ftrace: Reset fgd->hash in ftrace_graph_write()

fgd->hash is saved and then freed, but is never reset to either
ftrace_graph_hash nor ftrace_graph_notrace_hash. But if multiple writes are
performed, then the freed hash could be accessed again.

 # cd /sys/kernel/debug/tracing
 # head -1000 available_filter_functions > /tmp/funcs
 # cat /tmp/funcs > set_graph_function

Causes:

 general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
 Modules linked in:  [...]
 CPU: 2 PID: 1337 Comm: cat Not tainted 4.10.0-rc2-test-00010-g6b052e9 #32
 Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v02.05 05/07/2012
 task: ffff880113a12200 task.stack: ffffc90001940000
 RIP: 0010:free_ftrace_hash+0x7c/0x160
 RSP: 0018:ffffc90001943db0 EFLAGS: 00010246
 RAX: 6b6b6b6b6b6b6b6b RBX: 6b6b6b6b6b6b6b6b RCX: 6b6b6b6b6b6b6b6b
 RDX: 0000000000000002 RSI: 0000000000000001 RDI: ffff8800ce1e1d40
 RBP: ffff8800ce1e1d50 R08: 0000000000000000 R09: 0000000000006400
 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
 R13: ffff8800ce1e1d40 R14: 0000000000004000 R15: 0000000000000001
 FS:  00007f9408a07740(0000) GS:ffff88011e500000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000aee1f0 CR3: 0000000116bb4000 CR4: 00000000001406e0
 Call Trace:
  ? ftrace_graph_write+0x150/0x190
  ? __vfs_write+0x1f6/0x210
  ? __audit_syscall_entry+0x17f/0x200
  ? rw_verify_area+0xdb/0x210
  ? _cond_resched+0x2b/0x50
  ? __sb_start_write+0xb4/0x130
  ? vfs_write+0x1c8/0x330
  ? SyS_write+0x62/0xf0
  ? do_syscall_64+0xa3/0x1b0
  ? entry_SYSCALL64_slow_path+0x25/0x25
 Code: 01 48 85 db 0f 84 92 00 00 00 b8 01 00 00 00 d3 e0 85 c0 7e 3f 83 e8 01 48 8d 6f 10 45 31 e4 4c 8d 34 c5 08 00 00 00 49 8b 45 08 <4a> 8b 34 20 48 85 f6 74 13 48 8b 1e 48 89 ef e8 20 fa ff ff 48
 RIP: free_ftrace_hash+0x7c/0x160 RSP: ffffc90001943db0
 ---[ end trace 999b48216bf4b393 ]---
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -4858,10 +4858,13 @@ ftrace_graph_write(struct file *file, const char __user *ubuf,
 		if (!new_hash)
 			ret = -ENOMEM;
 
-		if (fgd->type == GRAPH_FILTER_FUNCTION)
+		if (fgd->type == GRAPH_FILTER_FUNCTION) {
 			rcu_assign_pointer(ftrace_graph_hash, new_hash);
-		else
+			fgd->hash = ftrace_graph_hash;
+		} else {
 			rcu_assign_pointer(ftrace_graph_notrace_hash, new_hash);
+			fgd->hash = ftrace_graph_notrace_hash;
+		}
 
 		mutex_unlock(&graph_lock);