[PATCH 6.4 737/800] watch_queue: prevent dangling pipe pointer

Greg Kroah-Hartman
2023-07-16T20:27:18Z

From: Siddh Raman Pant <code@siddh.me>

commit 943211c87427f25bd22e0e63849fb486bb5f87fa upstream.

NULL the dangling pipe reference while clearing watch_queue.

If not done, a reference to a freed pipe remains in the watch_queue,
as this function is called before freeing a pipe in free_pipe_info()
(see line 834 of fs/pipe.c).

The sole use of wqueue->defunct is for checking if the watch queue has
been cleared, but wqueue->pipe is also NULLed while clearing.

Thus, wqueue->defunct is superfluous, as wqueue->pipe can be checked
for NULL. Hence, the former can be removed.

Tested with keyutils testsuite.

Cc: stable@vger.kernel.org # 6.1
Signed-off-by: Siddh Raman Pant <code@siddh.me>
Acked-by: David Howells <dhowells@redhat.com>
Message-Id: <20230605143616.640517-1-code@siddh.me>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/watch_queue.h | 3 +--
kernel/watch_queue.c | 12 ++++++------
2 files changed, 7 insertions(+), 8 deletions(-)

--- a/include/linux/watch_queue.h
+++ b/include/linux/watch_queue.h
@@ -38,7 +38,7 @@ struct watch_filter {
struct watch_queue {
struct rcu_head rcu;
struct watch_filter __rcu *filter;
- struct pipe_inode_info *pipe; /* The pipe we're using as a buffer */
+ struct pipe_inode_info *pipe; /* Pipe we use as a buffer, NULL if queue closed */
struct hlist_head watches; /* Contributory watches */
struct page **notes; /* Preallocated notifications */
unsigned long *notes_bitmap; /* Allocation bitmap for notes */
@@ -46,7 +46,6 @@ struct watch_queue {
spinlock_t lock;
unsigned int nr_notes; /* Number of notes */
unsigned int nr_pages; /* Number of pages in notes[] */
- bool defunct; /* T when queues closed */
};

/*
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -42,7 +42,7 @@ MODULE_AUTHOR("Red Hat, Inc.");
static inline bool lock_wqueue(struct watch_queue *wqueue)
{
spin_lock_bh(&wqueue->lock);
- if (unlikely(wqueue->defunct)) {
+ if (unlikely(!wqueue->pipe)) {
spin_unlock_bh(&wqueue->lock);
return false;
}
@@ -104,9 +104,6 @@ static bool post_one_notification(struct
unsigned int head, tail, mask, note, offset, len;
bool done = false;

- if (!pipe)
- return false;
-
spin_lock_irq(&pipe->rd_wait.lock);

mask = pipe->ring_size - 1;
@@ -603,8 +600,11 @@ void watch_queue_clear(struct watch_queu
rcu_read_lock();
spin_lock_bh(&wqueue->lock);

- /* Prevent new notifications from being stored. */
- wqueue->defunct = true;
+ /*
+ * This pipe can be freed by callers like free_pipe_info().
+ * Removing this reference also prevents new notifications.
+ */
+ wqueue->pipe = NULL;

while (!hlist_empty(&wqueue->watches)) {
watch = hlist_entry(wqueue->watches.first, struct watch, queue_node);