[PATCH] ftrace/x86: Fix function graph tracer reset path

[Namhyung Kim]

On my system, simply enabling and disabling function graph tracer can
crash the kernel.  I don't know how it worked until now.

The ftrace_disable_ftrace_graph_caller() modifies jmp instruction at
ftrace_graph_call assuming it's a 5 bytes near jmp (e9 <offset>).
However it's a short jmp consisting of 2 bytes only (eb <offset>).  And
ftrace_stub() is located just below the ftrace_graph_caller so
modification above breaks the instruction resulting in kernel oops on
the ftrace_stub() with the invalid opcode like below:

  # cd /sys/kernel/trace
  # echo function_graph > current_tracer
  # echo nop > current_tracer

  [   78.122055] invalid opcode: 0000 [#1] SMP
  [   78.125125] Modules linked in: x86_pkg_temp_thermal kvm_intel kvm irqbypass crc32c_intel pcspkr iwldvm iwlwifi
  [   78.128241] CPU: 2 PID: 17 Comm: migration/2 Not tainted 4.6.0-rc4+ #36
  [   78.131310] Hardware name: LENOVO 4286A74/4286A74, BIOS 8DET56WW (1.26 ) 12/01/2011
  [   78.134369] task: ffff88040bec4240 ti: ffff88040bee4000 task.ti: ffff88040bee4000
  [   78.137412] RIP: 0010:[<ffffffff818939a8>]  [<ffffffff818939a8>] ftrace_stub+0x0/0x8
  [   78.140477] RSP: 0018:ffff88040bee7e48  EFLAGS: 00010246
  [   78.143489] RAX: ffff88040bec4240 RBX: ffffffff8107a7b0 RCX: 000000000000001f
  [   78.146512] RDX: 0000000000000000 RSI: ffff88041e2929d8 RDI: ffff88040bee7e50
  [   78.149581] RBP: ffff88040bee7e80 R08: ffff88040bee4000 R09: 0000000000000000
  [   78.152647] R10: 00000000000318b7 R11: ffff8800d661f800 R12: ffff88040d8011b0
  [   78.155679] R13: ffffffff81e43620 R14: ffff88040bda8588 R15: ffffffff81e503e0
  [   78.158675] FS:  0000000000000000(0000) GS:ffff88041e280000(0000) knlGS:0000000000000000
  [   78.161699] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [   78.164690] CR2: 00007fadb22dde1d CR3: 00000000d5ce2000 CR4: 00000000000406e0
  [   78.167691] Stack:
  [   78.170658]  ffffffff8110b3ee ffffffff8188de90 00000009161d55f6 00000012306fc2e4
  [   78.173710]  0000000000000000 ffff880400000000 ffff88040bec4240 ffff88040bee7ec8
  [   78.176783]  ffffffff81893bbd 0000000000000000 ffff88040bec4240 ffffffff81893ba8
  [   78.179863] Call Trace:
  [   78.182853]  [<ffffffff8110b3ee>] ? ftrace_return_to_handler+0x8e/0x100
  [   78.185909]  [<ffffffff8188de90>] ? __schedule+0xae0/0xae0
  [   78.188941]  [<ffffffff81893bbd>] return_to_handler+0x15/0x27
  [   78.192001]  [<ffffffff81893ba8>] ? ftrace_graph_caller+0xa8/0xa8
  [   78.195091]  [<ffffffff8107a6f0>] ? sort_range+0x30/0x30
  [   78.198138]  [<ffffffff810778a9>] kthread+0xc9/0xe0
  [   78.201143]  [<ffffffff81891a12>] ret_from_fork+0x22/0x40
  [   78.204138]  [<ffffffff810777e0>] ? kthread_worker_fn+0x170/0x170
  [   78.207129] Code: 8b 44 24 48 48 8b 7c 24 70 48 8b 74 24 68 48 8b 54 24 60
                       48 8b 4c 24 58 48 8b 44 24 50 48 8b 6c 24 20 48 81 c4 d0
                       00 00 00 e9 fd <ff> ff ff 80 00 00 00 00 9c 55 ff 74 24 18
                       55 48 89 e5 ff 74 24
  [   78.213997] RIP  [<ffffffff818939a8>] ftrace_stub+0x0/0x8
  [   78.217190]  RSP <ffff88040bee7e48>
  [   78.220374] ---[ end trace 0af2f0d9f7301011 ]---

Looking at the code dump, the ftrace_stub instruction was overwritten as
<ff>.  Below is disassembly output of related code.

  $ objdump -d --start-address=0xffffffff818939a6 --stop-address=0xffffffff818939b0 vmlinux

  vmlinux:     file format elf64-x86-64

  Disassembly of section .text:

  ffffffff818939a6 <ftrace_epilogue>:
  ffffffff818939a6:	eb 00			jmp    ffffffff818939a8 <ftrace_stub>

  ffffffff818939a8 <ftrace_stub>:
  ffffffff818939a8:	c3			retq
  ffffffff818939a9:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)

As you can see ftrace_epilogue (same as ftrace_graph_caller) is 2 byte
ahead of ftrace_stub.  And it's replaced by a jump to ftrace_stub() by
ftrace_disable_ftrace_graph_caller: "e9 <0xfffffffd>".  Pads 3 bytes
after ftrace_epilogue to prevent ftrace_stub from being overwritten.

Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 arch/x86/kernel/mcount_64.S | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kernel/mcount_64.S b/arch/x86/kernel/mcount_64.S
index ed48a9f465f8..0e6af57a713a 100644
--- a/arch/x86/kernel/mcount_64.S
+++ b/arch/x86/kernel/mcount_64.S
@@ -180,6 +180,15 @@ GLOBAL(ftrace_epilogue)
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 GLOBAL(ftrace_graph_call)
 	jmp ftrace_stub
+	/*
+	 * The above jmp is generated as a short jump which occupies 2 bytes
+	 * but ftrace_enable/disable_ftrace_graph_caller() assumes it's a
+	 * near jump which occupies 5 bytes so breaks ftrace_stub() below.
+	 * Add 3 bytes padding to avoid that.
+	 */
+	nop
+	nop
+	nop
 #endif
 
 GLOBAL(ftrace_stub)
-- 
2.8.0