linux-kernel - Re: [PATCH] misc: bcm_vk: Fix possible null-pointer dereferences in bcm_vk

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Message-ID: <CADm8TemcWPmEQvAvPAD-rLWwR6RbH+14NryMuxAA=JwpByU=9w@mail.gmail.com>
Date: Fri, 12 Dec 2025 07:42:15 +0800
From: Tuo Li <islituo@...il.com>
To: Scott Branden <scott.branden@...adcom.com>
Cc: bcm-kernel-feedback-list@...adcom.com, arnd@...db.de, 
	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] misc: bcm_vk: Fix possible null-pointer dereferences in bcm_vk_read()

Hi Scott,

Thanks for your reply!

On Fri, Dec 12, 2025 at 3:18 AM Scott Branden
<scott.branden@...adcom.com> wrote:
>
> Hi Tuo,
>
> On Wed, Dec 10, 2025 at 10:36 PM Tuo Li <islituo@...il.com> 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@...il.com>
> > ---
> >  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