[PATCH] tracing/eprobe: Update cond flag before enabling trigger

[Rafael Mendonca <rafaelmendsr@gmail.com>]

The eprobe's trigger, for the event it is attached to, needs access to the
trace record in order to perform its function. This need is indicated by
the TRIGGER_COND flag and properly set by the update_cond_flag() function.
Currently, the cond flag is set after calling
trace_event_trigger_enable_disable(), which leaves a small time gap in
which the eprobe's trigger could be invoked without a trace record, causing
a NULL pointer dereference issue.

This issue can be easily reproduced by enabling an event probe for
kmem/mm_page_alloc using any of its fields as such:

root@localhost:/sys/kernel/tracing# echo 'e kmem/mm_page_alloc $gfp_flags' > dynamic_events
root@localhost:/sys/kernel/tracing# echo 1 > events/eprobes/enable
[   30.847000] general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] PREEMPT SMP KASAN PTI
[   30.848618] KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
[   30.849498] CPU: 0 PID: 223 Comm: bash Not tainted 6.1.0-rc5+ #98
[...] stripped
[   30.850623] RIP: 0010:get_event_field.isra.0+0x1b8/0x3f0
[...] stripped
[   30.856173] Call Trace:
[   30.856340]  <TASK>
[   30.856489]  process_fetch_insn+0x1289/0x19b0
[   30.856789]  ? write_comp_data+0x2f/0x90
[   30.857061]  ? __sanitizer_cov_trace_pc+0x25/0x60
[   30.857379]  ? __trace_eprobe_create+0x1d30/0x1d30
[   30.857701]  ? __sanitizer_cov_trace_pc+0x25/0x60
[   30.858019]  ? write_comp_data+0x2f/0x90
[   30.858289]  ? write_comp_data+0x2f/0x90
[   30.858562]  eprobe_trigger_func+0x1253/0x2180
[   30.858867]  ? kasan_poison.part.0+0x3c/0x50
[   30.859165]  ? process_fetch_insn+0x19b0/0x19b0
[   30.859473]  ? get_page_from_freelist+0x10c1/0x29c0
[   30.859806]  ? rcu_read_lock_bh_held+0xd0/0xd0
[   30.860114]  ? write_comp_data+0x2f/0x90
[   30.860387]  event_triggers_call+0x293/0x410
[   30.860683]  __trace_trigger_soft_disabled+0xbb/0xe0
[   30.861019]  trace_event_raw_event_mm_page_alloc+0xb5/0xc0
[   30.861389]  __alloc_pages+0x4cd/0x760
[   30.861651]  ? __alloc_pages_slowpath.constprop.0+0x2480/0x2480
[   30.862047]  ? tracepoint_probe_register_prio_may_exist+0x100/0x100
[   30.862464]  ? __sanitizer_cov_trace_pc+0x25/0x60
[   30.862780]  ? write_comp_data+0x2f/0x90
[   30.863055]  trace_buffered_event_enable+0xc6/0x440
[   30.863381]  __ftrace_event_enable_disable+0x16a/0x880
[   30.863728]  trace_event_enable_disable+0x2a/0x40
[   30.864043]  trace_event_trigger_enable_disable.part.0+0x70/0x90
[   30.864446]  trace_event_trigger_enable_disable+0x31/0xc0
[   30.864806]  eprobe_register+0x49a/0xdf0
[   30.865080]  ? disable_eprobe+0x370/0x370
[   30.865360]  ? mutex_unlock+0x16/0x20
[   30.865618]  ? write_comp_data+0x2f/0x90
[   30.865897]  __ftrace_event_enable_disable+0x4c1/0x880
[   30.866247]  __ftrace_set_clr_event_nolock+0x297/0x390
[   30.866598]  __ftrace_set_clr_event+0x43/0x70
[   30.866901]  system_enable_write+0x1f7/0x2a0
[   30.867214]  ? event_enable_write+0x290/0x290
[   30.867521]  ? rcu_read_lock_held+0xc0/0xc0
[   30.867842]  ? write_comp_data+0x2f/0x90
[   30.868176]  ? write_comp_data+0x2f/0x90
[   30.868467]  vfs_write+0x311/0xe50
[   30.868752]  ? event_enable_write+0x290/0x290
[   30.869067]  ? kernel_write+0x5b0/0x5b0
[   30.869329]  ? lockdep_hardirqs_on_prepare+0x410/0x410
[   30.869675]  ? lock_is_held_type+0xaf/0x120
[   30.869954]  ? close_fd+0x6e/0xb0
[   30.870182]  ? write_comp_data+0x2f/0x90
[   30.870453]  ? __sanitizer_cov_trace_pc+0x25/0x60
[   30.870772]  ? write_comp_data+0x2f/0x90
[   30.871043]  ? write_comp_data+0x2f/0x90
[   30.871315]  ksys_write+0x158/0x2a0
[   30.871558]  ? __ia32_sys_read+0xc0/0xc0
[   30.871827]  ? syscall_enter_from_user_mode+0x25/0x70
[   30.872169]  ? do_syscall_64+0x3b/0x90
[   30.872432]  __x64_sys_write+0x7c/0xc0
[   30.872691]  ? syscall_enter_from_user_mode+0x25/0x70
[   30.873028]  do_syscall_64+0x60/0x90
[   30.873279]  ? syscall_exit_to_user_mode+0x4a/0x60
[   30.873597]  ? do_syscall_64+0x6d/0x90
[   30.873857]  ? fpregs_assert_state_consistent+0x90/0xf0
[   30.874214]  ? syscall_exit_to_user_mode+0x4a/0x60
[   30.874535]  ? do_syscall_64+0x6d/0x90
[   30.874794]  ? do_syscall_64+0x6d/0x90
[   30.875118]  ? do_syscall_64+0x6d/0x90
[   30.875379]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[   30.875712] RIP: 0033:0x7f7cc14da8f3
[...] stripped
[   30.879875]  </TASK>
[...] stripped
[   30.881930] ---[ end trace 0000000000000000 ]---
[   30.882237] RIP: 0010:get_event_field.isra.0+0x1b8/0x3f0
[...] stripped
[   30.887734] note: bash[223] exited with preempt_count 2

