tracing: Fix potential null-pointer-access of entry in list 'tr->err_log'

Entries in list 'tr->err_log' will be reused after entry number
exceed TRACING_LOG_ERRS_MAX.

The cmd string of the to be reused entry will be freed first then
allocated a new one. If the allocation failed, then the entry will
still be in list 'tr->err_log' but its 'cmd' field is set to be NULL,
later access of 'cmd' is risky.

Currently above problem can cause the loss of 'cmd' information of first
entry in 'tr->err_log'. When execute `cat /sys/kernel/tracing/error_log`,
reproduce logs like:
  [   37.495100] trace_kprobe: error: Maxactive is not for kprobe(null) ^
  [   38.412517] trace_kprobe: error: Maxactive is not for kprobe
    Command: p4:myprobe2 do_sys_openat2
            ^

Link: https://lore.kernel.org/linux-trace-kernel/20221114104632.3547266-1-zhengyejian1@huawei.com
Fixes: 1581a884b7ca ("tracing: Remove size restriction on tracing_log_err cmd strings")
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 5bd202d..a7fe0e1 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7803,6 +7803,7 @@ static struct tracing_log_err *get_tracing_log_err(struct trace_array *tr,
                                                   int len)
 {
        struct tracing_log_err *err;
+       char *cmd;
 
        if (tr->n_err_log_entries < TRACING_LOG_ERRS_MAX) {
                err = alloc_tracing_log_err(len);
@@ -7811,12 +7812,12 @@ static struct tracing_log_err *get_tracing_log_err(struct trace_array *tr,
 
                return err;
        }
-
+       cmd = kzalloc(len, GFP_KERNEL);
+       if (!cmd)
+               return ERR_PTR(-ENOMEM);
        err = list_first_entry(&tr->err_log, struct tracing_log_err, list);
        kfree(err->cmd);
-       err->cmd = kzalloc(len, GFP_KERNEL);
-       if (!err->cmd)
-               return ERR_PTR(-ENOMEM);
+       err->cmd = cmd;
        list_del(&err->list);
 
        return err;