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

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) --- */