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[Bug 1613817] Re: x86: ret, lret and iret with noncanonical IP saves wro
From: |
Thomas Huth |
Subject: |
[Bug 1613817] Re: x86: ret, lret and iret with noncanonical IP saves wrong IP on the exception stack |
Date: |
Tue, 10 Nov 2020 14:49:03 -0000 |
The QEMU project is currently considering to move its bug tracking to another
system. For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting older bugs to "Incomplete" now.
If you still think this bug report here is valid, then please switch the state
back to "New" within the next 60 days, otherwise this report will be marked as
"Expired". Or mark it as "Fix Released" if the problem has been solved with a
newer version of QEMU already. Thank you and sorry for the inconvenience.
** Changed in: qemu
Status: New => Incomplete
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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:
Incomplete
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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- [Bug 1613817] Re: x86: ret, lret and iret with noncanonical IP saves wrong IP on the exception stack,
Thomas Huth <=