then frees the old string.

debugfs_read_file_str() dereferences file->private_data without holding
an RCU read-side critical section: it loads the pointer, calls strlen()
on it, and then copies the string.  If a concurrent writer completes
synchronize_rcu() and kfree()s the old string between the load and the
use, the reader accesses freed memory.

Fix by wrapping the pointer dereference and copy inside
rcu_read_lock()/rcu_read_unlock(), using rcu_dereference() to annotate
the load.  Switch to kmemdup() with GFP_ATOMIC so the allocation stays
within the RCU critical section.

Tested on QEMU/KVM (7.3.0-rc4 + KASAN).  While KASAN did not
independently confirm this particular race window (it is a very narrow
reader-side window), the companion double-free in the write path
confirms that the RCU synchronization in this file is incomplete (see
the following patch).

Cc: stable@vger.kernel.org
Assisted-by: sashiko.dev <sashiko-bot@kernel.org>
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
---
 fs/debugfs/file.c | 25 +++++++++----------------
 1 file changed, 9 insertions(+), 16 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 08de6652a..ac4199e9f 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -1018,34 +1018,27 @@ ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
 			      size_t count, loff_t *ppos)
 {
 	struct dentry *dentry = F_DENTRY(file);
-	char *str, *copy = NULL;
-	int copy_len, len;
+	char *str, *copy;
+	int len;
 	ssize_t ret;
 
 	ret = debugfs_file_get(dentry);
 	if (unlikely(ret))
 		return ret;
 
-	str = *(char **)file->private_data;
+	rcu_read_lock();
+	str = rcu_dereference(*(char __rcu **)file->private_data);
 	len = strlen(str) + 1;
-	copy = kmalloc(len, GFP_KERNEL);
-	if (!copy) {
-		debugfs_file_put(dentry);
-		return -ENOMEM;
-	}
+	copy = kmemdup(str, len, GFP_ATOMIC);
+	rcu_read_unlock();
 
-	copy_len = strscpy(copy, str, len);
 	debugfs_file_put(dentry);
-	if (copy_len < 0) {
-		kfree(copy);
-		return copy_len;
-	}
-
-	copy[copy_len] = '\n';
+	if (!copy)
+		return -ENOMEM;
 
+	copy[len - 1] = '\n';
 	ret = simple_read_from_buffer(user_buf, count, ppos, copy, len);
 	kfree(copy);
-
 	return ret;
 }
 
-- 
2.43.0