CVE-2026-68377: net/sched: act_tunnel_key: Defer dst_release to RCU callback

In the Linux kernel, the following vulnerability has been resolved:

net/sched: acttunnelkey: Defer dstrelease to RCU callback

Fix a race-condition use-after-free in tunnelkeyreleaseparams().

The function releases the metadatadst of the old params synchronously via dstrelease() while deferring the params struct free with kfreercu(). A concurrent tunnelkeyact() reader on the datapath may still hold the old params pointer (under rcureadlockbh) and proceed to call dstclone(&params->tcftencmetadata->dst) after the writer's dstrelease has already pushed the dst's rcuref to RCUREFDEAD.

zdi-disclosures@trendmicro.com produced a poc which i (and Victor) verified that KASAN reports:

================================================================== BUG: KASAN: slab-use-after-free in instrumentatomicreadwrite include/linux/instrumented.h:112 BUG: KASAN: slab-use-after-free in atomicsubreturnrelease include/linux/atomic/atomic-instrumented.h:326 BUG: KASAN: slab-use-after-free in rcurefput include/linux/rcuref.h:109 BUG: KASAN: slab-use-after-free in rcurefput include/linux/rcuref.h:173 BUG: KASAN: slab-use-after-free in dstrelease+0x5b/0x370 net/core/dst.c:168 Write of size 4 at addr ffff88806158de40 by task poc/9388

CPU: 0 UID: 0 PID: 9388 Comm: poc Tainted: G W 7.1.0-rc7 #7 PREEMPT(lazy) Tainted: [W]=WARN Hardware name: QEMU Ubuntu 25.10 PC v2 (i440FX + PIIX, + 10.1 machine, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 Call Trace: <TASK> dumpstack lib/dumpstack.c:94 dumpstacklvl+0x100/0x190 lib/dumpstack.c:120 printaddressdescription mm/kasan/report.c:378 printreport+0x139/0x4ad mm/kasan/report.c:482 kasanreport+0xe4/0x1d0 mm/kasan/report.c:595 checkregioninline mm/kasan/generic.c:186 kasancheckrange+0x125/0x200 mm/kasan/generic.c:200 instrumentatomicreadwrite include/linux/instrumented.h:112 atomicsubreturnrelease include/linux/atomic/atomic-instrumented.h:326 rcurefput include/linux/rcuref.h:109 rcurefput include/linux/rcuref.h:173 dstrelease+0x5b/0x370 net/core/dst.c:168 refdstdrop include/net/dst.h:272 skbdstdrop include/net/dst.h:284 skbreleaseheadstate+0x293/0x400 net/core/skbuff.c:1163 skbreleaseall net/core/skbuff.c:1187 [..] Allocated by task 9391: kasansavestack+0x30/0x50 mm/kasan/common.c:57 kasansavetrack+0x14/0x30 mm/kasan/common.c:78 poisonkmallocredzone mm/kasan/common.c:398 kasankmalloc+0x9a/0xb0 mm/kasan/common.c:415 kasankmalloc include/linux/kasan.h:263 dokmallocnode mm/slub.c:5296 kmallocnoprof+0x2f1/0x830 mm/slub.c:5308 kmallocnoprof include/linux/slab.h:954 kzallocnoprof include/linux/slab.h:1188 offloadactionalloc+0x2f/0x130 net/core/flowoffload.c:35 tcfactionoffloadaddex+0x1ba/0x880 net/sched/actapi.c:258 tcfactionoffloadadd net/sched/actapi.c:293 tcfactioninit+0x66e/0xa20 net/sched/actapi.c:1547 tcfactionadd+0xf6/0x5d0 net/sched/actapi.c:2101 [..] Freed by task 9391: kasansavestack+0x30/0x50 mm/kasan/common.c:57 kasansavetrack+0x14/0x30 mm/kasan/common.c:78 kasansavefreeinfo+0x3b/0x70 mm/kasan/generic.c:584 poisonslabobject mm/kasan/common.c:253 kasanslabfree+0x6b/0x90 mm/kasan/common.c:285 kasanslabfree include/linux/kasan.h:235 slabfreehook mm/slub.c:2689 slabfree mm/slub.c:6251 kfree+0x21f/0x6b0 mm/slub.c:6566 tcfactionoffloadaddex+0x4ad/0x880 net/sched/actapi.c:284 tcfactionoffloadadd net/sched/actapi.c:293 tcfactioninit+0x66e/0xa20 net/sched/actapi.c:1547 tcfactionadd+0xf6/0x5d0 net/sched/actapi.c:2101

The buggy address belongs to the object at ffff88806158de00 which belongs to the cache kmalloc-256 of size 256 The buggy address is located 64 bytes inside of freed 256-byte region [ffff88806158de00, ffff88806158df00)

The buggy address belongs to the physical page: page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff88806158d600 pfn:0x6158c head: order:1 mapcount:0 entiremap ---truncated---