[PATCH 0/2] kprobes: Fix kretprobe incorrect stacking order problem

[Masami Hiramatsu]

Hello,

On recent talk with Andrea, I started more precise investigation on
the kernel panic with kretprobes on notrace functions, which Francis
had been reported last year ( https://lkml.org/lkml/2017/7/14/466 ).

At first, I tried to reproduce the issue. I picked up __fdget and
ftrace_ops_assist_func as probed functions.
With CONFIG_KPROBE_EVENTS_ON_NOTRACE=y, I could reproduce the kernel
panic as below.

=====
/sys/kernel/debug/tracing # echo "r:event_1 __fdget" >> kprobe_events 
/sys/kernel/debug/tracing # echo "r:event_2 ftrace_ops_assist_func" >> kprobe_events
/sys/kernel/debug/tracing # echo 1 > events/kprobes/enable 
[   70.491856] BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
[   70.493203] PGD 800000001c62e067 P4D 800000001c62e067 PUD 1b5bf067 PMD 0 
[   70.494247] Oops: 0010 [#1] PREEMPT SMP PTI
[   70.494918] CPU: 6 PID: 1210 Comm: sh Not tainted 4.20.0-rc3+ #58
[   70.495931] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.1-0-g8891697-prebuilt.qemu-project.org 04/01/2014
[   70.497906] RIP: 0010:0x10
[   70.498465] Code: Bad RIP value.
[   70.499077] RSP: 0018:ffffb1d4c0347e78 EFLAGS: 00010246
[   70.499959] RAX: 00000000fffffff7 RBX: 0000000000000000 RCX: 0000000000000000
[   70.501383] RDX: ffff88f19f9c4f80 RSI: ffffffffb7d75e12 RDI: ffffffffb7d0ede7
[   70.502501] RBP: 00007ffc7061af20 R08: 0000000080000002 R09: ffff88f19f9c4f80
[   70.503698] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000005401
[   70.504810] R13: 0000000000000000 R14: ffffb1d4c0347f58 R15: 0000000000000000
[   70.506028] FS:  0000000000922880(0000) GS:ffff88f19d380000(0000) knlGS:0000000000000000
[   70.507354] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   70.508271] CR2: ffffffffffffffe6 CR3: 000000001f916000 CR4: 00000000000006e0
[   70.509419] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   70.510803] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   70.511748] Call Trace:
[   70.512225]  ? ksys_ioctl+0x70/0x70
[   70.512884]  ? __fdget+0x5/0x10
[   70.513454]  ? __fdget+0x5/0x10
[   70.513980]  ? copy_oldmem_page_encrypted+0x20/0x20
[   70.514815]  ? __x64_sys_ioctl+0x16/0x20
[   70.515596]  ? do_syscall_64+0x50/0x100
[   70.516229]  ? entry_SYSCALL_64_after_hwframe+0x49/0xbe
[   70.517143] Modules linked in:
[   70.517806] CR2: 0000000000000010
[   70.518527] ---[ end trace ece844ac05189f10 ]---
[   70.519417] RIP: 0010:0x10
[   70.520026] Code: Bad RIP value.
[   70.520800] RSP: 0018:ffffb1d4c0347e78 EFLAGS: 00010246
[   70.521948] RAX: 00000000fffffff7 RBX: 0000000000000000 RCX: 0000000000000000
[   70.523315] RDX: ffff88f19f9c4f80 RSI: ffffffffb7d75e12 RDI: ffffffffb7d0ede7
[   70.524515] RBP: 00007ffc7061af20 R08: 0000000080000002 R09: ffff88f19f9c4f80
[   70.525702] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000005401
[   70.526715] R13: 0000000000000000 R14: ffffb1d4c0347f58 R15: 0000000000000000
[   70.527673] FS:  0000000000922880(0000) GS:ffff88f19d380000(0000) knlGS:0000000000000000
[   70.528896] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   70.529851] CR2: ffffffffffffffe6 CR3: 000000001f916000 CR4: 00000000000006e0
[   70.530922] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   70.531907] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Killed
=====

This seems the kernel is trying to execute incorrect address.

Next, I focused on the combination of probes. From Francis's report,
this issue caused by the combination of kretprobes, not kprobes.
I ensured that was true.

r __fdget & r ftrace_ops_assist_func => NG
p __fdget & p ftrace_ops_assist_func => OK
p __fdget & r ftrace_ops_assist_func => OK
r __fdget & p ftrace_ops_assist_func => OK

r: kretprobe, p: kprobe

This gave me a hint of what happened. I can explain the cause of this
issue as below;
 
Correct processing of kretprobe on probed-function.

<caller>
 -><probed-function>
   ->fentry
    ->ftrace_ops_assist_func()
     ->kprobe_ftrace_handler()
      ...->pre_handler_kretprobe()
           push the return address (caller) of probed-function to top of the
           kretprobe list and replace it with kretprobe_trampoline.
    <-(ftrace_ops_assist_func())
   <-(fentry)
  <-(probed-function)
[kretprobe_trampoline]
 ->tampoline_handler()
   pop the return address (caller) from top of the kretprobe list
 <-(trampoline_handler())
<caller>

When we put a kretprobe on ftrace_ops_assist_func(), below happens

<caller>
 -><probed-function>
   ->fentry
    ->ftrace_ops_assist_func()
     ->int3
      ->kprobe_int3_handler()
      ...->pre_handler_kretprobe()
           push the return address (*fentry*) of ftrace_ops_assist_func() to
           top of the kretprobe list and replace it with kretprobe_trampoline.
      <-kprobe_int3_handler()
     <-(int3)
     ->kprobe_ftrace_handler()
      ...->pre_handler_kretprobe()
           push the return address (caller) of probed-function to top of the
           kretprobe list and replace it with kretprobe_trampoline.
     <-(kprobe_ftrace_handler())
    <-(ftrace_ops_assist_func())
    [kretprobe_trampoline]
     ->tampoline_handler()
       pop the return address (caller) from top of the kretprobe list
     <-(trampoline_handler())
    <caller>
    [run caller with incorrect stack information]
   <-(<caller>)
  !!KERNEL PANIC!!

Therefore, this kernel panic happens only when we put 2 k*ret*probes on
ftrace_ops_assist_func() and other functions. If we put kprobes, it
doesn't cause any issue, since it doesn't change the return address.

To fix (or just avoid) this issue, we can introduce a frame pointer
verification to skip wrong order entries. And I also would like to
blacklist those functions because those are part of ftrace-based 
kprobe handling routine.

BTW, this is not all of issues. To remove CONFIG_KPROBE_EVENTS_ON_NOTRACE
I'm trying to find out other notrace functions which can cause
kernel crash by probing. Mostly done on x86, so I'll post it
after this series.

Thank you,

---

Masami Hiramatsu (2):
      x86/kprobes: Verify stack frame on kretprobe
      kprobes: Mark ftrace mcount handler functions nokprobe


 arch/x86/kernel/kprobes/core.c |   26 ++++++++++++++++++++++++++
 include/linux/kprobes.h        |    1 +
 kernel/trace/ftrace.c          |    5 ++++-
 3 files changed, 31 insertions(+), 1 deletion(-)

--
Masami Hiramatsu (Linaro) <mhiramat@xxxxxxxxxx>