net/sched: act_ct: don't WARN on benign flow_offload_alloc() failure

Overview

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

net/sched: act_ct: don't WARN on benign flow_offload_alloc() failure

flow_offload_alloc() returns NULL when the conntrack entry is dying (e.g. raced with a conntrack flush) or when the GFP_ATOMIC allocation fails; both are expected under load and neither is a kernel bug. This path runs from softirq on every committed packet, so with panic_on_warn=1 an unprivileged user can panic the box just by racing a conntrack flush against a tc ... action ct commit classifier.

Reproduced with a custom repro under QEMU: a small, fixed set of UDP flows through tc filter ... action ct commit on lo, raced against threads flooding bare ctnetlink CT_DELETE (flush) requests. Hits WARNING: net/sched/act_ct.c:437 (tcf_ct_flow_table_add(), inlined into tcf_ct_act() in this build) within ~15s on the unpatched kernel; same setup is clean on the patched kernel. The fix itself is behavior-preserving: both branches already did goto err_alloc before and after, only the WARN is removed.

Affected

• Linux >= 64ff70b80fd403110b67dd9f7184a604fdb0da43 < e8fcf3f08834e939b797311742ee72b1e2a08096
• Linux >= 64ff70b80fd403110b67dd9f7184a604fdb0da43 < af475f599270e59313552e6c6f3af8507da672b8
• Linux >= 64ff70b80fd403110b67dd9f7184a604fdb0da43 < 424ba52c0058deeb940d02cba52eb0aeb288db33
• Linux >= 64ff70b80fd403110b67dd9f7184a604fdb0da43 < d512c484bd93a5f2794aa03c9faeb7bcb82dbd4a
• Linux >= 64ff70b80fd403110b67dd9f7184a604fdb0da43 < d805560ad69cb266d669341ca01e26bffb0d2c67
• Linux >= 64ff70b80fd403110b67dd9f7184a604fdb0da43 < 00734f3684fcec42c319bedfe68a5d357c720b53
• Linux >= 64ff70b80fd403110b67dd9f7184a604fdb0da43 < 650787233c2fd6e515a7c5f371019a9fa080ccd2
• Linux >= 64ff70b80fd403110b67dd9f7184a604fdb0da43 < 47abe7a5c4eb53269aca3506446f851572a059a3
• Linux 5.7

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.