CVE-2024-53044 — net/sched: sch_api: fix xa_insert() error path in tcf_block_get_ext()

Affected: Linux

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

net/sched: sch_api: fix xa_insert() error path in tcf_block_get_ext()

This command:

$ tc qdisc replace dev eth0 ingress_block 1 egress_block 1 clsact Error: block dev insert failed: -EBUSY.

fails because user space requests the same block index to be set for both ingress and egress.

[ side note, I don't think it even failed prior to commit 913b47d3424e ("net/sched: Introduce tc block netdev tracking infra"), because this is a command from an old set of notes of mine which used to work, but alas, I did not scientifically bisect this ]

The problem is not that it fails, but rather, that the second time around, it fails differently (and irrecoverably):

$ tc qdisc replace dev eth0 ingress_block 1 egress_block 1 clsact Error: dsa_core: Flow block cb is busy.

[ another note: the extack is added by me for illustration purposes. the context of the problem is that clsact_init() obtains the same &q->ingress_block pointer as &q->egress_block, and since we call tcf_block_get_ext() on both of them, "dev" will be added to the block->ports xarray twice, thus failing the operation: once through the ingress block pointer, and once again through the egress block pointer. the problem itself is that when xa_insert() fails, we have emitted a FLOW_BLOCK_BIND command through ndo_setup_tc(), but the offload never sees a corresponding FLOW_BLOCK_UNBIND. ]

Even correcting the bad user input, we still can…

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CVE-2024-53044 — net/sched: sch_api: fix xa_insert() error path in tcf_block_get_ext()