[PATCH v4 1/2] debugfs: fix use-after-free in debugfs_read_file_str()

[Aldo Ariel Panzardo]

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 measuring the string length under rcu_read_lock(), allocating
with GFP_KERNEL outside the critical section, and then copying with
strscpy() under a second rcu_read_lock().  If the current string no
longer fits the allocated buffer, retry with a PAGE_SIZE allocation
which is the upper bound enforced by the write path.

Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: sashiko.dev <sashiko-bot@xxxxxxxxxx>
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@xxxxxxxxx>
---
 fs/debugfs/file.c | 39 +++++++++++++++++++++++++--------------
 1 file changed, 25 insertions(+), 14 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 08de6652a..f9a1f7739 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -1018,7 +1018,7 @@ 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;
+	char *str, *copy;
 	int copy_len, len;
 	ssize_t ret;
 
@@ -1026,26 +1026,37 @@ ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
 	if (unlikely(ret))
 		return ret;
 
-	str = *(char **)file->private_data;
-	len = strlen(str) + 1;
-	copy = kmalloc(len, GFP_KERNEL);
-	if (!copy) {
-		debugfs_file_put(dentry);
-		return -ENOMEM;
-	}
+	len = 0;
+	for (;;) {
+		rcu_read_lock();
+		str = rcu_dereference(*(char __rcu **)file->private_data);
+		len = max_t(int, strlen(str) + 1, len);
+		rcu_read_unlock();
+
+		copy = kmalloc(len, GFP_KERNEL);
+		if (!copy) {
+			debugfs_file_put(dentry);
+			return -ENOMEM;
+		}
+
+		rcu_read_lock();
+		str = rcu_dereference(*(char __rcu **)file->private_data);
+		copy_len = strscpy(copy, str, len);
+		rcu_read_unlock();
+
+		if (copy_len >= 0)
+			break;
 
-	copy_len = strscpy(copy, str, len);
-	debugfs_file_put(dentry);
-	if (copy_len < 0) {
 		kfree(copy);
-		return copy_len;
+		len = PAGE_SIZE;
 	}
 
-	copy[copy_len] = '\n';
+	debugfs_file_put(dentry);
 
+	copy[copy_len] = '\n';
+	len = copy_len + 1;
 	ret = simple_read_from_buffer(user_buf, count, ppos, copy, len);
 	kfree(copy);
-
 	return ret;
 }
 
-- 
2.43.0