perf top: Fix rare segfault in thread__comm_len()

In thread__comm_len(),strlen() is called outside of the
thread->comm_lock critical section,which may cause a UAF
problems if comm__free() is called by the process_thread
concurrently.

backtrace of the core file is as follows:

    (gdb) bt
    #0  __strlen_evex () at ../sysdeps/x86_64/multiarch/strlen-evex.S:77
    #1  0x000055ad15d31de5 in thread__comm_len (thread=0x7f627d20e300) at util/thread.c:320
    #2  0x000055ad15d4fade in hists__calc_col_len (h=0x7f627d295940, hists=0x55ad1772bfe0)
        at util/hist.c:103
    #3  hists__calc_col_len (hists=0x55ad1772bfe0, h=0x7f627d295940) at util/hist.c:79
    #4  0x000055ad15d52c8c in output_resort (hists=hists@entry=0x55ad1772bfe0, prog=0x0,
        use_callchain=false, cb=cb@entry=0x0, cb_arg=0x0) at util/hist.c:1926
    #5  0x000055ad15d530a4 in evsel__output_resort_cb (evsel=evsel@entry=0x55ad1772bde0,
        prog=prog@entry=0x0, cb=cb@entry=0x0, cb_arg=cb_arg@entry=0x0) at util/hist.c:1945
    #6  0x000055ad15d53110 in evsel__output_resort (evsel=evsel@entry=0x55ad1772bde0,
        prog=prog@entry=0x0) at util/hist.c:1950
    #7  0x000055ad15c6ae9a in perf_top__resort_hists (t=t@entry=0x7ffcd9cbf4f0) at builtin-top.c:311
    #8  0x000055ad15c6cc6d in perf_top__print_sym_table (top=0x7ffcd9cbf4f0) at builtin-top.c:346
    #9  display_thread (arg=0x7ffcd9cbf4f0) at builtin-top.c:700
    torvalds#10 0x00007f6282fab4fa in start_thread (arg=<optimized out>) at pthread_create.c:443
    torvalds#11 0x00007f628302e200 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

The reason is that strlen() get a pointer to a memory that has been freed.

The string pointer is stored in the structure comm_str, which corresponds
to a rb_tree node,when the node is erased, the memory of the string is also freed.

In thread__comm_len(),it gets the pointer within the thread->comm_lock critical section,
but passed to strlen() outside of the thread->comm_lock critical section, and the perf
process_thread may called comm__free() concurrently, cause this segfault problem.

The process is as follows:

display_thread                                  process_thread
--------------                                  --------------

thread__comm_len
  -> thread__comm_str
       # held the comm read lock
    -> __thread__comm_str(thread)
       # release the comm read lock
                                                thread__delete
                                                     # held the comm write lock
                                                  -> comm__free
                                                    -> comm_str__put(comm->comm_str)
                                                      -> zfree(&cs->str)
                                                     # release the comm write lock
      # The memory of the string pointed
        to by comm has been free.
    -> thread->comm_len = strlen(comm);

This patch expand the critical section range of thread->comm_lock in thread__comm_len(),
to make strlen() called safe.

Signed-off-by: Wenyu Liu <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Feilong Lin <[email protected]>
Cc: Hewenliang <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Yunfeng Ye <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

tools/perf/util/thread.c

@@ -311,17 +311,30 @@ const char *thread__comm_str(struct thread *thread)
 	return str;
 }
 
+static int __thread__comm_len(struct thread *thread, const char *comm)
+{
+	if (!comm)
+		return 0;
+	thread->comm_len = strlen(comm);
+
+	return thread->comm_len;
+}
+
 /* CHECKME: it should probably better return the max comm len from its comm list */
 int thread__comm_len(struct thread *thread)
 {
-	if (!thread->comm_len) {
-		const char *comm = thread__comm_str(thread);
-		if (!comm)
-			return 0;
-		thread->comm_len = strlen(comm);
+	int comm_len = thread->comm_len;
+
+	if (!comm_len) {
+		const char *comm;
+
+		down_read(&thread->comm_lock);
+		comm = __thread__comm_str(thread);
+		comm_len = __thread__comm_len(thread, comm);
+		up_read(&thread->comm_lock);
 	}
 
-	return thread->comm_len;
+	return comm_len;
 }
 
 size_t thread__fprintf(struct thread *thread, FILE *fp)