Re: [PATCH v3] Bluetooth: btmtk: Fix null pointer when processing coredump

[Paul Menzel <pmenzel@xxxxxxxxxxxxx>]

Dear Chris,


Am 12.07.23 um 10:53 schrieb Chris Lu (陸稚泓):
> On Wed, 2023-07-12 at 08:11 +0200, Paul Menzel wrote:
> >   	
> > External email : Please do not click links or open attachments until
> > you have verified the sender or the content.

(It’d be nice if you removed such (automatically added) phrases from 
your reply.)

> Thanks for your review and feedback to Mediatek's Bluetooth driver
> code.

Thank you for your reply.

> > Am 12.07.23 um 07:18 schrieb Chris Lu:
> > > There may be a potential null pointer risk if offset value is
> > > less than 0 when doing memcmp in btmtk_process_coredump().
> > > Checking offset is valid before doing memcmp.
> >
> > Use imperative mood: Check offset …
> >
> > > Signed-off-by: Chris Lu <chris.lu@xxxxxxxxxxxx>
> > > Co-developed-by: Sean Wang <sean.wang@xxxxxxxxxxxx>
> > > Signed-off-by: Sean Wang <sean.wang@xxxxxxxxxxxx>
> > > ---
> > > v2: fix typo
> > > v3: fix bot checking error
> > > ---
> > >    drivers/bluetooth/btmtk.c | 16 ++++++++--------
> > >    1 file changed, 8 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drivers/bluetooth/btmtk.c b/drivers/bluetooth/btmtk.c
> > > index 786f775196ae..0f290430ae0e 100644
> > > --- a/drivers/bluetooth/btmtk.c
> > > +++ b/drivers/bluetooth/btmtk.c
> > > @@ -370,7 +370,7 @@ EXPORT_SYMBOL_GPL(btmtk_register_coredump);
> > >    int btmtk_process_coredump(struct hci_dev *hdev, struct sk_buff *skb)
> > >    {
> > >    struct btmediatek_data *data = hci_get_priv(hdev);
> > > -int err;
> > > +int err, offset;
> > >    
> > >    if (!IS_ENABLED(CONFIG_DEV_COREDUMP))
> > >    return 0;
> > > @@ -392,15 +392,15 @@ int btmtk_process_coredump(struct hci_dev
> > *hdev, struct sk_buff *skb)
> > >    if (err < 0)
> > >    break;
> > >    data->cd_info.cnt++;
> > > +offset = skb->len - sizeof(MTK_COREDUMP_END);
> >
> > For `sizeof()` shouldn’t you use `size_t`? But that is unsigned of
> > course. Maybe ssize_t then?
>
> yes, it's better to use ssize_t or size_t, I'll change declaratins of
> offset from int to ssize_t.
>
> > >    
> > >    /* Mediatek coredump data would be more than MTK_COREDUMP_NUM */
> > > -if (data->cd_info.cnt > MTK_COREDUMP_NUM &&
> > > -    skb->len > sizeof(MTK_COREDUMP_END) &&
> > > -    !memcmp((char *)&skb->data[skb->len - sizeof(MTK_COREDUMP_END)],
> > > -    MTK_COREDUMP_END, sizeof(MTK_COREDUMP_END) - 1)) {
> > > -bt_dev_info(hdev, "Mediatek coredump end");
> > > -hci_devcd_complete(hdev);
> > > -}
> > > +if (data->cd_info.cnt > MTK_COREDUMP_NUM && offset > 0)
> >
> > Why not keep it like before, and just add the condition `skb->len <
> > sizeof(MTK_COREDUMP_END)`? The compiler is probably going to optimize
> > so the value is not calculated twice.

> The reason why I send this patch is when I backport devcoredump feature
> to specific project with older kernel version, the compiler might not
> so optimized that it would cause kernel panic when run into memcmp.
> As a result, make sure `skb->len > sizeof(MTK_COREDUMP_END) ` before
> doing memcmp part can avoid null pointer issue.
> Besides, only in condition 'data->cd_info.cnt > MTK_COREDUMP_NUM &&
> offset > 0' need to do memcmp to check the end of coredump. Driver do
> noting with condition `skb->len < sizeof(MTK_COREDUMP_END) ` that
> additional condiction is not really necessary.

Just to avoid misunderstandings, my point, to add the comparison and get 
rid of the variable `offset`.


Kind regards,

Paul


> > > +if (!memcmp((char *)&skb->data[offset], MTK_COREDUMP_END,
> > > +    sizeof(MTK_COREDUMP_END) - 1)) {
> > > +bt_dev_info(hdev, "Mediatek coredump end");
> > > +hci_devcd_complete(hdev);
> > > +}
> > >    
> > >    break;
> > >    }