[PATCH net] net/sched: em_canid: add length check before reading CAN ID

[PATCH net] net/sched: em_canid: add length check before reading CAN ID

[ssrane_b23]

From: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>

Add a check to verify that the skb has at least sizeof(canid_t) bytes
before reading the CAN ID from skb->data. This prevents reading
uninitialized memory when processing malformed packets that don't
contain a valid CAN frame.

Reported-by: syzbot+5d8269a1e099279152bc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5d8269a1e099279152bc
Fixes: f057bbb6f9ed ("net: em_canid: Ematch rule to match CAN frames according to their identifiers")
Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
---
 net/sched/em_canid.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/sched/em_canid.c b/net/sched/em_canid.c
index 5337bc462755..a9b6cab70ff1 100644
--- a/net/sched/em_canid.c
+++ b/net/sched/em_canid.c
@@ -99,6 +99,9 @@ static int em_canid_match(struct sk_buff *skb, struct tcf_ematch *m,
 	int i;
 	const struct can_filter *lp;
 
+	if (skb->len < sizeof(canid_t))
+		return 0;
+
 	can_id = em_canid_get_id(skb);
 
 	if (can_id & CAN_EFF_FLAG) {
-- 
2.34.1

Re: [PATCH net] net/sched: em_canid: add length check before reading CAN ID

[Eric Dumazet]

On Tue, Nov 25, 2025 at 7:46 PM <ssrane_b23@ee.vjti.ac.in> wrote:
>
> From: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
>
> Add a check to verify that the skb has at least sizeof(canid_t) bytes
> before reading the CAN ID from skb->data. This prevents reading
> uninitialized memory when processing malformed packets that don't
> contain a valid CAN frame.
>
> Reported-by: syzbot+5d8269a1e099279152bc@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=5d8269a1e099279152bc
> Fixes: f057bbb6f9ed ("net: em_canid: Ematch rule to match CAN frames according to their identifiers")
> Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
> ---
>  net/sched/em_canid.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/net/sched/em_canid.c b/net/sched/em_canid.c
> index 5337bc462755..a9b6cab70ff1 100644
> --- a/net/sched/em_canid.c
> +++ b/net/sched/em_canid.c
> @@ -99,6 +99,9 @@ static int em_canid_match(struct sk_buff *skb, struct tcf_ematch *m,
>         int i;
>         const struct can_filter *lp;
>
> +       if (skb->len < sizeof(canid_t))
> +               return 0;
> +

Please keep in mind that this test is not enough, even if it may
prevent a particular syzbot repro from triggering a bug.

Take a look at pskb_may_pull() for details.

Re: [PATCH net] net/sched: em_canid: add length check before reading CAN ID

[SHAURYA RANE]

You're right, checking skb->len alone doesn’t guarantee that the
required bytes are present in the linear data area, especially for
fragmented skbs. I’ll switch this to a proper pskb_may_pull(skb,
sizeof(canid_t)) check to ensure the CAN ID is safely accessible.
I’ll send a v2 patch with the corrected validation.
Thanks,
Shaurya

On Wed, Nov 26, 2025 at 9:46 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Tue, Nov 25, 2025 at 7:46 PM <ssrane_b23@ee.vjti.ac.in> wrote:
> >
> > From: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
> >
> > Add a check to verify that the skb has at least sizeof(canid_t) bytes
> > before reading the CAN ID from skb->data. This prevents reading
> > uninitialized memory when processing malformed packets that don't
> > contain a valid CAN frame.
> >
> > Reported-by: syzbot+5d8269a1e099279152bc@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=5d8269a1e099279152bc
> > Fixes: f057bbb6f9ed ("net: em_canid: Ematch rule to match CAN frames according to their identifiers")
> > Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
> > ---
> >  net/sched/em_canid.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/net/sched/em_canid.c b/net/sched/em_canid.c
> > index 5337bc462755..a9b6cab70ff1 100644
> > --- a/net/sched/em_canid.c
> > +++ b/net/sched/em_canid.c
> > @@ -99,6 +99,9 @@ static int em_canid_match(struct sk_buff *skb, struct tcf_ematch *m,
> >         int i;
> >         const struct can_filter *lp;
> >
> > +       if (skb->len < sizeof(canid_t))
> > +               return 0;
> > +
>
> Please keep in mind that this test is not enough, even if it may
> prevent a particular syzbot repro from triggering a bug.
>
> Take a look at pskb_may_pull() for details.