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[Qemu-devel] [Bug 1613817] Re: x86: ret, lret and iret with noncanonical


From: Vda-linux
Subject: [Qemu-devel] [Bug 1613817] Re: x86: ret, lret and iret with noncanonical IP saves wrong IP on the exception stack
Date: Tue, 16 Aug 2016 17:09:39 -0000

# qemu-system-x86_64 --version
QEMU emulator version 2.6.92(qemu-2.7.0-0.1.rc2.fc26), Copyright (c) 2003-2008 
Fabrice Bellard

Running it like this:

qemu-system-x86_64 -no-reboot -kernel "$bzImage" -initrd initramfs.cpio
-append "panic=1"

(i.e. no KVM, no unusual options)

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1613817

Title:
  x86: ret, lret and iret with noncanonical IP saves wrong IP on the
  exception stack

Status in QEMU:
  New

Bug description:
  This test program:

  # compile with: gcc -nostartfiles -nostdlib
  _start:         .globl  _start
                  mov     %ss,%eax
                  push    %rax
                  push    %rsp
                  pushf
                  mov     %cs,%eax
                  push    %rax
                  mov     $0x1234567812345678,%rax
                  push    %rax
  //qemu bug: ip=1234567812345678, should be ip=0000000000400abc:
                  iretq
  1:
                  jmp     1b

  should segfault on IRET instruction because return address on stack is invalid
  (it is not canonical). And it does, both on native CPU and in qemu.
  But there is a difference: on native CPU, it fails before instruction is 
executed,
  IOW: saved IP points to the failed IRET:

  # strace -i ./bad_ip_in_iret 
  [00007fa609805d57] execve("./bad_ip_in_iret", ["./bad_ip_in_iret"], [/* 54 
vars */]) = 0
  [00000000004000e7] --- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, 
si_addr=0} ---
   ^^^^^^^^^^^^^^^^-NOTE THIS
  [????????????????] +++ killed by SIGSEGV (core dumped) +++

  
  In qemu, evidently instruction succeeds, and then emulated CPU throws an 
exception because fetching instructions from non-canonical addresses is not 
allowed:

  / # strace -i ./bad_ip_in_iret
  [000000000041a790] execve("./bad_ip_in_iret", ["./bad_ip_in_iret"], [/* 5 
vars */]) = 0
  [1234567812345678] --- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, 
si_addr=0} ---
   ^^^^^^^^^^^^^^^^-NOTE THIS
  [????????????????] +++ killed by SIGSEGV +++
  Segmentation fault

  Thus, the emulation is not the same as real CPU.

  This is not specific to IRET, the same happens with "far return" LRET,
  and with ordinary RET instructions as well.
  In qemu:

  / # strace -i ./bad_ip_in_lret
  [000000000041a790] execve("./bad_ip_in_lret", ["./bad_ip_in_lret"], [/* 5 
vars */]) = 0
  [1234567812345678] --- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, 
si_addr=0} ---
  [????????????????] +++ killed by SIGSEGV +++
  Segmentation fault
  / # strace -i ./bad_ip_in_ret
  [000000000041a790] execve("./bad_ip_in_ret", ["./bad_ip_in_ret"], [/* 5 vars 
*/]) = 0
  [1234567812345678] --- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, 
si_addr=0} ---
  [????????????????] +++ killed by SIGSEGV +++
  Segmentation fault

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