netfilter: nft_flow_offload: drop flowtable reference on init error path

Overview

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

netfilter: nft_flow_offload: drop flowtable reference on init error path

nft_flow_offload_init() bumps the flowtable use count with nft_use_inc() before calling nf_ct_netns_get(). When the latter fails, the error is returned as-is and the reference is leaked.

The upper layers do not balance it either: nf_tables_newexpr() clears expr->ops when the expression init callback fails, so the nft_expr_more() iteration in nft_rule_expr_deactivate() and nf_tables_rule_destroy() stops right before the failed expression and its ->destroy callback, which would drop the reference, never runs.

Each failed rule addition therefore leaks one flowtable reference and the flowtable can no longer be removed: NFT_MSG_DELFLOWTABLE keeps reporting -EBUSY even though no rule references it.

Save the nf_ct_netns_get() return value and undo the nft_use_inc() when it fails, restoring the inc/dec pairing within nft_flow_offload_init() itself.

Affected

• Linux >= a3c90f7a2323b331ae816d5b0633e68148e25d04 < 64ac0cb574c740bf723affb79b646eeba920db6b
• Linux >= a3c90f7a2323b331ae816d5b0633e68148e25d04 < c5f91098edee98c015f78488460a0609e62582a5
• Linux >= a3c90f7a2323b331ae816d5b0633e68148e25d04 < 4cd378423a33194a773c267fc58606d28fce95cf
• Linux >= a3c90f7a2323b331ae816d5b0633e68148e25d04 < 747928b5d4ff8c8de55138a54c13980e4bb88578
• Linux 4.16

Remediation

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