That happens because enable_eprobe() will eventually trigger the
kmem/mm_page_alloc trace event:

- enable_eprobe [trace_eprobe.c]
  - trace_event_trigger_enable_disable [trace_events_trigger.c]
    - trace_event_enable_disable [trace_events.c]
      - __ftrace_event_enable_disable [trace_events.c]
        - trace_buffered_event_enable [trace.c]
          - alloc_pages_node [gfp.h]
	   ...
            - __alloc_pages [page_alloc.c]
              - trace_mm_page_alloc // eprobe event file without TRIGGER_COND bit set

By the time kmem/mm_page_alloc trace event is hit, the eprobe event file
does not have the TRIGGER_COND flag set yet, which causes the eprobe's
trigger to be invoked (through the trace_trigger_soft_disabled() path)
without a trace record, causing a NULL pointer dereference when fetching
the event fields.

Fix this by setting the cond flag beforehand when enabling the eprobe's
trigger.

Fixes: 7491e2c44278 ("tracing: Add a probe that attaches to trace events")
Signed-off-by: Rafael Mendonca <rafaelmendsr@gmail.com>
---
 kernel/trace/trace_eprobe.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace_eprobe.c b/kernel/trace/trace_eprobe.c
index 5dd0617e5df6..5004c6f6f232 100644
--- a/kernel/trace/trace_eprobe.c
+++ b/kernel/trace/trace_eprobe.c
@@ -677,8 +677,8 @@ static int enable_eprobe(struct trace_eprobe *ep,
 
 	list_add_tail_rcu(&trigger->list, &file->triggers);
 
-	trace_event_trigger_enable_disable(file, 1);
 	update_cond_flag(file);
+	trace_event_trigger_enable_disable(file, 1);
 
 	return 0;
 }