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Date:   Sat, 31 Oct 2020 12:35:29 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Pradeep P V K <ppvk@...eaurora.org>,
        Miklos Szeredi <mszeredi@...hat.com>
Subject: [PATCH 5.4 33/49] fuse: fix page dereference after free

From: Miklos Szeredi <mszeredi@...hat.com>

commit d78092e4937de9ce55edcb4ee4c5e3c707be0190 upstream.

After unlock_request() pages from the ap->pages[] array may be put (e.g. by
aborting the connection) and the pages can be freed.

Prevent use after free by grabbing a reference to the page before calling
unlock_request().

The original patch was created by Pradeep P V K.

Reported-by: Pradeep P V K <ppvk@...eaurora.org>
Cc: <stable@...r.kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/fuse/dev.c |   28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -786,15 +786,16 @@ static int fuse_try_move_page(struct fus
 	struct page *newpage;
 	struct pipe_buffer *buf = cs->pipebufs;
 
+	get_page(oldpage);
 	err = unlock_request(cs->req);
 	if (err)
-		return err;
+		goto out_put_old;
 
 	fuse_copy_finish(cs);
 
 	err = pipe_buf_confirm(cs->pipe, buf);
 	if (err)
-		return err;
+		goto out_put_old;
 
 	BUG_ON(!cs->nr_segs);
 	cs->currbuf = buf;
@@ -834,7 +835,7 @@ static int fuse_try_move_page(struct fus
 	err = replace_page_cache_page(oldpage, newpage, GFP_KERNEL);
 	if (err) {
 		unlock_page(newpage);
-		return err;
+		goto out_put_old;
 	}
 
 	get_page(newpage);
@@ -853,14 +854,19 @@ static int fuse_try_move_page(struct fus
 	if (err) {
 		unlock_page(newpage);
 		put_page(newpage);
-		return err;
+		goto out_put_old;
 	}
 
 	unlock_page(oldpage);
+	/* Drop ref for ap->pages[] array */
 	put_page(oldpage);
 	cs->len = 0;
 
-	return 0;
+	err = 0;
+out_put_old:
+	/* Drop ref obtained in this function */
+	put_page(oldpage);
+	return err;
 
 out_fallback_unlock:
 	unlock_page(newpage);
@@ -869,10 +875,10 @@ out_fallback:
 	cs->offset = buf->offset;
 
 	err = lock_request(cs->req);
-	if (err)
-		return err;
+	if (!err)
+		err = 1;
 
-	return 1;
+	goto out_put_old;
 }
 
 static int fuse_ref_page(struct fuse_copy_state *cs, struct page *page,
@@ -884,14 +890,16 @@ static int fuse_ref_page(struct fuse_cop
 	if (cs->nr_segs == cs->pipe->buffers)
 		return -EIO;
 
+	get_page(page);
 	err = unlock_request(cs->req);
-	if (err)
+	if (err) {
+		put_page(page);
 		return err;
+	}
 
 	fuse_copy_finish(cs);
 
 	buf = cs->pipebufs;
-	get_page(page);
 	buf->page = page;
 	buf->offset = offset;
 	buf->len = count;


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