AZL-95012

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

net/sched: serialize qdiscrtablist against concurrent get/put

qdiscgetrtab() and qdiscputrtab() mutate the process-global singly linked list qdiscrtablist and a plain non-atomic 'int refcnt' with no lock. This was only safe because every caller historically held the RTNL mutex, which serialized all rate-table lookups, inserts and frees.

That invariant no longer holds. clsflower sets TCFPROTOOPSDOITUNLOCKED, so tcnewtfilter() keeps rtnlheld == false for it and sets TCAACTFLAGSNORTNL. That flag propagates through tcfextsvalidateex() -> tcfactioninit() -> tcfactioninit1() -> tcfpoliceinit(), which calls qdiscgetrtab()/qdiscputrtab() with the RTNL mutex NOT held. Two RTMNEWTFILTER requests on different CPUs, each adding a flower filter with a police action carrying the same rate, then race on qdiscrtablist and on the non-atomic refcnt, leading to a use-after-free / double-free of the kmalloc-2k struct qdiscratetable. qdiscrtab_list is a single global (not per-netns), so the corrupted object is shared system-wide.

BUG: KASAN: slab-use-after-free in qdiscputrtab+0x12f/0x160 qdiscputrtab+0x12f/0x160 tcfpoliceinit+0xda9/0x1590 tcfactioninit1+0x460/0x6b0 tcfactioninit+0x439/0xa40 tcfextsvalidateex+0x42d/0x550 flchange+0xddd/0x7da0 tcnewtfilter+0xaa7/0x2420 rtnetlinkrcv_msg+0x95e/0xe90 which belongs to the cache kmalloc-2k of size 2048

Protect qdiscrtablist and the refcount with a dedicated spinlock. The (sleeping, GFPKERNEL) allocation in qdiscgetrtab() is performed before taking the lock; if a concurrent inserter added an identical table in the meantime the freshly allocated one is freed under the lock, so no duplicate is leaked. qdiscput_rtab() now decrements the refcount and unlinks under the same lock.