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.
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 <[email protected]>
---
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
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 <[email protected]>
> ---
> 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;
>
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 <[email protected]>
> ---
> 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
>
>
> -----原始邮件-----
> 发件人: "Vivek Goyal" <[email protected]>
> 发送时间: 2021-03-24 01:10:03 (星期三)
> 收件人: "Lv Yunlong" <[email protected]>
> 抄送: [email protected], [email protected], [email protected], [email protected], [email protected]
> 主题: Re: [PATCH] fuse: Fix a potential double free in virtio_fs_get_tree
>
> 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 <[email protected]>
> > ---
> > 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
> >
> >
>
Ok, thanks.
It should be a false positive.