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

[Steven Rostedt]

On Wed, 12 Aug 2026 01:39:03 +0800
Hui Su <sh_def@xxxxxxx> wrote:

> 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>".

Thanks but it's actually easier to trigger it this way:

  echo ':mod:trace_events_kunit_missing' > /sys/kernel/tracing/set_event
  echo '!foo_bar:mod:trace_events_kunit_missing' >> /sys/kernel/tracing/set_event

as '>>' doesn't truncate.

I'll queue it up and start testing it.

-- Steve


> 
> Fixes: b355247df104 ("tracing: Cache \":mod:\" events for modules not loaded yet")
> Reported-by: syzbot+4d3143c8e28f6266c636@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/lkml/6a7a6b7f.9c11d2ce.289b96.00f8.GAE@xxxxxxxxxx/
> Signed-off-by: Hui Su <sh_def@xxxxxxx>
> ---
>  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 &&