NACK: [SRU][F][PATCH 1/1] net_sched: hfsc: Fix a UAF vulnerability in class handling

[Ian Whitfield]

On Wed, May 07, 2025 at 06:28:52PM -0700, Tim Whisonant wrote:
> From: Cong Wang <xiyou.wangcong at gmail.com>
> 
> This patch fixes a Use-After-Free vulnerability in the HFSC qdisc class
> handling. The issue occurs due to a time-of-check/time-of-use condition
> in hfsc_change_class() when working with certain child qdiscs like netem
> or codel.
> 
> The vulnerability works as follows:
> 1. hfsc_change_class() checks if a class has packets (q.qlen != 0)
> 2. It then calls qdisc_peek_len(), which for certain qdiscs (e.g.,
>    codel, netem) might drop packets and empty the queue
> 3. The code continues assuming the queue is still non-empty, adding
>    the class to vttree
> 4. This breaks HFSC scheduler assumptions that only non-empty classes
>    are in vttree
> 5. Later, when the class is destroyed, this can lead to a Use-After-Free
> 
> The fix adds a second queue length check after qdisc_peek_len() to verify
> the queue wasn't emptied.
> 
> Fixes: 21f4d5cc25ec ("net_sched/hfsc: fix curve activation in hfsc_change_class()")
> Reported-by: Gerrard Tai <gerrard.tai at starlabs.sg>
> Reviewed-by: Konstantin Khlebnikov <koct9i at gmail.com>
> Signed-off-by: Cong Wang <xiyou.wangcong at gmail.com>
> Reviewed-by: Jamal Hadi Salim <jhs at mojatatu.com>
> Link: https://patch.msgid.link/20250417184732.943057-2-xiyou.wangcong@gmail.com
> Signed-off-by: Jakub Kicinski <kuba at kernel.org>
> (cherry picked from commit 3df275ef0a6ae181e8428a6589ef5d5231e58b5c)
> CVE-2025-37797
> Signed-off-by: Tim Whisonant <tim.whisonant at canonical.com>
> ---
>  net/sched/sch_hfsc.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
> index 9ebae0d07a9c6..eabc62df6f4e4 100644
> --- a/net/sched/sch_hfsc.c
> +++ b/net/sched/sch_hfsc.c
> @@ -959,6 +959,7 @@ hfsc_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
>  
>  	if (cl != NULL) {
>  		int old_flags;
> +		int len = 0;
>  
>  		if (parentid) {
>  			if (cl->cl_parent &&
> @@ -989,9 +990,13 @@ hfsc_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
>  		if (usc != NULL)
>  			hfsc_change_usc(cl, usc, cur_time);
>  
> +		if (cl->qdisc->q.qlen != 0)
> +			len = qdisc_peek_len(cl->qdisc);
> +		/* Check queue length again since some qdisc implementations
> +		 * (e.g., netem/codel) might empty the queue during the peek
> +		 * operation.
> +		 */
>  		if (cl->qdisc->q.qlen != 0) {
> -			int len = qdisc_peek_len(cl->qdisc);
> -
>  			if (cl->cl_flags & HFSC_RSC) {
>  				if (old_flags & HFSC_RSC)
>  					update_ed(cl, len);
> -- 
> 2.43.0
> 
> 
> -- 
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The subject of this patch needs the full [F/J/N/O/P] to be picked up by our
tooling for all kernels, and it currently only has [F].