From: Hui Su Date: Tue, 11 Aug 2026 17:39:03 +0000 (+0800) Subject: tracing: Fix NULL pointer dereference in module event cache removal X-Git-Tag: v7.2~25^2~1 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b69859204d4db3acd86c1c2dadcef0d77b451933;p=thirdparty%2Fkernel%2Flinux.git 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: echo ':mod:trace_events_kunit_missing' > /sys/kernel/tracing/set_event echo '!foo_bar:mod:trace_events_kunit_missing' >> /sys/kernel/tracing/set_event The second write must be a concatenation (">>") to not include O_TRUNC as that would cause ftrace_clear_events() to clear the cached modules lines. 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:". Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260811173902.1927376-2-sh_def@163.com 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/ Signed-off-by: Hui Su Signed-off-by: Steven Rostedt --- 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 &&