Re: [PATCH] misc: bcm_vk: Fix possible null-pointer dereferences in bcm_vk_read()

[Scott Branden]

Hi Tuo,

On Thu, Dec 11, 2025 at 3:42 PM Tuo Li <islituo@xxxxxxxxx> wrote:
>
> Hi Scott,
>
> Thanks for your reply!
>
> On Fri, Dec 12, 2025 at 3:18 AM Scott Branden
> <scott.branden@xxxxxxxxxxxx> wrote:
> >
> > Hi Tuo,
> >
> > On Wed, Dec 10, 2025 at 10:36 PM Tuo Li <islituo@xxxxxxxxx> wrote:
> > >
> > > In the function bcm_vk_read(), the pointer entry is checked, indicating
> > > that it can be NULL. If entry is NULL and rc is set to -EMSGSIZE, the
> > > following code may cause null-pointer dereferences:
> > >
> > >   struct vk_msg_blk tmp_msg = entry->to_h_msg[0];
> > >   set_msg_id(&tmp_msg, entry->usr_msg_id);
> > >   tmp_msg.size = entry->to_h_blks - 1;
> > >
> > > To prevent these possible null-pointer dereferences, copy to_h_msg,
> > > usr_msg_id, and to_h_blks from iter into temporary variables, and return
> > > these temporary variables to the application instead of accessing them
> > > through a potentially NULL entry.
> > >
> > > Signed-off-by: Tuo Li <islituo@xxxxxxxxx>
> > > ---
> > >  drivers/misc/bcm-vk/bcm_vk_msg.c | 12 ++++++++----
> > >  1 file changed, 8 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/misc/bcm-vk/bcm_vk_msg.c b/drivers/misc/bcm-vk/bcm_vk_msg.c
> > > index 1f42d1d5a630..665a3888708a 100644
> > > --- a/drivers/misc/bcm-vk/bcm_vk_msg.c
> > > +++ b/drivers/misc/bcm-vk/bcm_vk_msg.c
> > > @@ -1010,6 +1010,9 @@ ssize_t bcm_vk_read(struct file *p_file,
> > >         struct device *dev = &vk->pdev->dev;
> > >         struct bcm_vk_msg_chan *chan = &vk->to_h_msg_chan;
> > >         struct bcm_vk_wkent *entry = NULL, *iter;
> > > +       struct vk_msg_blk tmp_msg;
> > > +       u32 tmp_usr_msg_id;
> > > +       u32 tmp_blks;
> > >         u32 q_num;
> > >         u32 rsp_length;
> > >
> > > @@ -1034,6 +1037,9 @@ ssize_t bcm_vk_read(struct file *p_file,
> > >                                         entry = iter;
> > >                                 } else {
> > >                                         /* buffer not big enough */
> > > +                                       tmp_msg = iter->to_h_msg[0];
> > > +                                       tmp_usr_msg_id = iter->usr_msg_id;
> > > +                                       tmp_blks = iter->to_h_blks;
> > >                                         rc = -EMSGSIZE;
> > >                                 }
> > >                                 goto read_loop_exit;
> > > @@ -1052,14 +1058,12 @@ ssize_t bcm_vk_read(struct file *p_file,
> > >
> > >                 bcm_vk_free_wkent(dev, entry);
> > >         } else if (rc == -EMSGSIZE) {
> > > -               struct vk_msg_blk tmp_msg = entry->to_h_msg[0];
> > > -
> > >                 /*
> > >                  * in this case, return just the first block, so
> > >                  * that app knows what size it is looking for.
> > >                  */
> > > -               set_msg_id(&tmp_msg, entry->usr_msg_id);
> > > -               tmp_msg.size = entry->to_h_blks - 1;
> > You can just change entry to iter on these 2 lines here?
> >
> > > +               set_msg_id(&tmp_msg, tmp_usr_msg_id);
> > > +               tmp_msg.size = tmp_blks - 1;
> > >                 if (copy_to_user(buf, &tmp_msg, VK_MSGQ_BLK_SIZE) != 0) {
> > >                         dev_err(dev, "Error return 1st block in -EMSGSIZE\n");
> > >                         rc = -EFAULT;
> > > --
> > > 2.43.0
> > >
>
> Thanks for the suggestion! My understanding is that you are suggesting
> replacing entry with iter in the rc == -EMSGSIZE path. However, I am not
> sure whether the objects referenced by iter can be modified or freed by
> other threads, since this path runs outside the protection of chan->pendq
> lock. If that can happen, accessing the objects via iter may lead to a data
> race or a use-after-free.
>
> Could you please help confirm whether this scenario is possible? If my
> concern is unfounded, I will prepare a v2 patch and replace entry with
> iter as you suggested.
>
> Thanks again for your time and helpful feedback!
>
> Sincerely,
> Tuo Li

Portions of your response sound like it is coming from an AI tool.
Are you using such?
I am just commenting on your patch.
Could you please examine the code to answer your questions?

Regards,
Scott

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