vm_page_fault: unhandled page fault in kernel space when executing "iret"

[i80and]

While fuzzing Haiku hrev51728 x86-64, I found that the iret instruction sometimes triggers a kernel panic:

echo 'int main(void) { asm("iret;"); }' > panic.c
gcc panic.c -o panic
while true; do ./panic; done

Within a few executions, I get kicked into the KDL:

PANIC: vm_page_fault: unhandled page fault in kernel space at 0x200, ip 0x48694000

I observe this both in VirtualBox as well as on an actual x86-64 machine.

[i80and]

Backtrace

[mmlr]

I have investigated this and found the reason for the panic as well as a possible fix. But I don't yet fully understand why exactly it happens and need to investigate some more to be sure that the fix is proper.

[waddlesplash]

Confirmed this happens on x86_gcc2h as well.

[waddlesplash]

Was fixed in hrev52345.