Re: [PATCH] fuse: Fix a potential double free in virtio_fs_get_tree

[Connor Kuehl]

On 3/23/21 12:18 AM, Lv Yunlong wrote:
In virtio_fs_get_tree, fm is allocated by kzalloc() and
assigned to fsc->s_fs_info by fsc->s_fs_info=fm statement.
If the kzalloc() failed, it will goto err directly, so that

Right, I follow this so far.

fsc->s_fs_info must be non-NULL and fm will be freed.

But this I don't follow in the context of the stuff that happens in out_err.

But later fm is freed again when virtio_fs_fill_super() fialed.
I think the statement if (fsc->s_fs_info) {kfree(fm);} is
misplaced.

I'm not sure this can double free, because:

* If fm = kzalloc[..] fails, the function bails early.

* If sget_fc() fails, the function cleans up fm and fc and bails early.

* If sget_fc() succeeds and allocated a new superblock, fc->s_fs_info 
pointer is moved to sb->s_fs_info and fc->s_fs_info is set to NULL, so 
the first free hasn't happened yet.

* If sget_fc() succeeds and somehow returns an existing superblock 
(which I think is tested by checking if fc->s_fs_info is not NULL, since 
otherwise it'd have been moved to the superblock and set to NULL in 
sget_fc), I think sb->s_root would not be NULL, therefore the flow of 
control wouldn't enter the if-block where virtio_fs_fill_super could 
fail which means the code won't reach the double free.

That's just my reading of it though, and I'm wondering if that makes 
sense to others :-)

One last comment inline:

My patch puts this statement in the correct palce to avoid
double free.

Signed-off-by: Lv Yunlong <lyl2...@mail.ustc.edu.cn>
---
  fs/fuse/virtio_fs.c | 10 ++++++----
  1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 8868ac31a3c0..727cf436828f 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -1437,10 +1437,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
  
  	fsc->s_fs_info = fm;
        sb = sget_fc(fsc, virtio_fs_test_super, set_anon_super_fc);
-       if (fsc->s_fs_info) {
-               fuse_conn_put(fc);
-               kfree(fm);
-       }
+
        if (IS_ERR(sb))
                return PTR_ERR(sb);

By removing the check from here, it now looks like if sget_fc() fails, 
then this early return will leak fm's memory and fc's reference.

Connor

  
@@ -1457,6 +1454,11 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
                sb->s_flags |= SB_ACTIVE;
        }
  
+	if (fsc->s_fs_info) {
+               fuse_conn_put(fc);
+               kfree(fm);
+       }
+
        WARN_ON(fsc->root);
        fsc->root = dget(sb->s_root);
        return 0;