[PATCH] tracing: Fix NULL pointer dereference in func_set_flag()

[Yuanhe Shu]

func_set_flag() dereferences tr->current_trace_flags before verifying
that the current tracer is actually the function tracer. When the active
tracer has been switched away from "function" (e.g., to "wakeup_rt"),
tr->current_trace_flags can be NULL, leading to a NULL pointer
dereference and kernel crash.

The call chain that triggers this is:

  trace_options_write()
    -> __set_tracer_option()
      -> trace->set_flag()          /* func_set_flag */

In func_set_flag(), the first operation is:

  if (!!set == !!(tr->current_trace_flags->val & bit))

This dereferences tr->current_trace_flags unconditionally. The safety
check that guards against a non-function tracer:

  if (tr->current_trace != &function_trace)
      return 0;

is placed *after* the dereference, which is too late.

This was observed with the following crash dump:

  BUG: unable to handle page fault at 0000000000000000
  RIP: func_set_flag+0xd

  Call Trace:
   __set_tracer_option+0x27
   trace_options_write+0x75
   vfs_write+0x12a
   ksys_write+0x66
   do_syscall_64+0x5b

  RIP: ffffffff914c973d  RSP: ff67ec88b01dfdf0  RFLAGS: 00010202
  RAX: 0000000000000000  RBX: ff3a826e80354580  RCX: 0000000000000001
  RDX: 0000000000000001  RSI: 0000000000000000  RDI: ffffffff93918080

The disassembly confirms the fault:

  func_set_flag+0:   mov 0x1f08(%rdi), %rax  ; RAX = tr->current_trace_flags = NULL
  func_set_flag+13:  mov (%rax), %eax        ; page fault: dereference NULL

At the time of the crash:
  tr->current_trace_flags = 0x0 (NULL)
  tr->current_trace = wakeup_rt_tracer (not function_trace)

The scenario is that a process opens a function tracer option file (such
as "func_stack_trace"), then the current tracer is switched to another
tracer (e.g., "wakeup_rt"), which sets current_trace_flags to NULL. When
the process subsequently writes to the option file, func_set_flag() is
invoked and crashes on the NULL dereference.

Fix this by moving the current_trace check before the
current_trace_flags dereference, so that func_set_flag() returns early
when the function tracer is not active.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 76680d0d2825 ("tracing: Have function tracer define options per instance")
Signed-off-by: Yuanhe Shu <xiangzao@xxxxxxxxxxxxxxxxx>
---
 kernel/trace/trace_functions.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
index f283391a4dc8..cd37f2013758 100644
--- a/kernel/trace/trace_functions.c
+++ b/kernel/trace/trace_functions.c
@@ -458,12 +458,12 @@ func_set_flag(struct trace_array *tr, u32 old_flags, u32 bit, int set)
 	ftrace_func_t func;
 	u32 new_flags;
 
-	/* Do nothing if already set. */
-	if (!!set == !!(tr->current_trace_flags->val & bit))
+	/* We can change this flag only when current tracer is function. */
+	if (tr->current_trace != &function_trace)
 		return 0;
 
-	/* We can change this flag only when not running. */
-	if (tr->current_trace != &function_trace)
+	/* Do nothing if already set. */
+	if (!!set == !!(tr->current_trace_flags->val & bit))
 		return 0;
 
 	new_flags = (tr->current_trace_flags->val & ~bit) | (set ? bit : 0);
-- 
2.39.3