fuse: fix page stealing

commit 712a951025c0667ff00b25afc360f74e639dfabe 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: Frank Dinoff <fdinoff@google.com>
Link: https://lore.kernel.org/r/CAAmZXrsGg2xsP1CK+cbuEMumtrqdvD-NKnWzhNcvn71RV3c1yw@mail.gmail.com/
Fixes: dd3bb14f44a6 ("fuse: support splice() writing to fuse device")
Cc: <stable@vger.kernel.org> # v2.6.35
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 38a12b0e395fd214fb73e267cd88e9693165d050..75fad41bc6e1446fca9b1a12f2462afab5b1b9d6 100644 (file)
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -922,6 +922,13 @@ static int fuse_try_move_page(struct fuse_copy_state *cs, struct page **pagep)
                return err;
        }
 
+       /*
+        * Release while we have extra ref on stolen page.  Otherwise
+        * anon_pipe_buf_release() might think the page can be reused.
+        */
+       buf->ops->release(cs->pipe, buf);
+       buf->ops = NULL;
+
        page_cache_get(newpage);
 
        if (!(buf->flags & PIPE_BUF_FLAG_LRU))
@@ -2090,7 +2097,8 @@ static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
 out_free:
        for (idx = 0; idx < nbuf; idx++) {
                struct pipe_buffer *buf = &bufs[idx];
-               buf->ops->release(pipe, buf);
+               if (buf->ops)
+                       buf->ops->release(pipe, buf);
        }
        pipe_unlock(pipe);