[PATCH 5.9 58/74] fuse: fix page dereference after free

[Greg Kroah-Hartman]

From: Miklos Szeredi <mszer...@redhat.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 <p...@codeaurora.org>
Cc: <sta...@vger.kernel.org>
Signed-off-by: Miklos Szeredi <mszer...@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

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

--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -785,15 +785,16 @@ 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;
@@ -833,7 +834,7 @@ 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);
@@ -852,14 +853,19 @@ 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);
@@ -868,10 +874,10 @@ 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,
@@ -883,14 +889,16 @@ static int fuse_ref_page(struct fuse_cop
        if (cs->nr_segs >= cs->pipe->max_usage)
                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;