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

Greg Kroah-Hartman
2020-10-31T11:46:40Z

From: Miklos Szeredi <mszeredi@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 <ppvk@codeaurora.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@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;