CVE-2022-48644

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

net/sched: taprio: avoid disabling offload when it was never enabled

In an incredibly strange API design decision, qdisc->destroy() gets
called even if qdisc->init() never succeeded, not exclusively since
commit 87b60cfacf9f (“net_sched: fix error recovery at qdisc creation”),
but apparently also earlier (in the case of qdisc_create_dflt()).

The taprio qdisc does not fully acknowledge this when it attempts full
offload, because it starts off with q->flags = TAPRIO_FLAGS_INVALID in
taprio_init(), then it replaces q->flags with TCA_TAPRIO_ATTR_FLAGS
parsed from netlink (in taprio_change(), tail called from taprio_init()).

But in taprio_destroy(), we call taprio_disable_offload(), and this
determines what to do based on FULL_OFFLOAD_IS_ENABLED(q->flags).

But looking at the implementation of FULL_OFFLOAD_IS_ENABLED()
(a bitwise check of bit 1 in q->flags), it is invalid to call this macro
on q->flags when it contains TAPRIO_FLAGS_INVALID, because that is set
to U32_MAX, and therefore FULL_OFFLOAD_IS_ENABLED() will return true on
an invalid set of flags.

As a result, it is possible to crash the kernel if user space forces an
error between setting q->flags = TAPRIO_FLAGS_INVALID, and the calling
of taprio_enable_offload(). This is because drivers do not expect the
offload to be disabled when it was never enabled.

The error that we force here is to attach taprio as a non-root qdisc,
but instead as child of an mqprio root qdisc:

$ tc qdisc add dev swp0 root handle 1:
mqprio num_tc 8 map 0 1 2 3 4 5 6 7
queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 hw 0
$ tc qdisc replace dev swp0 parent 1:1
taprio num_tc 8 map 0 1 2 3 4 5 6 7
queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 base-time 0
sched-entry S 0x7f 990000 sched-entry S 0x80 100000
flags 0x0 clockid CLOCK_TAI
Unable to handle kernel paging request at virtual address fffffffffffffff8
[fffffffffffffff8] pgd=0000000000000000, p4d=0000000000000000
Internal error: Oops: 96000004 [#1] PREEMPT SMP
Call trace:
taprio_dump+0x27c/0x310
vsc9959_port_setup_tc+0x1f4/0x460
felix_port_setup_tc+0x24/0x3c
dsa_slave_setup_tc+0x54/0x27c
taprio_disable_offload.isra.0+0x58/0xe0
taprio_destroy+0x80/0x104
qdisc_create+0x240/0x470
tc_modify_qdisc+0x1fc/0x6b0
rtnetlink_rcv_msg+0x12c/0x390
netlink_rcv_skb+0x5c/0x130
rtnetlink_rcv+0x1c/0x2c

Fix this by keeping track of the operations we made, and undo the
offload only if we actually did it.

I’ve added “bool offloaded” inside a 4 byte hole between “int clockid”
and “atomic64_t picos_per_byte”. Now the first cache line looks like
below:

$ pahole -C taprio_sched net/sched/sch_taprio.o
struct taprio_sched {
struct Qdisc * * qdiscs; /* 0 8 /
struct Qdisc * root; / 8 8 /
u32 flags; / 16 4 /
enum tk_offsets tk_offset; / 20 4 /
int clockid; / 24 4 /
bool offloaded; / 28 1 */

    /* XXX 3 bytes hole, try to pack */

    atomic64_t                 picos_per_byte;       /*    32     0 */

    /* XXX 8 bytes hole, try to pack */

    spinlock_t                 current_entry_lock;   /*    40     0 */

    /* XXX 8 bytes hole, try to pack */

    struct sched_entry *       current_entry;        /*    48     8 */
    struct sched_gate_list *   oper_sched;           /*    56     8 */
    /* --- cacheline 1 boundary (64 bytes) --- */