io_uring: Fix registered ring file refcount leak

Currently, io_uring_unreg_ringfd() (which cleans up registered rings) is
only called on exit, but __io_uring_free (which frees the tctx in which the
registered ring pointers are stored) is also called on execve (via
begin_new_exec -> io_uring_task_cancel -> __io_uring_cancel ->
io_uring_cancel_generic -> __io_uring_free).

This means: A process going through execve while having registered rings
will leak references to the rings' `struct file`.

Fix it by zapping registered rings on execve(). This is implemented by
moving the io_uring_unreg_ringfd() from io_uring_files_cancel() into its
callee __io_uring_cancel(), which is called from io_uring_task_cancel() on
execve.

This could probably be exploited *on 32-bit kernels* by leaking 2^32
references to the same ring, because the file refcount is stored in a
pointer-sized field and get_file() doesn't have protection against
refcount overflow, just a WARN_ONCE(); but on 64-bit it should have no
impact beyond a memory leak.

Cc: stable@vger.kernel.org
Fixes: e7a6c00 ("io_uring: add support for registering ring file descriptors")
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20241218-uring-reg-ring-cleanup-v1-1-8f63e999045b@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: thejh

include/linux/io_uring.h

@@ -15,10 +15,8 @@ bool io_is_uring_fops(struct file *file);
 
 static inline void io_uring_files_cancel(void)
 {
-	if (current->io_uring) {
-		io_uring_unreg_ringfd();
+	if (current->io_uring)
 		__io_uring_cancel(false);
-	}
 }
 static inline void io_uring_task_cancel(void)
 {

io_uring/io_uring.c

@@ -3214,6 +3214,7 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
 
 void __io_uring_cancel(bool cancel_all)
 {
+	io_uring_unreg_ringfd();
 	io_uring_cancel_generic(cancel_all, NULL);
 }