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: [email protected]
Closes:
https://lore.kernel.org/lkml/[email protected]/
Signed-off-by: Hui Su <[email protected]>
---
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
