[Qemu-devel] [Bug 1824344] [NEW] x86: retf or iret pagefault sets wrong error code

[Zdeněk Sojka]

Public bug reported:

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 <address@hidden>
Date:   Wed Apr 10 15:38:59 2019 +0100

    Update version for v4.0.0-rc3 release
    
    Signed-off-by: Peter Maydell <address@hidden>

** Affects: qemu
     Importance: Undecided
         Status: New


** Tags: amd64 i386

** Attachment added: "dmesg screenshot"
   https://bugs.launchpad.net/bugs/1824344/+attachment/5254916/+files/dmesg.png

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

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

Status in QEMU:
  New

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 <address@hidden>
  Date:   Wed Apr 10 15:38:59 2019 +0100

      Update version for v4.0.0-rc3 release
      
      Signed-off-by: Peter Maydell <address@hidden>

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