* Re: [PATCH] fuse: Fix a potential double free in virtio_fs_get_tree
[not found] <20210323051831.13575-1-lyl2019@mail.ustc.edu.cn>
@ 2021-03-23 14:33 ` Connor Kuehl
2021-03-23 17:10 ` Vivek Goyal
1 sibling, 0 replies; 2+ messages in thread
From: Connor Kuehl @ 2021-03-23 14:33 UTC (permalink / raw)
To: Lv Yunlong, vgoyal, stefanha, miklos
Cc: linux-fsdevel, linux-kernel, virtualization
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 <lyl2019@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;
>
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^ permalink raw reply [flat|nested] 2+ messages in thread* Re: [PATCH] fuse: Fix a potential double free in virtio_fs_get_tree
[not found] <20210323051831.13575-1-lyl2019@mail.ustc.edu.cn>
2021-03-23 14:33 ` [PATCH] fuse: Fix a potential double free in virtio_fs_get_tree Connor Kuehl
@ 2021-03-23 17:10 ` Vivek Goyal
1 sibling, 0 replies; 2+ messages in thread
From: Vivek Goyal @ 2021-03-23 17:10 UTC (permalink / raw)
To: Lv Yunlong; +Cc: linux-fsdevel, virtualization, linux-kernel, stefanha, miklos
On Mon, Mar 22, 2021 at 10:18:31PM -0700, 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
> fsc->s_fs_info must be non-NULL and fm will be freed.
sget_fc() will either consume fsc->s_fs_info in case a new super
block is allocated and set fsc->s_fs_info. In that case we don't
free fc or fm.
Or, sget_fc() will return with fsc->s_fs_info set in case we already
found a super block. In that case we need to free fc and fm.
In case of error from sget_fc(), fc/fm need to be freed first and
then error needs to be returned to caller.
if (IS_ERR(sb))
return PTR_ERR(sb);
If we allocated a new super block in sget_fc(), then next step is
to initialize it.
if (!sb->s_root) {
err = virtio_fs_fill_super(sb, fsc);
}
If we run into errors here, then fc/fm need to be freed.
So current code looks fine to me.
Vivek
>
> 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.
>
> My patch puts this statement in the correct palce to avoid
> double free.
>
> Signed-off-by: Lv Yunlong <lyl2019@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);
>
> @@ -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;
> --
> 2.25.1
>
>
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^ permalink raw reply [flat|nested] 2+ messages in thread