[BUGFIX PATCH tip/master 0/3] kprobes: Fix a possible deadlock in kretprobe

[Masami Hiramatsu <mhiramat@kernel.org>]

Hi,

This series will fix a possible deadlock case in kretprobe
on x86, arm, arm64. Since kretprobe has been optimized on
those arch, they have similar possible deadlock issue.

Problem
=====

The deadlock senario is when a user puts 2 kretprobes, 
one on normal function and one on a function which can be
called from NMI or FIQ where normal interrupt disabled.
(we don't recommend it, but possible.) In this case, if
the kernel hits the 1st kretprobe on a normal function
return which calls trampoline_handler(), acquire a 
spinlock on the hash table in kretprobe_hash_lock() and
disable irqs.
After that, if NMI(or FIQ on arm/arm64) is occurred and
the 2nd kretprobe is kicked, it also calls
trampoline_handler() and tries to acquire the same 
spinlock (since the hash is based on current task, same
as the 1st kretprobe), it causes a deadlock on the
spinlock.
Note that this is very rare case, but theoretically happens.

Reason and Affected Arch
=====

Actually, this bug has been introduced by kretprobe-booster,
which removes a kprobe from return trampoline code, but also
resets current kprobe, which can be a stopper for the nested
k(ret)probes. So, currently only x86, arm, and arm64 are
affected, because other arch have not implemented the
kretprobe-booster.

Solution
=====

To fix this issue, I introduced a dummy kprobe which is set
as a current kprobe while holding the kretprobe-hash lock.
With that, if an NMI/FIQ occurred and 2nd kretprobe's kprobe
is kicked (to modify the return address, a kprobe is kicked
when the target function is called), the kprobe (and the 2nd
kretprobe also) is skipped because it detects there is
another kprobe is running.
    
This reentrance detection and nested kprobe blocker had
existed when the original kretprobe was implemented by
using kprobe on trampoline code. This fix just revived it.

Thank you,

---

Masami Hiramatsu (3):
      kprobes/x86: Fix a possible deadlock case in kretprobe
      kprobes/arm64: Fix a possible deadlock case in kretprobe
      kprobes/arm: Fix a possible deadlock case in kretprobe


 arch/arm/probes/kprobes/core.c     |   12 ++++++++++--
 arch/arm64/kernel/probes/kprobes.c |   12 ++++++++++--
 arch/x86/kernel/kprobes/core.c     |   13 ++++++++++---
 3 files changed, 30 insertions(+), 7 deletions(-)

--
Masami Hiramatsu