[Bug 1824344] Re: x86: retf or iret pagefault sets wrong error code

[Thomas Huth]

This is an automated cleanup. This bug report has been moved to QEMU's
new bug tracker on gitlab.com and thus gets marked as 'expired' now.
Please continue with the discussion here:

 https://gitlab.com/qemu-project/qemu/-/issues/265


** Changed in: qemu
       Status: New => Expired

** Bug watch added: gitlab.com/qemu-project/qemu/-/issues #265
   https://gitlab.com/qemu-project/qemu/-/issues/265

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https://bugs.launchpad.net/bugs/1824344

Title:
  x86: retf or iret pagefault sets wrong error code

Status in QEMU:
  Expired

Bug description:
  With a x86_64 or i386 guest, non-KVM, when trying to execute a
  "iret/iretq/retf" instruction in userspace with invalid stack pointer
  (under a protected mode OS, like Linux), wrong bits are set in the
  pushed error code; bit 2 is not set, indicating the error comes from
  kernel space.

  If the guest OS is using this flag to decide whether this was a kernel
  or user page fault, it will mistakenly decide a kernel has irrecoverably
  faulted, possibly causing guest OS panic.

  
  How to reproduce the problem a guest (non-KVM) Linux:
  Note, on recent Linux kernel version, this needs a CPU with SMAP support
  (eg. -cpu max)

  $ cat tst.c
  int main()
  {
  __asm__ volatile (
  "mov $0,%esp\n"
  "retf"
  );
  return 0;
  }

  $ gcc tst.c
  $ ./a.out
  Killed

  
  "dmesg" shows the kernel has in fact triggered a "BUG: unable to handle
  kernel NULL pointer dereference...", but it has "recovered" by killing
  the faulting process (see attached screenshot).

  
  Using self-compiled qemu from git:
  commit 532cc6da74ec25b5ba6893b5757c977d54582949 (HEAD -> master, tag: 
v4.0.0-rc3, origin/master, origin/HEAD)
  Author: Peter Maydell <peter.maydell@linaro.org>
  Date:   Wed Apr 10 15:38:59 2019 +0100

      Update version for v4.0.0-rc3 release
      
      Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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