tracing/histograms: Fix memory leak problem

JIRA: https://issues.redhat.com/browse/RHEL-81008
CVE: CVE-2022-49648

commit 7edc394
Author: Zheng Yejian <[email protected]>
Date:   Mon Jul 11 09:47:31 2022 +0800

    tracing/histograms: Fix memory leak problem

    This reverts commit 46bbe5c.

    As commit 46bbe5c ("tracing: fix double free") said, the
    "double free" problem reported by clang static analyzer is:
      > In parse_var_defs() if there is a problem allocating
      > var_defs.expr, the earlier var_defs.name is freed.
      > This free is duplicated by free_var_defs() which frees
      > the rest of the list.

    However, if there is a problem allocating N-th var_defs.expr:
      + in parse_var_defs(), the freed 'earlier var_defs.name' is
        actually the N-th var_defs.name;
      + then in free_var_defs(), the names from 0th to (N-1)-th are freed;

                            IF ALLOCATING PROBLEM HAPPENED HERE!!! -+
                                                                     \
                                                                      |
              0th           1th                 (N-1)-th      N-th    V
              +-------------+-------------+-----+-------------+-----------
    var_defs: | name | expr | name | expr | ... | name | expr | name | ///
              +-------------+-------------+-----+-------------+-----------

    These two frees don't act on same name, so there was no "double free"
    problem before. Conversely, after that commit, we get a "memory leak"
    problem because the above "N-th var_defs.name" is not freed.

    If enable CONFIG_DEBUG_KMEMLEAK and inject a fault at where the N-th
    var_defs.expr allocated, then execute on shell like:
      $ echo 'hist:key=call_site:val=$v1,$v2:v1=bytes_req,v2=bytes_alloc' > \
    /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    Then kmemleak reports:
      unreferenced object 0xffff8fb100ef3518 (size 8):
        comm "bash", pid 196, jiffies 4295681690 (age 28.538s)
        hex dump (first 8 bytes):
          76 31 00 00 b1 8f ff ff                          v1......
        backtrace:
          [<0000000038fe4895>] kstrdup+0x2d/0x60
          [<00000000c99c049a>] event_hist_trigger_parse+0x206f/0x20e0
          [<00000000ae70d2cc>] trigger_process_regex+0xc0/0x110
          [<0000000066737a4c>] event_trigger_write+0x75/0xd0
          [<000000007341e40c>] vfs_write+0xbb/0x2a0
          [<0000000087fde4c2>] ksys_write+0x59/0xd0
          [<00000000581e9cdf>] do_syscall_64+0x3a/0x80
          [<00000000cf3b065c>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

    Link: https://lkml.kernel.org/r/[email protected]

    Cc: [email protected]
    Fixes: 46bbe5c ("tracing: fix double free")
    Reported-by: Hulk Robot <[email protected]>
    Suggested-by: Steven Rostedt <[email protected]>
    Reviewed-by: Tom Zanussi <[email protected]>
    Signed-off-by: Zheng Yejian <[email protected]>
    Signed-off-by: Steven Rostedt (Google) <[email protected]>

Signed-off-by: CKI Backport Bot <[email protected]>

Author: CKI Backport Bot

kernel/trace/trace_events_hist.c

@@ -4008,6 +4008,8 @@ static int parse_var_defs(struct hist_trigger_data *hist_data)
 
 			s = kstrdup(field_str, GFP_KERNEL);
 			if (!s) {
+				kfree(hist_data->attrs->var_defs.name[n_vars]);
+				hist_data->attrs->var_defs.name[n_vars] = NULL;
 				ret = -ENOMEM;
 				goto free;
 			}