[Qemu-devel] [Bug 1661815] Re: Stack address is returned from function translate_one

[shqking]

Thanks for your reply.

Inspired by this issue in apache httpd 
(https://bz.apache.org/bugzilla/show_bug.cgi?id=59844#c0),
we customized a checker based on the Clang Static Analyzer to detect such 
undefined behavior.

Yes. 
After examining the code carefully, we didn't find any place where the 
"->fields" is accessed, either. However, we think this kind of defect seems 
like a 'time bomb' and we'd better fix it just to be on the safe side.

** Bug watch added: bz.apache.org/bugzilla/ #59844
   https://bz.apache.org/bugzilla/show_bug.cgi?id=59844

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https://bugs.launchpad.net/bugs/1661815

Title:
  Stack address is returned from function translate_one

Status in QEMU:
  New

Bug description:
  The vulnerable version is qemu-2.8.0, and the vulnerable function is
  in "target-s390x/translate.c".

  The code snippet is as following.

  static ExitStatus translate_one(CPUS390XState *env, DisasContext *s)
  {
      const DisasInsn *insn;
      ExitStatus ret = NO_EXIT;
      DisasFields f;
      ...
      s->fields = &f;
      ...
      s->pc = s->next_pc;
      return ret;
  }

  A stack address, i.e. the address of local variable "f" is returned
  from current function through the output parameter "s->fields" as a
  side effect.

  This issue is one kind of undefined behaviors, according the C
  Standard, 6.2.4 [ISO/IEC 9899:2011]
  
(https://www.securecoding.cert.org/confluence/display/c/DCL30-C.+Declare+objects+with+appropriate+storage+durations)

  This dangerous defect may lead to an exploitable vulnerability.
  We suggest sanitizing "s->fields" as null before return.

  Note that this issue is reported by shqking and Zhenwei Zou together.

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