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

From: Robert Henry
Subject: [Bug 1824344] Re: x86: retf or iret pagefault sets wrong error code
Date: Tue, 17 Mar 2020 03:11:11 -0000

This appears to be similar to
https://bugs.launchpad.net/qemu/+bug/1866892 (and much simpler)

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  x86: retf or iret pagefault sets wrong error code

Status in QEMU:

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"
  return 0;

  $ gcc tst.c
  $ ./a.out

  "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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