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