[PATCH] tracing: Fix NULL pointer dereference in module event cache removal

A module-only event filter such as ":mod:foo" is cached with a NULL
event_mod->match when foo has not been loaded. If a later write tries to
remove a specific match from the same module, remove_cache_mod() passes
the NULL cached match to strcmp(), causing a NULL pointer dereference.

The issue can be reproduced from userspace by keeping one set_event file
descriptor open across both writes:

  mount -t tracefs tracefs /sys/kernel/tracing
  exec 3>/sys/kernel/tracing/set_event
  printf ':mod:trace_events_kunit_missing\n' >&3
  printf '!foo_bar:mod:trace_events_kunit_missing\n' >&3

The descriptor must remain open because reopening set_event with O_TRUNC
calls ftrace_clear_events() and removes the cached module filter before
the second write.

The crash was reproduced on x86_64 QEMU while KUnit workers contended on
the event tracing path:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  RIP: 0010:strcmp+0x10/0x30
  Call Trace:
   __ftrace_set_clr_event_nolock+0x373/0x4a0
   ftrace_set_clr_event+0xf0/0x180
   ftrace_event_write+0xdf/0x110
   vfs_write+0xf6/0x440
   ksys_write+0x68/0xe0
   do_syscall_64+0xf9/0x540
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Check event_mod->match before comparing it, consistent with the existing
NULL checks for the cached system and event fields. The mismatched removal
continues to return -EINVAL; a broad cached module filter is removed with
"!:mod:<module>".

Fixes: b355247df104 ("tracing: Cache \":mod:\" events for modules not loaded yet")
Reported-by: syzbot+4d3143c8e28f6266c636@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/6a7a6b7f.9c11d2ce.289b96.00f8.GAE@google.com/ (local)
Signed-off-by: Hui Su <redacted>
---
 kernel/trace/trace_events.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index c01b10b99f67..032f741ba616 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c

@@ -945,7 +945,7 @@ static int remove_cache_mod(struct trace_array *tr, const char *mod,
 		if (strcmp(event_mod->module, mod) != 0)
 			continue;
 
-		if (match && strcmp(event_mod->match, match) != 0)
+		if (match && (!event_mod->match || strcmp(event_mod->match, match) != 0))
 			continue;
 
 		if (system &&

-- 
2.54.0