fuse: fix page stealing

stable inclusion
from linux-4.19.219
commit 65f1f3eb09a3907c5f5edaff6c473b99e49e6020

--------------------------------

commit 712a9510 upstream.

It is possible to trigger a crash by splicing anon pipe bufs to the fuse
device.

The reason for this is that anon_pipe_buf_release() will reuse buf->page if
the refcount is 1, but that page might have already been stolen and its
flags modified (e.g. PG_lru added).

This happens in the unlikely case of fuse_dev_splice_write() getting around
to calling pipe_buf_release() after a page has been stolen, added to the
page cache and removed from the page cache.

Fix by calling pipe_buf_release() right after the page was inserted into
the page cache.  In this case the page has an elevated refcount so any
release function will know that the page isn't reusable.
Reported-by: NFrank Dinoff <fdinoff@google.com>
Link: https://lore.kernel.org/r/CAAmZXrsGg2xsP1CK+cbuEMumtrqdvD-NKnWzhNcvn71RV3c1yw@mail.gmail.com/
Fixes: dd3bb14f ("fuse: support splice() writing to fuse device")
Cc: <stable@vger.kernel.org> # v2.6.35
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/fuse/dev.c

@@ -905,6 +905,12 @@ static int fuse_try_move_page(struct fuse_copy_state *cs, struct page **pagep)
 		goto out_put_old;
 	}
 
+	/*
+	 * Release while we have extra ref on stolen page.  Otherwise
+	 * anon_pipe_buf_release() might think the page can be reused.
+	 */
+	pipe_buf_release(cs->pipe, buf);
+
 	get_page(newpage);
 
 	if (!(buf->flags & PIPE_BUF_FLAG_LRU))
@@ -2054,8 +2060,12 @@ static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
 
 	pipe_lock(pipe);
 out_free:
-	for (idx = 0; idx < nbuf; idx++)
-		pipe_buf_release(pipe, &bufs[idx]);
+	for (idx = 0; idx < nbuf; idx++) {
+		struct pipe_buffer *buf = &bufs[idx];
+
+		if (buf->ops)
+			pipe_buf_release(pipe, buf);
+	}
 	pipe_unlock(pipe);
 
 	kvfree(bufs);