pipe: Fix missing lock in pipe_resize_ring()

pipe_resize_ring() needs to take the pipe->rd_wait.lock spinlock to
prevent post_one_notification() from trying to insert into the ring
whilst the ring is being replaced.

The occupancy check must be done after the lock is taken, and the lock
must be taken after the new ring is allocated.

The bug can lead to an oops looking something like:

 BUG: KASAN: use-after-free in post_one_notification.isra.0+0x62e/0x840
 Read of size 4 at addr ffff88801cc72a70 by task poc/27196
 ...
 Call Trace:
  post_one_notification.isra.0+0x62e/0x840
  __post_watch_notification+0x3b7/0x650
  key_create_or_update+0xb8b/0xd20
  __do_sys_add_key+0x175/0x340
  __x64_sys_add_key+0xbe/0x140
  do_syscall_64+0x5c/0xc0
  entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Selim Enes Karaduman @Enesdex working with Trend Micro Zero
Day Initiative.

Fixes: c73be61 ("pipe: Add general notification queue support")
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-17291
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: dhowells

fs/pipe.c

@@ -1245,30 +1245,33 @@ unsigned int round_pipe_size(unsigned long size)
 
 /*
  * Resize the pipe ring to a number of slots.
+ *
+ * Note the pipe can be reduced in capacity, but only if the current
+ * occupancy doesn't exceed nr_slots; if it does, EBUSY will be
+ * returned instead.
  */
 int pipe_resize_ring(struct pipe_inode_info *pipe, unsigned int nr_slots)
 {
 	struct pipe_buffer *bufs;
 	unsigned int head, tail, mask, n;
 
-	/*
-	 * We can shrink the pipe, if arg is greater than the ring occupancy.
-	 * Since we don't expect a lot of shrink+grow operations, just free and
-	 * allocate again like we would do for growing.  If the pipe currently
-	 * contains more buffers than arg, then return busy.
-	 */
-	mask = pipe->ring_size - 1;
-	head = pipe->head;
-	tail = pipe->tail;
-	n = pipe_occupancy(pipe->head, pipe->tail);
-	if (nr_slots < n)
-		return -EBUSY;
-
 	bufs = kcalloc(nr_slots, sizeof(*bufs),
 		       GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
 	if (unlikely(!bufs))
 		return -ENOMEM;
 
+	spin_lock_irq(&pipe->rd_wait.lock);
+	mask = pipe->ring_size - 1;
+	head = pipe->head;
+	tail = pipe->tail;
+
+	n = pipe_occupancy(head, tail);
+	if (nr_slots < n) {
+		spin_unlock_irq(&pipe->rd_wait.lock);
+		kfree(bufs);
+		return -EBUSY;
+	}
+
 	/*
 	 * The pipe array wraps around, so just start the new one at zero
 	 * and adjust the indices.
@@ -1300,6 +1303,8 @@ int pipe_resize_ring(struct pipe_inode_info *pipe, unsigned int nr_slots)
 	pipe->tail = tail;
 	pipe->head = head;
 
+	spin_unlock_irq(&pipe->rd_wait.lock);
+
 	/* This might have made more room for writers */
 	wake_up_interruptible(&pipe->wr_wait);
 	return 0;