tracing: Fix crash on synthetic stacktrace field usage

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

tracing: Fix crash on synthetic stacktrace field usage

When creating a synthetic event based on an existing synthetic event that

had a stacktrace field and the new synthetic event used that field a

kernel crash occurred:

~# cd /sys/kernel/tracing

~# echo 's:stack unsigned long stack[];' > dynamic_events

~# echo 'hist:keys=prev_pid:s0=common_stacktrace if prev_state & 3' >> events/sched/sched_switch/trigger

~# echo 'hist:keys=next_pid:s1=$s0:onmatch(sched.sched_switch).trace(stack,$s1)' >> events/sched/sched_switch/trigger

The above creates a synthetic event that takes a stacktrace when a task

schedules out in a non-running state and passes that stacktrace to the

sched_switch event when that task schedules back in. It triggers the

"stack" synthetic event that has a stacktrace as its field (called "stack").

~# echo 's:syscall_stack s64 id; unsigned long stack[];' >> dynamic_events

~# echo 'hist:keys=common_pid:s2=stack' >> events/synthetic/stack/trigger

~# echo 'hist:keys=common_pid:s3=$s2,i0=id:onmatch(synthetic.stack).trace(syscall_stack,$i0,$s3)' >> events/raw_syscalls/sys_exit/trigger

The above makes another synthetic event called "syscall_stack" that

attaches the first synthetic event (stack) to the sys_exit trace event and

records the stacktrace from the stack event with the id of the system call

that is exiting.

When enabling this event (or using it in a historgram):

~# echo 1 > events/synthetic/syscall_stack/enable

Produces a kernel crash!

BUG: unable to handle page fault for address: 0000000000400010

#PF: supervisor read access in kernel mode

#PF: error_code(0x0000) - not-present page

PGD 0 P4D 0

Oops: Oops: 0000 [#1] SMP PTI

CPU: 6 UID: 0 PID: 1257 Comm: bash Not tainted 6.16.3+deb14-amd64 #1 PREEMPT(lazy) Debian 6.16.3-1

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-debian-1.17.0-1 04/01/2014

RIP: 0010:trace_event_raw_event_synth+0x90/0x380

Code: c5 00 00 00 00 85 d2 0f 84 e1 00 00 00 31 db eb 34 0f 1f 00 66 66 2e 0f 1f 84 00 00 00 00 00 66 66 2e 0f 1f 84 00 00 00 00 00 <49> 8b 04 24 48 83 c3 01 8d 0c c5 08 00 00 00 01 cd 41 3b 5d 40 0f

RSP: 0018:ffffd2670388f958 EFLAGS: 00010202

RAX: ffff8ba1065cc100 RBX: 0000000000000000 RCX: 0000000000000000

RDX: 0000000000000001 RSI: fffff266ffda7b90 RDI: ffffd2670388f9b0

RBP: 0000000000000010 R08: ffff8ba104e76000 R09: ffffd2670388fa50

R10: ffff8ba102dd42e0 R11: ffffffff9a908970 R12: 0000000000400010

R13: ffff8ba10a246400 R14: ffff8ba10a710220 R15: fffff266ffda7b90

FS: 00007fa3bc63f740(0000) GS:ffff8ba2e0f48000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 0000000000400010 CR3: 0000000107f9e003 CR4: 0000000000172ef0

Call Trace:

<TASK>

? __tracing_map_insert+0x208/0x3a0

action_trace+0x67/0x70

event_hist_trigger+0x633/0x6d0

event_triggers_call+0x82/0x130

trace_event_buffer_commit+0x19d/0x250

trace_event_raw_event_sys_exit+0x62/0xb0

syscall_exit_work+0x9d/0x140

do_syscall_64+0x20a/0x2f0

? trace_event_raw_event_sched_switch+0x12b/0x170

? save_fpregs_to_fpstate+0x3e/0x90

? _raw_spin_unlock+0xe/0x30

? finish_task_switch.isra.0+0x97/0x2c0

? __rseq_handle_notify_resume+0xad/0x4c0

? __schedule+0x4b8/0xd00

? restore_fpregs_from_fpstate+0x3c/0x90

? switch_fpu_return+0x5b/0xe0

? do_syscall_64+0x1ef/0x2f0

? do_fault+0x2e9/0x540

? __handle_mm_fault+0x7d1/0xf70

? count_memcg_events+0x167/0x1d0

? handle_mm_fault+0x1d7/0x2e0

? do_user_addr_fault+0x2c3/0x7f0

entry_SYSCALL_64_after_hwframe+0x76/0x7e

The reason is that the stacktrace field is not labeled as such, and is

treated as a normal field and not as a dynamic event that it is.

In trace_event_raw_event_synth() the event is field is still treated as a

dynamic array, but the retrieval of the data is considered a normal field,

and the reference is just the meta data:

// Meta data is retrieved instead of a dynamic array

---truncated---