Re: [PATCH net] net/sched: mqprio: fix stack out-of-bounds write in tc entry parsing

[Simon Horman <horms@kernel.org>]

+ Ferenc and Vladimir

On Tue, Jul 22, 2025 at 04:51:21PM +0100, Maher Azzouzi wrote:
> From: MaherAzzouzi <maherazz04@gmail.com>

nit: space between your names please

> 
> TCA_MQPRIO_TC_ENTRY_INDEX is validated using
> NLA_POLICY_MAX(NLA_U32, TC_QOPT_MAX_QUEUE), which allows the value
> TC_QOPT_MAX_QUEUE (16). This leads to a 4-byte out-of-bounds stack write in
> the fp[] array, which only has room for 16 elements (0–15).
> 
> Fix this by changing the policy to allow only up to TC_QOPT_MAX_QUEUE - 1.
> 
> Fixes: f62af20bed2d ("net/sched: mqprio: allow per-TC user input of FP adminStatus")
> Reported-by: Maher Azzouzi <maherazz04@gmail.com>

I don't think there is any need to include a Reported-by tag if
you are also the patch author.

> Signed-off-by: Maher Azzouzi <maherazz04@gmail.com>

I agree with your analysis and that this is a good fix.

Reviewed-by: Simon Horman <horms@kernel.org>

I do think it is misleading to name this #define MAX,
but it's part of the UAPI so that ship has sailed.

It seems that taprio has a similar problem, but that it is
not a bug due to an additional check. I wonder if something
like this for net-next is appropriate to align it's implementation
wit that of maprio.

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 2b14c81a87e5..e759e43ad27e 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -998,7 +998,7 @@ static const struct nla_policy entry_policy[TCA_TAPRIO_SCHED_ENTRY_MAX + 1] = {
 
 static const struct nla_policy taprio_tc_policy[TCA_TAPRIO_TC_ENTRY_MAX + 1] = {
 	[TCA_TAPRIO_TC_ENTRY_INDEX]	   = NLA_POLICY_MAX(NLA_U32,
-							    TC_QOPT_MAX_QUEUE),
+							    TC_QOPT_MAX_QUEUE - 1),
 	[TCA_TAPRIO_TC_ENTRY_MAX_SDU]	   = { .type = NLA_U32 },
 	[TCA_TAPRIO_TC_ENTRY_FP]	   = NLA_POLICY_RANGE(NLA_U32,
 							      TC_FP_EXPRESS,
@@ -1698,19 +1698,15 @@ static int taprio_parse_tc_entry(struct Qdisc *sch,
 	if (err < 0)
 		return err;
 
-	if (!tb[TCA_TAPRIO_TC_ENTRY_INDEX]) {
+	if (NL_REQ_ATTR_CHECK(extack, opt, tb, TCA_TAPRIO_TC_ENTRY_INDEX)) {
 		NL_SET_ERR_MSG_MOD(extack, "TC entry index missing");
 		return -EINVAL;
 	}
 
 	tc = nla_get_u32(tb[TCA_TAPRIO_TC_ENTRY_INDEX]);
-	if (tc >= TC_QOPT_MAX_QUEUE) {
-		NL_SET_ERR_MSG_MOD(extack, "TC entry index out of range");
-		return -ERANGE;
-	}
-
 	if (*seen_tcs & BIT(tc)) {
-		NL_SET_ERR_MSG_MOD(extack, "Duplicate TC entry");
+		NL_SET_ERR_MSG_ATTR(extack, tb[TCA_TAPRIO_TC_ENTRY_INDEX],
+				    "Duplicate tc entry");
 		return -EINVAL;
 	}
 

> ---
>  net/sched/sch_mqprio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/sched/sch_mqprio.c b/net/sched/sch_mqprio.c
> index 51d4013b6121..f3e5ef9a9592 100644
> --- a/net/sched/sch_mqprio.c
> +++ b/net/sched/sch_mqprio.c
> @@ -152,7 +152,7 @@ static int mqprio_parse_opt(struct net_device *dev, struct tc_mqprio_qopt *qopt,
>  static const struct
>  nla_policy mqprio_tc_entry_policy[TCA_MQPRIO_TC_ENTRY_MAX + 1] = {
>       [TCA_MQPRIO_TC_ENTRY_INDEX]     = NLA_POLICY_MAX(NLA_U32,
> -                                                      TC_QOPT_MAX_QUEUE),
> +                                                      TC_QOPT_MAX_QUEUE - 1),
>       [TCA_MQPRIO_TC_ENTRY_FP]        = NLA_POLICY_RANGE(NLA_U32,
>                                                          TC_FP_EXPRESS,
>                                                          TC_FP_PREEMPTIBLE),
> --
> 2.34.1
>